Subject: smtpupd-09b.txt

From: Matti Aarnio <[email-masked]>
Date: Tue, 8 Dec 1998 00:35:57 +0200

As John has weird problems at posting this draft, and we are
running out of time before WG session(s), I am giving visible
spot for it at:

  ftp://ftp.funet.fi/pub/unix/incoming/smtpupd9b.txt

and also attaching it below here as is.
That spot should be considered temporary, and I *should* remember
to remove it away after IETF.  Policy at FUNET is not to publish
anything in incoming directories..

  /Matti Aarnio <[email-masked]>


INTERNET-DRAFT                                John C. Klensin, Editor
Expires in six months                         
December 6, 1998

                  Simple Mail Transfer Protocol 

                 draft-ietf-drums-smtpupd-09b.txt

Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working documents
of the Internet Engineering Task Force (IETF), its areas, and its working
groups.  Note that other groups may also distribute working documents as
Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and
may be updated, replaced, or obsoleted by other documents at any time.  It
is inappropriate to use Internet- Drafts as reference material or to cite
them other than as "work in progress."

To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe),
ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim),
ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

[[Sections marked with doubled brackets (e.g., "<<") are explicit
placeholders or known major loose ends. Please review these ASAP. The
sections with these placeholders are 4.1.2, 4.4, and X.2.]]

[[Appendix X will be removed before the document is submitted to the
IESG.]]

[[If consensus is reached on this document, it will be forwarded to the
IESG with the recommendation that it be processed onto the Standards
track.]]

Copyright Notice

Copyright (C) The Internet Society (1998).  All Rights Reserved.

                           Table of Contents

0. Abstract

1. Introduction

2. The SMTP Model
2.1 Basic Structure
2.2 The Extension Model
2.2.1 Background
2.2.2 Definition and Registration of Extensions
2.3 Terminology
2.3.1 Mail Objects
2.3.2 Senders and Receivers
2.3.3 Mail Agents
2.3.4 Host
2.3.5 Domain
2.3.6 Buffer and State Table
2.3.7 Lines
2.3.8 Originator, Delivery, Relay, and Gateway Systems
2.3.9 Message Content and Mail Data
2.3.10 Mailbox and Address
2.3.11 Reply
2.4 Syntax Principles
2.4.1 General Syntax and Transaction Model
2.4.2 Command and Reply Syntax

3. The SMTP Procedures: An Overview
3.1 Session Initiation
3.2 Client Initiation
3.3 Mail Transactions
3.4 Forwarding for Address Correction or Updating
3.5 Commands for Debugging Addresses
3.5.1 Overview
3.5.2 VRFY Normal Response
3.5.3 Meaning of VRFY or EXPN Success Response
3.5.4 Semantics and Applications of EXPN
3.6 Domains
3.7 Relaying
3.8 Mail Gatewaying
3.8.1 Header Fields in Gatewaying
3.8.2 Received Lines in Gatewaying
3.8.3 Addresses in Gatewaying
3.8.4 Other Header Fields in Gatewaying
3.8.5 Envelopes in Gatewaying
3.9 Terminating Sessions and Connections
3.10 Mailing Lists and Aliases
3.10.1 Alias
3.10.2 List

4. The SMTP Specifications
4.1 SMTP Commands
4.1.1 Command Semantics and Syntax
4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO)
4.1.1.2 MAIL (MAIL)
4.1.1.3 RECIPIENT (RCPT)
4.1.1.4 DATA (DATA)
4.1.1.5 RESET (RSET)
4.1.1.6 VERIFY (VRFY)
4.1.1.7 EXPAND (EXPN)
4.1.1.8 HELP (HELP)
4.1.1.9 NOOP (NOOP)
4.1.1.10 QUIT (QUIT)
4.1.2 Lower-level Syntax
4.1.3 Address Literals
4.1.4 Order of Commands
4.1.5 Private-use Commands
4.2  SMTP Replies
4.2.1 Reply Code Severities and Theory
4.2.2 Reply Codes by Function Groups
4.2.3  Reply Codes in Numeric Order
4.2.4 Reply Code 502
4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
4.3 Sequencing of Commands and Replies
4.3.1 Sequencing Overview
4.3.2 Command-Reply Sequences
4.4 Trace Information
4.5 Additional Implementation Issues
4.5.1 Minimum Implementation
4.5.2 Transparency
4.5.3 Sizes and Timeouts
4.5.4 Queuing Strategies
4.5.4.1 Sending Strategy
4.5.4.2 Receiving Strategy
4.5.5 Messages with a null reverse-path

5. Address Resolution and Mail Handling

6. Problem Detection and Handling
6.1 Reliable Delivery and Replies by Email
6.2 Loop Detection
6.3 Compensating for Irregularities

7. Security Considerations
7.1 Mail Security and Spoofing
7.2 "Blind" Copies
7.3 VRFY, EXPN, and Security
7.4 Information Disclosure in Announcements
7.5 Information Disclosure in Trace Fields
7.6 Scope of Operation of SMTP Servers

8. IANA Considerations

9. References

10. Editors' Addresses

11. Acknowledgments

A. TCP Transport Service
B. Generating SMTP Commands from RFC 822 Headers 
C. Source Routes
D. Scenarios
E. Other Gateway Issues
F. Deprecated Features of RFC 821
X. Change Summary and Loose Ends (Temporary)

0. Abstract

This document is a self-contained specification of the basic protocol for
the Internet electronic mail transport, consolidating and updating:

 - the original SMTP specification of RFC 821 [RFC-821],

 - domain name system requirements and implications for mail transport from
   RFC 1035 [RFC-DNS] and RFC 974 [RFC-974],

 - the clarifications and applicability statements in RFC 1123 [RFC-1123],
   and

 - material drawn from the SMTP Extension mechanisms [SMTPEXT].

It replaces RFC 821, RFC 974, and the mail transport materials of RFC 1123.
However, RFC 821 specifies some features that are not in significant use in
the Internet of the mid-1990s and (in appendices) some additional transport
models.  Those sections are omitted here in the interest of clarity and
brevity; readers needing them should refer to RFC 821.

It also includes some additional material from RFC 1123 that required
amplification.  This material has been identified in multiple ways, mostly
by tracking flaming on various lists and newsgroups and problems of unusual
readings or interpretations that have turned up as the SMTP extensions have
been deployed.  Where this specification moves beyond consolidation and
actually differs from earlier documents, it supersedes them technically as
well as textually.

Although SMTP was designed as a mail transport and delivery protocol, this
specification also contains information that is important to its use as a
"mail posting" protocol, as recommended for POP [RFC-POP2, RFC-POP3] and
IMAP [RFC-IMAP4].

Section 2.3 provides definitions of terms specific to this document. Except
when the historical terminology is necessary for clarity, this document
uses the current "client" and "server" terminology to identify the sending
and receiving SMTP processes, respectively.

A companion document discusses message headers, message bodies and formats
and structures for them, and their relationship - [MSGFMT].

1. Introduction

The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer
mail reliably and efficiently.

SMTP is independent of the particular transmission subsystem and requires
only a reliable ordered data stream channel.  While this document
specifically discusses transport over TCP, other transports are possible.
Appendices to RFC 821 describe some of them.

An important feature of SMTP is its capability to transport mail across
transport service environments, usually referred to as "SMTP mail relaying"
(see section 3.8).  A transport service  environment might consist of the
mutually-TCP-accessible hosts on  the public Internet, the
mutually-TCP-accessible hosts on a firewall-isolated private TCP/IP
Intranet, or hosts in some other LAN or WAN environment utilizing a
different transport-level protocol.  It is important to realize that
"transport service environments" are one-to-one with usual definitions of
"networks".  A process can communicate directly with another process, and
transport mail using this protocol, through any mutually known and
connected transport service. Conversely, mail can be relayed or gatewayed
between processes in two different transport service environments by a
process known and connected to each of the two transport service
environments. The Mail eXchanger mechanisms of the domain name system
[RFC-DNS, and section 5 of this document] allows the identity of hosts
supporting SMTP relay and gateway processes to be specified.

2. The SMTP Model

2.1 Basic Structure

The SMTP design can be pictured as:

               +----------+                +----------+
   +------+    |          |                |          |
   | User |<-->|          |      SMTP      |          |
   +------+    |  Sender- |Commands/Replies| Receiver-|
   +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
   | File |<-->|          |    and Mail    |          |<-->| File |
   |System|    |          |                |          |    |System|
   +------+    +----------+                +----------+    +------+
                SMTP client                SMTP server

When an SMTP client has a message to transmit, it establishes a two-way
transmission channel to an SMTP server. The role of an SMTP client is to
transfer mail messages to one or more SMTP servers, or report its failure
to do so.

The means by which a mail message is transferred to an SMTP client, and how
that client determines the domain name(s) to which mail messages are to be
transferred is a local matter, and is not addressed by this document. In
some cases, the domain name(s) transferred to, or determined by, an SMTP
client will identify the final destination(s) of the mail message. In other
cases, common with SMTP clients associated with implementations of the POP
[RFC-POP2, RFC-POP3] or IMAP [RFC-IMAP4] protocols, or when the SMTP client
is inside an isolated transport service environment, the domain name
determined will identify an intermediate destination through which all mail
messages are to be relayed.  SMTP clients that transfer all traffic,
regardless of the target domain names associated with the individual
messages, or that do not maintain queues for retrying message transmissions
that initially cannot be completed, may otherwise conform to this
specification but are not considered fully-capable. Fully-capable SMTP
implementations, including the relays used by these less capable ones, and
their destinations, are expected to support all of the queuing, retrying,
and alternate address functions discussed in this specification.

The means by which an SMTP client, once it has determined a target domain
name, determines the identity of an SMTP server to which a copy of a
message is to be transferred, and then performs that transfer, is covered
by this document. To effect a mail transfer to an SMTP server, an SMTP
client establishes a two-way transmission channel to that SMTP server. An
SMTP client determines the address of an appropriate host running an SMTP
server by resolving a destination domain name to either an intermediate
Mail eXchanger host or a final target host.

An SMTP server may be either the ultimate destination or an intermediate
"relay" (that is, it may assume the role of an SMTP client after receiving
the message) or "gateway" (that is, it may transport the message further
using some protocol other than SMTP).  SMTP commands are generated by the
SMTP client and sent to the SMTP server.  SMTP replies are sent from the
SMTP server to the SMTP client in response to the commands.

Once the transmission channel is established and initial handshaking
completed, the SMTP client normally initiates a mail transaction. Such a
transaction consists of a series of commands to specify the originator and
destination of the mail and transmission of the message content (including
any headers or other structure) itself. When the same message is sent to
multiple recipients, this protocol encourages the transmission of only one
copy of the data for all recipients at the same destination (or
intermediate relay) host.

The server responds to each command with a reply; replies may indicate that
the command was accepted, that additional commands are expected, or that a
temporary or permanent error condition exists. Commands specifying the
sender or recipients may include server-permitted SMTP service extension
requests as discussed in section 2.2.  The dialog is purposely lock-step,
one-at-a-time, although this can be modified by mutually-agreed extension
requests such as in [RFC-Pipeline].

Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
transactions. In addition, an SMTP client may use a connection to an SMTP
server for ancillary services such as verification of email addresses or
retrieval of mailing list subscriber addresses.

As suggested above, this protocol provides mechanisms for the transmission
of mail.  This transmission normally occurs directly from the sending
user's host to the receiving user's host when the two hosts are connected
to the same transport service.  When they are not connected to the same
transport service, transmission occurs via one or more relay SMTP servers.
An intermediate host that acts as either an SMTP relay or as a gateway into
some other transmission environment is usually selected through the use of
the domain name service (DNS) Mail eXchanger mechanism.

To provide relay capability, the SMTP server is supplied with the name of
the ultimate destination host as well as the destination mailbox name.
Usually, intermediate hosts are determined via the DNS MX record, not by
explicit "source" routing (see appendices C and F.2).

2.2 The Extension Model

2.2.1 Background

In an effort that started in 1990, approximately a decade after RFC 821 was
completed, the protocol was modified with a "service extensions" model that
permits the client and server to agree to utilize shared functionality
beyond the original SMTP requirements. The SMTP extension mechanism defines
a means whereby an extended SMTP client and server may recognize each
other, and the server can inform the client as to the service extensions
that it supports.

Contemporary SMTP implementations MUST support the basic extension
mechanisms. For instance, servers MUST support the EHLO command even if
they do not implement any specific extensions and clients SHOULD
preferentially utilize EHLO rather than HELO. (However, for compatibility
with older conforming implementations, SMTP clients and servers MUST
support the original HELO mechanisms as a fallback.)  Unless the different
characteristics of HELO must be identified for interoperability purposes,
this document discusses only EHLO.

SMTP is widely deployed and high-quality implementations have proven to be
very robust. However, the Internet community now considers some services to
be important that were not anticipated when the protocol was first
designed.  If support for those services is to be added, it must be done in
a way that permits older implementations to continue working acceptably.
The extension framework consists of:

 - The SMTP command EHLO, superseding the earlier HELO,

 - a registry of SMTP service extensions,

 - additional parameters to the SMTP MAIL FROM and RCPT TO commands, and

 - optional replacements for verbs defined in this protocol, such as for
   DATA (see [RFC-BDAT]).

SMTP's strength comes primarily from its simplicity.  Experience with many
protocols has shown that protocols with few options tend towards ubiquity,
whereas protocols with many options tend towards obscurity.

Each and every extension, regardless of its benefits, must be carefully
scrutinized with respect to its implementation, deployment, and
interoperability costs. In many cases, the cost of extending the SMTP
service will likely outweigh the benefit.

2.2.2 Definition and Registration of Extensions

The IANA maintains a registry of SMTP service extensions.  A corresponding
EHLO keyword value is associated with each extension. Each service
extension registered with the IANA must be defined in a formal
standards-track or IESG-approved experimental protocol document.  The
definition must include:

 - the textual name of the SMTP service extension;

 - the EHLO keyword value associated with the extension;

 - the syntax and possible values of parameters associated with the
   EHLO keyword value;

 - any additional SMTP verbs associated with the extension (additional
   verbs will usually be, but are not required to be, the same as the
   EHLO keyword value);

 - any new parameters the extension associates with the MAIL FROM or
   RCPT TO verbs;

 - a description of how support for the extension affects the behavior
   of a server and client SMTP; and,

 - the increment by which the extension is increasing the maximum
   length of the commands MAIL FROM and/or RCPT TO, over that specified
   in RFC 821.

In addition, any EHLO keyword value starting with an upper or lower case
"X" refers to a local SMTP service extension used exclusively through
bilateral agreement.  Keywords beginning with "X" MUST NOT be used in a
registered service extension.  Conversely, keyword values presented in the
EHLO response that do not begin with "X" MUST correspond to a standard,
standards-track, or IESG-approved experimental SMTP service extension
registered with IANA.  A conforming server MUST NOT offer non-"X"-prefixed
keyword values that are not described in a registered extension.

Additional verbs and parameter names are bound by the same rules as EHLO
keywords; specifically, verbs beginning with "X" are local extensions that
may not be registered or standardized.  Conversely, verbs not beginning
with "X" must always be registered.

2.3 Terminology

Most of the terminology in this document is common in the Internet at the
time of its writing.  However, the following terms and concepts are used
in special ways here, or represent differences in terminology between RFC
821 and this document, and should be understood before reading further.
These definitions are normative, that is, they contain specifications to
which SMTP implementations are required to conform.

The terms "MUST" and "SHOULD" (and "MUST NOT" and "SHOULD NOT") are used
in the same general sense here as in the Host Requirements Standards
[RFC-1123].  Specifically, "MUST" or "MUST NOT" identify absolute
requirements for conformance to this specification.  Implementations that
do not conform to them lie outside the scope of this specification and
often will not interoperate properly with SMTP implementations that do
conform.  Implementations that are fully conforming also adhere to all
"SHOULD" and "SHOULD NOT" requirements.  Implementations that adhere to
all "MUST" ("MUST NOT") but not to all of these are considered to be
partially conforming.  Such implementations may interoperate properly with
fully conforming ones and with each other, but this will typically be the
case only if great care is taken.  Consequently, an implementation should
violation "SHOULD" ("SHOULD NOT") requirements only under exceptional and
well-understood circumstances. "SHOULD" (and sometimes "MUST")
requirements are often imposed by this specification when experience has
shown that following such requirements or restrictions leads, in practice,
to better interoperation, or smoother operation of the Internet email
infrastructure. As a consequence, some of these statements constitute
recommended practices, rather than the statistically most common practice
at the time of this writing.  Statements using "MAY" describe features or
styles of doing things that may be followed, or not, at the discretion of
the implementation, normally without causing significant interoperability
problems.  

2.3.1 Mail Objects

SMTP transports a mail object. A mail object contains an envelope and
content.

The SMTP envelope is sent as a series of SMTP protocol units (described in
section 3). It consists of an originator address (to which error reports
should be directed); a delivery mode (e.g., deliver to recipient
mailboxes); one or more recipient addresses; and optional protocol
extension material.

The SMTP content is sent in the SMTP DATA protocol unit and has two parts:
the headers and the body. If the content conforms to existing standards,
the headers form a collection of field/value pairs structured as described
in [MSGFMT]; the body, if structured, is defined according to MIME
[RFC-MIME]. The content is textual in nature, expressed using the US-ASCII
repertoire [US-ASCII]. Although SMTP extensions (such as [8BitMIME]) may
relax this restriction for the content body, the content headers are always
encoded using the US-ASCII repertoire. The algorithm defined in
[RFC-INTLHDR] is used to represent header values outside the US-ASCII
repertoire, while still encoding them using the US-ASCII repertoire.

2.3.2 Senders and Receivers

In RFC 821, the two hosts participating in an SMTP transaction were
described as the "SMTP-sender" and "SMTP-receiver".  This document has been
changed to reflect current industry terminology and hence refers to them as
the "SMTP client" (or sometimes just "the client") and "SMTP server" (or
just "the server"), respectively.  Since a given host may act both as
server and client in a relay situation, "receiver" and "sender" terminology
is still used where needed for clarity.

2.3.3 Mail Agents

Additional mail system terminology became common after RFC 821 was
published and, where convenient, is used in this specification.  In
particular, SMTP servers and clients provide a mail transport service and
therefore act as Mail Transfer Agents (MTAs).  Mail User Agents (MUAs or
UAs) are normally thought of as the sources and targets of mail.  At the
source, an MUA might collect mail to be transmitted from a user and hand it
off to an MTA; the final ("delivery") MTA would be thought of as handing
the mail off to an MUA (or at least transferring responsibility to it).
However, while these terms are used with at least the appearance of great
precision in other environments, the implied boundaries between MUAs and
MTAs often do not accurately match common, and conforming, practices with
Internet mail.  Hence, the reader should be cautious about inferring the
strong relationships and responsibilities that might be implied if these
terms were used elsewhere.

2.3.4 Host

For the purposes of this specification, a host is a computer system
attached to the Internet (or, in some cases, to a private TCP/IP network)
and supporting the SMTP protocol.  Hosts are known by names (see "domain");
identifying them by numerical address is discouraged.

2.3.5 Domain

A domain (or domain name) consists of one or more dot-separated components,
each consisting of a sequence of letters, digits, and hyphens.  Domain
names are used as names of hosts and of other entities in the domain name
hierarchy. For example, a  domain may refer to an alias (label of a CNAME
RR) or the label of Mail eXchanger records to be used to deliver mail
instead of representing a host name. See [RFC-DNS] and section 5.

The domain name, as described in this document and in [RFC-DNS], is the
entire, fully-qualified name (often referred to as an "FQDN").  A domain
name that is not in FQDN form is no more than a local alias.  Local aliases
MUST NOT appear in any SMTP transaction.

2.3.6 Buffer and State Table

SMTP sessions are stateful, with both parties carefully maintaining a
common view of the current state.  In this document we model this state by
a virtual "buffer" and a "state table" on the server which may be used by
the client to, for example, "clear the buffer" or "reset the state table,"
causing the information in the buffer to be discarded and the state to be
returned to some previous state

2.3.7 Lines

SMTP commands and, unless altered by a service extension, message data, are
transmitted in "lines".  Lines consist of zero or more data characters
terminated by the sequence ASCII character "CR" (hex value 0D) followed
immediately by ASCII character "LF" (hex value 0A). This termination
sequence is denoted as <CRLF> in this document. Conforming implementations
MUST NOT recognize or generate any other character or character sequence as
a line terminator.

2.3.8 Originator, Delivery, Relay, and Gateway Systems

This specification makes a distinction among four types of SMTP systems,
based on the role those systems play in transmitting electronic mail.  An
"originating" system (sometimes called an SMTP originator) introduces mail
into the Internet or, more generally, into a transport service environment.
A "delivery" SMTP system is one that receives mail from a transport service
environment and hands it to a mail user agent or deposits it in a message
store which a mail user agent is expected to subsequently access.  A
"relay" SMTP system (usually referred to just as a "relay") receives mail
from an SMTP client and transmits it, without modification to the message
data other than adding trace information, to another SMTP server for
further relaying or for delivery.

A "gateway" SMTP system (usually referred to just as a "gateway") receives
mail from a client system in one transport environment and transmits it to
a server system in another transport environment. Differences in protocols
or message semantics between the transport environments on either side of a
gateway may require that the gateway system perform transformations to the
message that are not permitted to SMTP relay systems.

2.3.9 Message Content and Mail Data

The terms "message content" and "mail data" are used interchangeably in
this document to describe the material transmitted after the DATA command
is accepted and before the end of data indication is transmitted.  Message
content includes message headers and the possibly-structured message body.
The MIME specification [RFC-MIME] provides the Standard mechanisms for
structured message bodies.

2.3.10 Mailbox and Address

As used in this specification, an "address" is a character string that
identifies a user to whom mail will be sent or a location into which mail
will be deposited.  The term "mailbox" refers to that depository.  The two
terms are typically used interchangeably unless the distinction between the
location in which mail is placed (the mailbox) and a reference to it (the
address) is important.  An address normally consists of user and domain
specifications.  The standard mailbox naming convention is defined to be
"local-part@domain": contemporary usage permits a much broader set of
applications than simple "user names" and, consequently, the local-part is
interpreted and assigned semantics only by the host specified in the domain
part of the address.

2.3.11 Reply

An SMTP reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a command.
The general form of a reply is a numeric completion code (indicating
failure or success) usually followed by a text string.  The codes are for
use by programs and the text is usually intended for human users.

2.4 Syntax Principles

2.4.1 General Syntax and Transaction Model

SMTP commands and replies have a rigid syntax. All commands begin with a
four letter command verb. All Replies begin with a three digit numeric
code. In some commands and replies, arguments MUST follow the verb or reply
code. Some commands do not accept arguments (after the verb), and some
reply codes are followed, sometimes optionally, by free form text. In both
cases, where text appears, it is separated from the verb or reply code by a
<SP>. Complete definitions of commands and replies appear in section 4.

Verbs and argument values are not case sensitive, with the sole exception
of a mailbox local-part. That is, a command verb, an argument value other
than a mailbox local-part, and free form text MAY be encoded in upper case,
lower case, or any mixture of upper and lower case with no impact on its
meaning.  This is NOT true of a mailbox local-part. The local-part of a
mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations
MUST take care to preserve the case of mailbox local-parts.  Mailbox
domains are not case sensitive.  However, exploiting the case sensitivity
of mailbox local-parts impedes interoperability and is discouraged.

Commands and replies are composed of characters from the ASCII character
set [US-ASCII]. When the transport service provides an 8-bit byte (octet)
transmission channel, each 7-bit character is transmitted right justified
in an octet with the high order bit cleared to zero. More specifically, the
unextended SMTP service provides seven bit transport only.  An originating
SMTP client which has not successfully negotiated an appropriate extension
with a particular server MUST NOT transmit messages with information in the
high-order bit of octets.  If such messages are transmitted in violation of
this rule, receiving SMTP servers MAY clear the high-order bit or reject
the message as invalid.  In general, a relay SMTP SHOULD assume that the
message content it has received is valid and, assuming that the envelope
permits doing so, relay it without inspecting that content.  Of course, if
the content is mislabeled and the data path cannot accept the actual
content, this may result in ultimate delivery of a severely garbled message
to the recipient.  Delivery SMTP systems MAY reject ("bounce") such
messages rather than deliver them.  No sending SMTP system is permitted to
send envelope commands in any character set other than US-ASCII; receiving
systems SHOULD reject such commands, normally using "500 syntax error -
invalid character" replies.

Eight-bit message content transmission MAY be requested of the server by a
client using extended SMTP facilities, notably the "8BITMIME" extension
[8BITMIME].  8BITMIME SHOULD be supported by SMTP servers. However, it MUST
not be construed as authorization to transmit unrestricted eight bit
material.  8BITMIME MUST NOT be requested by senders for material with the
high bit on that is not in MIME format with an appropriate content-transfer
encoding; servers MAY reject such messages.

The metalinguistic notation used in this document corresponds to the
"Augmented BNF" used in other Internet mail system documents. The reader
who is not familiar with that syntax should consult [ABNF]. Metalanguage
terms used in running text are surrounded by pointed brackets (e.g.,
<CRLF>) for clarity.

2.4.2 Command and Reply Syntax

The commands consist of a command verb followed by an argument field.
Command verbs are four alphabetic characters and are case insensitive.

This also applies to any symbols representing parameter values, such as
"TO" or "to" for the forward-path.  Command verbs and the argument fields
are separated by one or more spaces.  However, case is important in the
local-part within the reverse-path and forward-path arguments.  In
particular, for some hosts the user "smith" is different from the user
"Smith".

A few SMTP servers, in violation of this specification (and RFC 821)
require that command verbs and certain argument text (such as the
forward-path and reverse-path in RCPT and MAIL commands) be encoded by
clients in upper case. Implementations MAY wish to employ this encoding to
accommodate those servers.

The argument field consists of a variable length character string ending
with the character sequence <CRLF>.  The receiver will take no action until
this sequence is received.

The syntax for each command is shown with the discussion of that command.
Common elements and parameters are shown in section 4.1.2.

3. The SMTP Procedures: An Overview

This section contains descriptions of the procedures used in SMTP: session
initiation, the mail transaction, forwarding mail, verifying mailbox names
and expanding mailing lists, and the opening and closing exchanges.
Comments on relaying, a note on mail domains, and a discussion of changing
roles are included at the end of this section.  Several complete scenarios
are presented in appendix D.

3.1 Session Initiation

An SMTP session is initiated when a client opens a connection to a server
and the server responds with an opening message.

SMTP server implementations MAY include identification of their software
and version information in the connection greeting reply after the 220
code, a practice that permits more efficient isolation and repair of any
problems. Implementations MAY make provision for SMTP servers to disable
the software and version announcement where it causes security concerns.
While some systems also identify their contact point for mail problems,
this is not a substitute for maintaining the required "postmaster" address
(see [MSGFMT]).

The SMTP protocol allows a server to formally reject a transaction while
still allowing the initial connection as follows: a 554 response MAY be
given in the initial connection opening message instead of the 220.  A
server taking this approach MUST still wait for the client to send a QUIT
(see section 4.1.1.10) before closing the connection and SHOULD respond to
any intervening commands with "503 bad sequence of commands".  Since an
attempt to make an SMTP connection to such a system is probably in error, a
server returning a 554 response on connection opening SHOULD provide enough
information in the reply text to facilitate debugging of the sending
system.

3.2 Client Initiation

Once the server has sent the welcoming message and the client has received
it, the client normally sends the EHLO command to the server, indicating
the client's identity.  In addition to opening the session, use of EHLO
indicates that the client is able to process service extensions and
requests that the server provide a list of the extensions it supports.
Older SMTP systems which are unable to support service extensions and
contemporary clients which do not require service extensions in the mail
session being initiated, MAY use HELO instead of EHLO.  Servers MUST NOT
return the extended EHLO-style response to a HELO command.

In the EHLO command the host sending the command identifies itself; the
command may be interpreted as saying "Hello, I am <domain>" (and, in the
case of EHLO, "and I support service extension requests").

3.3 Mail Transactions

There are three steps to SMTP mail transactions.  The transaction starts
with a MAIL command which gives the sender identification.  A series of one
or more RCPT commands follows giving the receiver information.  Then a DATA
command initiates transfer of the mail data and is terminated by the "end
of mail" data indicator, which also confirms the transaction.

The first step in the procedure is the MAIL command.

   MAIL FROM:<reverse-path> [ <mail-parameters> ] <CRLF>

This command tells the SMTP-receiver that a new mail transaction is
starting and to reset all its state tables and buffers, including any
recipients or mail data. The <reverse-path> contains the source mailbox
(between "<" and ">" brackets, which can be used to report errors (see
section 4.2 for a discussion of error reporting).  If accepted, the
SMTP server returns a 250 OK reply.  If the mailbox specification is
not acceptable for some reason, the server MUST return a reply
indicating whether the failure is permanent (i.e., will occur again if
the client tries to send the same address again) or temporary (i.e.,
the address might be accepted if the client tries again later). Despite
the apparent scope of this requirement, there are circumstances in
which the acceptability of the reverse-path may not be determined until
one or more forward-paths (in RCPT commands) can be examined.  In those
cases, the server MAY reasonably accept the reverse-path (with a 250
reply) and then report problems after the forward-paths are received
and examined.  Normally, failures produce 550 or 553 replies.

Historically, the <reverse-path> can contain more than just a mailbox,
however, contemporary systems SHOULD NOT use source routing (see appendix
C).

The optional <mail-parameters> are associated with negotiated SMTP service
extensions (see section 2.2).

The second step in the procedure is the RCPT command.

   RCPT TO:<forward-path> [ <SP> <rcpt-parameters> ] <CRLF>

This command gives a forward-path (normally a mailbox and domain,
always surrounded by "<" and ">" brackets) identifying one recipient.
If accepted, the SMTP server returns a 250 OK reply and stores the
forward-path.  If the recipient is known not to be a deliverable
address, the SMTP server returns a 550 reply, typically with a string
such as "no such user - " and the mailbox name (other circumstances and
reply codes are possible).  This step of the procedure can be repeated
any number of times.

The <forward-path> can contain more than just a mailbox.  Historically, the
<forward-path> can be a source routing list of hosts and the destination
mailbox, however, contemporary SMTP clients SHOULD NOT utilize source
routes (see appendix C). Servers MUST be prepared to encounter a list of
source routes in the forward path, but SHOULD ignore the routes or MAY
decline to support the relaying they imply.  Similarly, servers MAY decline
to accept mail that is destined for other hosts or systems.  These
restrictions make a server useless as a relay for clients that do not
support full SMTP functionality.  Consequently, restricted-capability
clients MUST NOT assume that any SMTP server on the Internet can be used as
their mail processing  (relaying) site. If RCPT TO appears without a
previous MAIL FROM, the server MUST return a 503 "Bad sequence of commands"
response. The optional <rcpt-parameters> are associated with negotiated
SMTP service extensions (see section 2.2).

The third step in the procedure is the DATA command (or some alternative
specified in a service extension).

   DATA <CRLF>

If accepted, the SMTP server returns a 354 Intermediate reply and considers
all succeeding lines up to but not including the end of mail data indicator
to be the message text.  When the end of text is successfully received and
stored the SMTP-receiver sends a 250 OK reply.

Since the mail data is sent on the transmission channel, the end of mail
data must be indicated so that the command and reply dialog can be resumed.
SMTP indicates the end of the mail data by sending a line containing only a
"." (period or full stop).  A transparency procedure is used to prevent
this from interfering with the user's text (see section 4.5.2).

The end of mail data indicator also confirms the mail transaction and tells
the SMTP server to now process the stored recipients and mail data.  If
accepted, the SMTP server returns a 250 OK reply. The DATA command can fail
in only two ways:

 - If there was no MAIL FROM, or no RCPT TO, command, or all such commands
   were rejected, the server MAY return a "command out of sequence" (503)
   reply.  If that reply is received, the client MUST NOT send the message
   data; more generally, message data MUST NOT be sent unless a 354 reply
   is received.

 - If the verb is initially accepted and the 354 reply issued, the DATA
   command should fail only if the mail transaction was incomplete (for
   example, no recipients), or if resources were unavailable.

However, in practice, some servers do not perform recipient verification
until after the message text is received.  These servers SHOULD treat a
failure for one or more recipients as a "subsequent failure" and return a
mail message as discussed in section 6.  Using a "550 mailbox not found"
(or equivalent) reply code after the data are accepted makes it difficult
or impossible for the client to determine which recipients failed.

When RFC 822 format is being used, the mail data include the memo header
items such as Date, Subject, To, Cc, From [MSGFMT].  Server SMTP systems
SHOULD NOT reject messages based on perceived defects in the RFC 822 or
MIME [RFC-MIME] message header or message body.  In particular, they MUST
NOT reject messages in which the numbers of Resent- fields do not match or
Resent-to appears without Resent-from and/or Resent-date.

Mail transaction commands MUST be used in the order discussed above.

3.4 Forwarding for Address Correction or Updating

Forwarding support is most often required to consolidate and simplify
addresses within, or relative to, some enterprise and less frequently to
establish addresses to link a person's prior address with current one.
Silent forwarding of messages (without server notification to the sender),
for security or non-disclosure purposes, is common in the contemporary
Internet.

In both the enterprise and the "new address" cases, information hiding
(and sometimes security) considerations argue against exposure of the
"final" address through the SMTP protocol as a side-effect of the
forwarding activity.  This may be especially important when the final
address may not even be reachable by the sender.  Consequently, the
"forwarding" mechanisms described in section 3.2 of RFC 821, and
especially the 251 (corrected destination) reply code from RCPT TO are
deprecated: Servers SHOULD NOT provide that service or return that code.

3.5 Commands for Debugging Addresses

3.5.1 Overview

SMTP provides commands to verify a user name or obtain the content of a
mailing list.  This is done with the VRFY and EXPN commands, which have
character string arguments.  Implementations SHOULD support VRFY and EXPN
(however, see section 3.5.2 and 7.3).

For the VRFY command, the string is a user name or a user name and domain
(see below). If a normal (i.e., 250) response is returned, the response MAY
include the full name of the user and MUST include the mailbox of the user.
It MUST be in either of the following forms:

   User Name <local-part@domain>
   local-part@domain

When a name that is the argument to VRFY could identify more than one
mailbox, the server MAY either note the ambiguity or identify the
alternatives.  In other words, any of the following are legitimate
response to VRFY:

   553 User ambiguous

or

   553- Ambiguous;  Possibilities are
   553-Joe Smith <[email-masked]>
   553-Harry Smith <[email-masked]>
   553 Melvin Smith <[email-masked]>

or

   553-Ambiguous;  Possibilities 
   553- <[email-masked]>
   553- <[email-masked]>
   553 <[email-masked]>

Under normal circumstances, a client receiving a 553 reply would be
expected to expose the result to the user.  Use of exactly the forms
given, and the "user ambiguous" or "ambiguous" keywords, possibly
supplemented by extended reply codes such as those described in
[RFC-REPLY], will facilitate automated translation into other languages as
needed. Of course, a client that was highly automated or that was
operating in another language than English, might choose to try to
translate the response, to return some other indication to the user than
the literal text of the reply, or to take some automated action such as
consulting a directory service for additional information before reporting
to the user.

For the EXPN command, the string identifies a mailing list, and the
successful (i.e., 250) multiline response MAY include the full name of the
users and MUST give the mailboxes on the mailing list.

In some hosts the distinction between a mailing list and an alias for a
single mailbox is a bit fuzzy, since a common data structure may hold both
types of entries, and it is possible to have mailing lists of one mailbox.
If a request is made to verify a mailing list, a positive response MAY be
given if a message so addressed would be delivered to everyone on the list,
otherwise an error SHOULD be reported (e.g., "550 That is a mailing list,
not a user" or "252 Unable to verify members of mailing list").  If a
request is made to expand a user name, the server MAY return a positive
response consisting of a list containing one name, or an error MAY be
reported (e.g., "550 That is a user name, not a mailing list").

In the case of a successful multiline reply (normal for EXPN) exactly one
mailbox is to be specified on each line of the reply.  The case of an
ambiguous request is discussed above.

"User name" is a fuzzy term and has been used deliberately.  An
implementation of the VRFY or EXPN commands MUST include at least
recognition of local mailboxes as "user names".  However, since current
Internet practice often results in a single host handling mail for
multiple domains, hosts, especially hosts that provide this functionality,
SHOULD accept the "local-part@domain" form as a "user name"; hosts MAY
also choose to recognize other strings as "user names".

The case of expanding a mailbox list requires a multiline reply, such as:

   C: EXPN Example-People
   S: 250-Jon Postel <[email-masked]>
   S: 250-Fred Fonebone <[email-masked]>
   S: 250 Sam Q. Smith <[email-masked]>

or

   C EXPN Executive-Washroom-List
   S: 550 Access Denied to You.

The character string arguments of the VRFY and EXPN commands cannot be
further restricted due to the variety of implementations of the user name
and mailbox list concepts.  On some systems it may be appropriate for the
argument of the EXPN command to be a file name for a file containing a
mailing list, but again there are a variety of file naming conventions in
the Internet.  Similarly, historical variations in what is returned by
these commands are such that the response SHOULD be interpreted very
carefully, if at all, and SHOULD generally only be used for diagnostic
purposes.

3.5.2 VRFY Normal Response

When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply MUST normally include the mailbox name.
"<local-part@domain>", where "domain" is a fully qualified domain name,
MUST appear in the syntax.  In exceptional circumstances, free-form text
MAY be returned.  In order to facilitate parsing by both computers and
people, addresses SHOULD appear in pointed brackets.  When addresses,
rather than free-form debugging information, are returned, EXPN and VRFY
MUST return only valid domain addresses that are usable in SMTP RCPT
commands.  Consequently, if an address implies delivery to a program or
other system, the mailbox name used to reach that target MUST be given.
Paths (explicit source routes) MUST NOT be returned by VRFY or EXPN.

Server implementations SHOULD support both VRFY and EXPN. For security
reasons, implementations MAY provide local installations a way to disable
either or both of these commands through configuration options or the
equivalent.  When these commands are supported, they are not required to
work across relays when relaying is supported.  Since they were both
optional in RFC 821, they MUST be listed as service extensions in an EHLO
response, if they are supported.

3.5.3 Meaning of VRFY or EXPN Success Response

A server MUST NOT return a 220 code in response to a VRFY or EXPN command
unless it has actually verified the address.  In particular, a server MUST
NOT return 220 if all it has done is to verify that the syntax given is
valid.  In that case, 502 (Command not implemented) or 500 (Syntax error,
command unrecognized) SHOULD be returned.  As stated elsewhere,
implementation of VRFY and EXPN are strongly recommended.
Hence, implementations that return 500 or 502 for VRFY are not in full
compliance with this specification.

There may be circumstances where an address appears to be valid but cannot
reasonably be verified in real time, particularly when a server is acting
as a mail exchanger for another server or domain. "Apparent validity" in
this case would normally involve at least syntax checking and might
involve verification that any domains specified were ones to which the
host expected to be able to relay mail.  In these situations, reply code
252 SHOULD be returned.  These cases parallel the discussion of RCPT
verification discussed in section 2.1.  Implementations generally SHOULD
be more aggressive about address verification in the case of VRFY than in
the case of RCPT, even if it takes a little longer to do so.

3.5.4 Semantics and Applications of EXPN

EXPN is often very useful in debugging and understanding problems with
mailing lists and multiple-target-address aliases. Some systems have
attempted to use source expansion of mailing lists as a means of
eliminating duplicates.  The propagation of aliasing systems with mail on
the Internet, for hosts (typically with MX and CNAME DNS records), for
mailboxes (various types of local host aliases), and in various proxying
arrangements, has made it nearly impossible for these strategies to work,
and mail systems SHOULD NOT attempt them.

3.6 Domains

Only resolvable, fully-qualified, domain names (FQDNs) are permitted when
domain names are used in SMTP.  In other words, names that can be resolved
to MX RRs or A RRs (as discussed in section 5) are permitted, as are CNAME
RRs whose targets can be resolved, in turn, to MX or A RRs.  Local
nicknames or unqualified names MUST NOT be used.  There are two exceptions
to the rule requiring FQDNs:

 - The domain name given in the EHLO command MUST BE either a primary host
   name (a domain name that resolves to an A RR) or, if the host has no
   name, an address literal as described in section 4.1.1.1.

 - The reserved mailbox name "postmaster" may be used in a RCPT TO command
   without domain qualification (see section 4.1.1.3) and MUST be accepted
   if so used.

3.7 Relaying

In general, the availability of Mail eXchanger records in the domain name
system [RFC-DNS] makes the use of explicit source routes in the Internet
mail system unnecessary.  Many historical problems with their
interpretation have made their use undesirable.  SMTP clients SHOULD NOT
generate explicit source routes except under unusual circumstances.  SMTP
servers MAY decline to act as mail relays or to accept addresses that
specify source routes.  When route information is encountered, they are
also permitted to ignore the route information and simply send to the final
destination specified as the last element in the route and SHOULD do so.
There has been an invalid practice of using names that do not appear in the
DNS as destination names, with the senders counting on the intermediate
hosts specified in source routing to resolve any problems.  If source
routes are stripped, this practice will cause failures. This is one of
several reasons why SMTP clients MUST NOT generate invalid source routes or
depend on serial resolution of names.

When source routes are not used, the process described in RFC 821 for
constructing a reverse-path from the forward-path is not applicable and the
reverse-path at the time of delivery will simply be the address that
appeared in the MAIL command.

A relay SMTP server is usually the target of a DNS MX record that
designates it, rather than the final delivery system. The relay server may
accept or reject the task of relaying the mail in the same way it accepts
or rejects mail for a local user.  If it accepts the task, it then becomes
an SMTP client, establishes a transmission channel to the next SMTP server
specified in the DNS (according to the rules in section 5), and sends it
the mail.  If it declines to relay mail to a particular address for policy
reasons, a 550 response SHOULD be returned.

If an SMTP server has accepted the task of relaying the mail and later
finds that the destination is incorrect or that the mail cannot be
delivered for some other reason, then it MUST construct an "undeliverable
mail" notification message and send it to the originator of the
undeliverable mail (as indicated by the reverse-path).  Formats specified
for non-delivery reports by other standards (see, for example,
[RFC-NOTARY1]) SHOULD be used if possible.

This notification message must be from the SMTP server at the relay host
or the host that first determines that delivery cannot be accomplished.
Of course, SMTP servers MUST NOT send notification messages about problems
transporting notification messages.  One way to prevent loops in error
reporting is to specify a null reverse-path in the MAIL command of a
notification message.  When such a message is transmitted the reverse-path
MUST be set to null (see section 4.5.5 for additional discussion).  A MAIL
command with a null reverse-path appears as follows:

   MAIL FROM:<>

As discussed in section 2.4.1, a relay SMTP has no need to inspect or act
upon the headers or body of the message data and MUST NOT do so except
to add its own "Received:" header (section 4.4) and to perform simple
counting of the number of "Received:" headers in a message (section 6.2).

3.8 Mail Gatewaying

While the relay function discussed above operates within the Internet SMTP
transport service environment, MX records or various forms of explicit
routing may require that an intermediate SMTP server perform a translation
function between one transport service and another.  As discussed in
section 2.3.8, when such a system is at the boundary between two transport
service environments, we refer to it as a "gateway" or "gateway SMTP".

Gatewaying mail between different mail environments, such as different mail
formats and protocols, is complex and does not easily yield to
standardization.  However, some general requirements may be given for a
gateway between the Internet and another mail environment.

3.8.1 Header Fields in Gatewaying

Header fields MAY be rewritten when necessary as messages are gatewayed
across mail environment boundaries. This may involve inspecting the message
body or interpreting the local-part of the destination address in spite of
the prohibitions in section 2.4.1

Other mail systems gatewayed to the Internet often use a subset of RFC-822
headers or provide similar functionality with a different syntax, but some
of these mail systems do not have an equivalent to the SMTP envelope.
Therefore, when a message leaves the Internet environment, it may be
necessary to fold the SMTP envelope information into the message header.  A
possible solution would be to create new header fields to carry the
envelope information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however,
this would require changes in mail programs in foreign environments and
might risk disclosure of private information (see section 7.2).

3.8.2 Received Lines in Gatewaying

When forwarding a message into or out of the Internet environment, a
gateway MUST prepend a Received: line, but it MUST NOT alter in any way a
Received: line that is already in the header.

Received: fields of messages originating from other environments may not
conform exactly to this specification.  However, the most important use of
Received: lines is for debugging mail faults, and this debugging can be
severely hampered by well-meaning gateways that try to "fix" a Received:
line.  As another consequence of trace fields arising in non-SMTP
environments, receiving systems MUST NOT reject mail based on the format of
a trace field and SHOULD be extremely robust in the light of unexpected
information or formats in those fields.

The gateway SHOULD indicate the environment and protocol in the "via"
clauses of Received field(s) that it supplies.

3.8.3 Addresses in Gatewaying

>From the Internet side, the gateway SHOULD accept all valid address formats
in SMTP commands and in RFC-822 headers, and all valid RFC-822 messages.
Gateways are, of course, subject to the same rules for handling source
routes as those described for other SMTP systems in section 3.3.

3.8.4 Other Header Fields in Gatewaying

The gateway MUST ensure that all header fields of a message that it
forwards into the Internet meet the requirements for Internet mail.  In
particular, all addresses in "From:", "To:", "Cc:", etc., fields MUST be
transformed (if necessary) to satisfy RFC-822 syntax, MUST reference only
fully-qualified domain names, and MUST be effective and useful for sending
replies.  3.8.5 The translation algorithm used to convert mail from the
Internet protocols to another environment's protocol SHOULD ensure that
error messages from the foreign mail environment are delivered to the
return path from the SMTP envelope, not to the sender listed in the "From:"
field (or other fields) of the RFC-822 message.

3.8.5 Envelopes in Gatewaying

Similarly, when forwarding a message from another environment into the
Internet, the gateway SHOULD set the envelope return path in accordance
with an error message return address, if supplied by the foreign
environment.  If the foreign environment has no equivalent concept, the
gateway must select and use a best approximation, with the message
originator's address as the default of last resort.

3.9 Terminating Sessions and Connections

An SMTP connection is terminated when the client sends a QUIT command.  The
server responds with a positive reply code, after which it closes the
connection.

An SMTP server MUST NOT intentionally close the connection except:

 - After receiving a QUIT command and responding with a 221 reply.

 - After detecting the need to shutdown the SMTP service and returning a
   421 response code.  This response code can be issued after the server
   receives any command or, if necessary, asynchronously from command
   receipt (on the assumption that the client will receive it after the
   next command is issued).

In particular, a server that closes connections in response to commands
that are not understood is in violation of this specification. Servers are
expected to be tolerant of unknown commands, issuing a 500 reply and
awaiting further instructions from the client.

An SMTP server which is forcibly shut down via external means SHOULD
attempt to send a line containing a 421 response code to the SMTP client
before exiting.  The SMTP client will normally read the 421 response code
after sending its next command.

SMTP clients that experience a connection close, reset, or other
communications failure due to circumstances not under their control (in
violation of the intent of this specification but sometimes unavoidable)
SHOULD, to maintain the robustness of the mail system, treat the mail
transaction as if a 451 response had been received and act accordingly.

3.10 Mailing Lists and Aliases

An SMTP-capable host SHOULD support both the alias and the list models of
address expansion for multiple delivery.  When a message is delivered or
forwarded to each address of an expanded list form, the return address in
the envelope ("MAIL FROM:") MUST be changed to be the address of a person
or other entity who administers the list.  However, in this case, the
message header (see [MSGFMT]) MUST be left unchanged; in particular, the
"From" field of the message header is unaffected.

An important mail facility is a mechanism for multi-destination delivery
of a single message, by transforming (or "expanding" or "exploding") a
pseudo-mailbox address into a list of destination mailbox addresses. When
a message is sent to such a pseudo-mailbox (sometimes called an
"exploder"), copies are forwarded or redistributed to each mailbox in the
expanded list.  Servers SHOULD simply utilize the addresses on the list;
application of heuristics or other matching rules to eliminate some
addresses, such as that of the originator, is strongly discouraged.  We
classify such a pseudo-mailbox as an "alias" or a "list", depending upon
the expansion rules.

3.10.1 Alias

To expand an alias, the recipient mailer simply replaces the pseudo-mailbox
address in the envelope with each of the expanded addresses in turn; the
rest of the envelope and the message body are left unchanged.  The message
is then delivered or forwarded to each expanded address.

3.10.2 List

A mailing list may be said to operate by "redistribution" rather than
by "forwarding".  To expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with all of the expanded
addresses.  The return address in the envelope is changed so that all
error messages generated by the final deliveries will be returned to a
list administrator, not to the message originator, who generally has no
control over the contents of the list and will typically find error
messages annoying.

4. The SMTP Specifications

4.1 SMTP Commands

4.1.1 Command Semantics and Syntax

The SMTP commands define the mail transfer or the mail system function
requested by the user.  SMTP commands are character strings terminated by
<CRLF>.  The commands themselves are alphabetic characters terminated by
<SP> if parameters follow and <CRLF> otherwise.  (In the interest of
improved interoperability, SMTP receivers are encouraged to tolerate
trailing white space before the terminating <CRLF>.)  The syntax of the
local part of a mailbox must conform to receiver site conventions and the
syntax specified in section 4.1.2.  The SMTP commands are discussed below.
The SMTP replies are discussed in section 4.2.

A mail transaction involves several data objects which are communicated as
arguments to different commands.  The reverse-path is the argument of the
MAIL command, the forward-path is the argument of the RCPT command, and the
mail data is the argument of the DATA command.  These arguments or data
objects must be transmitted and held pending the confirmation communicated
by the end of mail data indication which finalizes the transaction.  The
model for this is that distinct buffers are provided to hold the types of
data objects, that is, there is a reverse-path buffer, a forward-path
buffer, and a mail data buffer.  Specific commands cause information to be
appended to a specific buffer, or cause one or more buffers to be cleared.

Several commands (RSET, DATA, QUIT) are specified as not permitting
parameters.   In the absence of specific extensions offered by the server
and accepted by the client, clients MUST NOT send such parameters and
servers SHOULD reject commands containing them as having invalid syntax.  

4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO)

These commands are used to identify the SMTP client to the SMTP server.
The argument field contains the fully-qualified domain name of the SMTP
client if one is available.  In situations in which the SMTP client system
does not have a meaningful domain name (e.g., when its address is
dynamically allocated and no reverse mapping record is available), the
client SHOULD send an address literal (see section 4.1.3), optionally
followed by information that will help to identify the client system.

The SMTP server identifies itself to the SMTP client in the connection
greeting reply and in the response to this command.

A client SMTP SHOULD start an SMTP session by issuing the EHLO command. If
the SMTP server supports the SMTP service extensions it will give a
successful response, a failure response, or an error response. If the SMTP
server, in violation of this specification, does not support any SMTP
service extensions it will generate an error response.  Older client SMTP
systems MAY, as discussed above, use HELO (as specified in RFC 821) instead
of EHLO, and servers MUST support the HELO command and reply properly to
it.  In any event, a client MUST issue HELO or EHLO before starting a mail
transaction.

These commands, and a "250 OK" reply to one of them, confirm that both the
SMTP client and the SMTP server are in the initial state, that is, there is
no transaction in progress and all state tables and buffers are cleared.

Normally, the response to EHLO will be a multiline reply.  Each line of the
response contains a keyword and, optionally, one or more parameters.  The
syntax for a positive response, using the ABNF notation and low-level
terminals of [ABNF], is:

   ehlo-ok-rsp  ::=      "250"    domain [ <SP> greeting ] <CRLF>
                  / (    "250-"   domain [ <SP> greeting ] <CRLF>
                      *( "250-"   ehlo-line                <CRLF> )
                         "250"    <SP> ehlo-line           <CRLF>    )
   
   greeting     ::= 1*<any character other than CR or LF>
   
   ehlo-line    ::= ehlo-keyword *( <SP> ehlo-param )
   
   ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                ; syntax and values depend on ehlo-keyword

   ehlo-param   ::= 1*<any CHAR excluding <SP> and all
                       control characters (US-ASCII 0-31
                       inclusive)>
   
   ALPHA        ::= any one of the 52 alphabetic characters, i.e.,
                    (A through Z in upper case, and, a through z in lower
					case) 

Although EHLO keywords may be specified in upper, lower, or mixed case,
they MUST always be recognized and processed in a case-insensitive manner.
This is simply an extension of practices specified in RFC 821 and section
2.4.1.

4.1.1.2 MAIL (MAIL)

This command is used to initiate a mail transaction in which the mail data
is delivered to one or more mailboxes.  The argument field contains a
reverse-path.  In general, the MAIL command may be sent only when no mail
transaction is in progress, see section 4.1.4.

The reverse-path consists of the sender mailbox or a list of hosts.  In
some types of reporting messages for which a reply is likely to cause a
mail loop (for example, mail delivery and nondelivery notifications), the
reverse-path may be null (see section 3.7).

This command clears the reverse-path buffer, the forward-path buffer, and
the mail data buffer; and inserts the reverse-path information from this
command into the reverse-path buffer.

If service extensions were negotiated, the MAIL command may also carry
parameters associated with a particular service extension.

Syntax:
   
or
   "MAIL FROM:<>" [ <SP> Mail-parameters ] 

4.1.1.3 RECIPIENT (RCPT)

This command is used to identify an individual recipient of the mail data;
multiple recipients are specified by multiple use of this command.

The forward-path normally consists of the required destination mailbox or
mailboxes.  Sending systems SHOULD not generate the optional list of hosts
known as a source route.  Receiving systems MUST recognize source route
syntax but SHOULD strip off the source route specification and utilize the
domain name associated with the mailbox as if the source route had not been
provided.

Similarly, relay hosts SHOULD strip or ignore source routes, and names MUST
NOT be copied into the reverse-path. When mail reaches its ultimate
destination (the forward-path contains only a destination mailbox), the
SMTP server inserts it into the destination mailbox in accordance with its
host mail conventions.

For example, mail received at relay host xyz.com with envelope commands

   MAIL FROM:<[email-masked]>
   RCPT TO:<@hosta.int,@jkl.org:[email-masked]>

will normally be sent directly on to host d.bar.org with envelope commands

   MAIL FROM:<[email-masked]>
   RCPT TO:<[email-masked]>

As provided in appendix C, xyz.com MAY also choose to relay the message to
jkl.org, using the envelope commands

   MAIL FROM:<[email-masked]>
   RCPT TO:<@jkl.org:[email-masked]>

Of course, since hosts are not required to relay mail at all, xyz.com may
also reject the message entirely when the RCPT TO command is received,
using a 550 code (since this is a "policy reason").

If service extensions were negotiated, the RCPT TO command may also carry
parameters associated with a particular service extension offered by the
server.  The client MUST NOT transmit parameters other than those
associated with a service extension offered by the server in its EHLO
response.

Syntax:
   "RCPT TO:" Forward-path  [ <SP> Rcpt-parameters ]
or
   "RCPT TO:<Postmaster>" [ <SP> Rcpt-parameters ]

4.1.1.4 DATA (DATA)

The receiver treats the lines (strings ending in <CRLF> sequences, as
described in section 2.3.7) following the command as mail data from the
sender. This command causes the mail data to be appended to the mail data
buffer.  The mail data may contain any of the 128 ASCII character codes,
although experience has indicated that use of control characters other than
SP, HT, CR, and LF (especially the ASCII "Null" character) may cause
problems and SHOULD be avoided when possible.

The mail data is terminated by a line containing only a period, that is,
the character sequence "<CRLF>.<CRLF>" (see section 4.5.2).  This is the
end of mail data indication.  Note that the first <CRLF> of this
terminating sequence is also the <CRLF> that ends the final line of the
data (message text) or, if there was no data, ends the DATA command itself.
An extra <CRLF> MUST NOT be added, as that would cause an empty line to be
added to the message. The only exception to this rule would arise if the
message body were passed to the originating SMTP-sender with a final "line"
that did not end in <CRLF>; in that case, the originating SMTP system MUST
either reject the message as invalid or add <CRLF> in order to have the
receiving SMTP server recognize the "end of data" condition.

The custom of accepting lines ending only in <LF>, as a concession to
non-conforming behavior on the part of some UNIX systems, has proven to
cause more interoperability problems than it solves, and SMTP server
systems MUST NOT do this, even in the name of improved robustness.  In
particular, the sequence "<LF>.<LF>" (bare line feeds, without carriage
returns) MUST NOT be treated as equivalent to <CRLF>.<CRLF> as the end of
mail data indication.

Receipt of the end of mail data indication requires the server to process
the stored mail transaction information.  This processing consumes the
information in the reverse-path buffer, the forward-path buffer, and the
mail data buffer, and on the completion of this command these buffers are
cleared.  If the processing is successful, the receiver MUST send an OK
reply.  If the processing fails the receiver MUST send a failure reply. The
SMTP model does not allow for partial failures at this point: either the
message is accepted by the server for delivery and a positive response is
returned or it is not accepted and a failure reply is returned.  Errors
that are diagnosed subsequently MUST be reported in a mail message, as
discussed in section 4.4 In sending a positive completion reply to the end
of data indication, the receiver takes full responsibility for the message
(see section 6.1).

When the SMTP server accepts a message either for relaying or for final
delivery, it inserts a trace record (also referred to interchangeably as a
"time stamp line" or "Received" line) at the top of the mail data.  This
trace record indicates the identity of the host that sent the message, the
identity of the host that received the message (and is inserting this time
stamp), and the date and time the message was received.  Relayed messages
will have multiple time stamp lines.  Details for formation of these lines,
including their syntax, is specified in section 4.4.

4.1.1.5 RESET (RSET)

This command specifies that the current mail transaction will be aborted.
Any stored sender, recipients, and mail data MUST be discarded, and all
buffers and state tables cleared.  The receiver MUST send a "250 OK" reply
to a RSET command with no arguments.  A reset command may be issued by the
client at any time.  It is effectively equivalent to a NOOP if issued
immediately after EHLO, before EHLO is issued in the session, after an
end-of-data indicator has been sent and acknowledged, or immediately before
a QUIT. In other situations, it restores the state to that immediately
after the most recent EHLO.  An SMTP server MUST NOT close the connection
as the result of receiving a RSET; that action is reserved for QUIT (see
section 4.1.1.10).

Since EHLO implies some additional processing and response by the server,
RSET will normally be more efficient than reissuing that command, even
though the formal semantics are the same.

There are circumstances, contrary to the intent of this specification, in
which an SMTP server may receive an indication that the underlying TCP
connection has been closed or reset.  To preserve the robustness of the
mail system, SMTP servers SHOULD be prepared for this condition and SHOULD
treat it as if a QUIT had been received before the connection disappeared.

Syntax:
   "RSET"

4.1.1.6 VERIFY (VRFY)

This command asks the receiver to confirm that the argument identifies a
user or mailbox.  If it is a user name, information is returned as
specified in section 3.5.

This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer.

Syntax: 
   "VRFY" <SP> String

4.1.1.7 EXPAND (EXPN)

This command asks the receiver to confirm that the argument identifies a
mailing list, and if so, to return the membership of that list.  If the
command is successful, a reply is returned containing information as
described in section 3.5.  This reply will have multiple lines except in
the trivial case of a one-member list.

This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer.

Syntax:
   "EXPN" <SP> String

4.1.1.8 HELP (HELP)

This command causes the server to send helpful information to the client.
The command MAY take an argument (e.g., any command name) and return more
specific information as a response.

This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer.

SMTP servers SHOULD support HELP without arguments and MAY support it with
arguments.

Syntax:
   "HELP" [ <SP> String ]

4.1.1.9 NOOP (NOOP)

This command does not affect any parameters or previously entered commands.
It specifies no action other than that the receiver send an OK reply.

This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer.  If a parameter string is specified,
servers SHOULD ignore it.

Syntax:
   "NOOP" [ <SP> String ]

4.1.1.10 QUIT (QUIT)

This command specifies that the receiver MUST send an OK reply, and then
close the transmission channel.

The receiver MUST NOT intentionally close the transmission channel until it
receives and replies to a QUIT command (even if there was an error).  The
sender MUST NOT intentionally close the transmission channel until it sends
a QUIT command and receives the reply (even if there was an error response
to a previous command).  If the connection is closed prematurely due to
violations of the above or system or network failure, the server MUST
cancel any pending transaction, but not undo any previously completed
transaction, and generally MUST act as if the command or transaction in
progress had received a temporary error (i.e., a 4yz response).

Syntax: 
   "QUIT"

4.1.2 Lower-level Syntax

The syntax of the argument fields of the above commands (using the syntax
specified in [ABNF] where applicable) is given below.  Some of the
productions given below are used only in conjunction with source routes as
described in appendix C.  Terminals not defined in this document, such as
ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in the "core" syntax
(section 6) of [ABNF] or in the syntax of [MSGFMT].

   Reverse-path = Path

   Forward-path = Path

   Path = "<" [ A-d-l ":" ] Mailbox ">"

   A-d-l = At-domain *( "," A-d-l )    ; Note that this form, the
                                 so-called "source route", MUST BE
                                 accepted, SHOULD NOT be generated,
                                 and SHOULD be ignored.

   At-domain = "@" Domain

   Mail-parameters = *( <SP> Keyword "=" Argument )

   Rcpt-parameters =  *( <SP> Keyword "=" Argument )

   Keyword  = Ldh-str
   Argument = Atom

   Domain = sub-domain 1*("." sub-domain) / address-literal

   sub-domain = let-dig *(Ldh-str)
   address-literal = "[" IPv4-address-literal / 
                  IPv6-address-literal / General-address-literal "]"
   IPv4-address-literal = snum 3*3("." snum)
   IPv6-address-literal = "IPv6" <SP> IPv6-addr-string
   IPv6-addr-string = String         ; IPv6 address in standard form
                                       [IPv6AddrSpec].  Since this
                                       form uses colon characters,
                                       the String will actually need
                                       to be quoted in all cases.
   General-address-literal = Standardized-tag <SP> String
   Standardized-tag = Ldh-str        ; Specified in a
                                       standards-track RFC 
                                       and registered with IANA
   snum = 1*3Digit                   ; representing a decimal integer
                                       value in the range 0 through 255 
   let-dig = Alpha / Digit
   ldh-str = *( Alpha / Digit / "-" ) let-dig

   Mailbox = Local-part "@" Domain

   Local-part = Dot-string / Quoted-string

   Dot-string = Atom [ "." Atom ]

While the above definition for Local-part is relatively permissive, for
maximum interoperability, a host that expects to receive mail SHOULD avoid
defining mailboxes where the Local-part requires (or uses) the
Quoted-string form or where the Local-part is case-sensitive.  For any
purposes that require generating or comparing Local-parts (e.g., to
specific mailbox names), all quoted forms MUST be treated as equivalent and
the sending system SHOULD transmit the form that uses the minimum quoting
possible.

Systems MUST NOT define mailboxes in such a way as to require the use of
non-ASCII characters (octets with the high order bit set to one) or ASCII
"control characters" (decimal value 0-31 and 127).  These characters MUST
NOT be used in MAIL FROM or RCPT TO commands or other commands that require
mailbox names.

   String = Atom / Quoted-string

   special = <<Msg-fmt-special>> /     [[placeholder, see above]]
             the control characters (ASCII codes 0 through 31
             inclusive and 127)

Note that the backslash, "\", is a quote character, which is used to
indicate that the next character is to be used literally (instead of its
normal interpretation).  For example, "Joe\,Smith" indicates a single nine
character user field with the comma being the fourth character of the
field.

To promote interoperability and consistent with long-standing guidance
about conservative use of the DNS in naming and applications (e.g., see
section 2.3.1 of the base DNS document [RFC-1015]), characters outside the
set of alphas, digits, and hyphen MUST NOT appear in domain name labels
for SMTP clients or servers.  In particular, the underscore character is
not permitted.  SMTP servers that receive a command in which illegal
character codes have been employed, and for which there are no other
reasons for rejection, MUST reject that command with a 501 response.

4.1.3 Address Literals

Sometimes a host is not known to the domain name system and communication
(and, in particular, communication to report and repair the error) is
blocked.  To bypass this barrier a special literal form of the address is
allowed as an alternative to a domain name.  For IPv4 addresses, this form
uses four small decimal integers separated by dots and enclosed by brackets
such as [123.255.37.2], which indicates an (IPv4) Internet Address in
sequence-of-octets form.  For IPv6 and other forms of addressing that might
eventually be standardized, the form consists of a standardized "tag" that
identifies the address syntax, a space, and the address itself, in a format
specified as part of the IPv6 standards [IPv6AddrString].

4.1.4 Order of Commands

There are restrictions on the order in which these commands may be used.

A session that will contain mail transactions MUST first be initialized by
the use of the EHLO command.  An SMTP server SHOULD accept commands for
non-mail transactions (e.g., VRFY or EXPN) without this initialization.

An EHLO command MAY be issued by a client later in the session.  If it is
issued after the session begins, the SMTP server MUST clear all buffers and
reset the state exactly as if a RSET command had been issued.  In other
words, the sequence of RSET followed immediately by EHLO is redundant, but
not harmful other than in the performance cost of executing unnecessary
commands.

If the EHLO command is not acceptable to the SMTP server, 501, 500, or 502
failure replies MUST be returned as appropriate.  The SMTP server MUST stay
in the same state after transmitting these replies that it was in before
the EHLO was received.

The SMTP client MUST ensure that the domain parameter to the EHLO command
is a valid principal host name (not a CNAME or MX name) for its host.  If
this is not possible (e.g., when the client's address is dynamically
assigned and the client does not have an obvious name), an address literal
SHOULD be substituted for the domain name and supplemental information
provided that will assist in identifying the client.

An SMTP server MAY verify that the domain name parameter in the EHLO
command actually corresponds to the IP address of the client. However, the
server MUST NOT refuse to accept a message if the verification fails: the
information about verification failure is for logging and tracing only.

The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
during a session, or without previously initializing a session.  SMTP
servers SHOULD process these normally (that is, not return a 503 code) even
if no EHLO command has yet been received; clients SHOULD open a session
with EHLO before sending these commands.

If these rules are followed, the example in RFC 821 that shows "550 access
denied to you" in response to an EXPN command is incorrect unless an EHLO
command precedes the EXPN or the denial of access is based on the client's
IP address or other authentication or authorization-determining mechanisms.

The MAIL command (or the obsolete SEND, SOML, or SAML commands) begins a
mail transaction.  Once started, a mail transaction consists of a
transaction beginning command, one or more RCPT commands, and a DATA
command, in that order.  A mail transaction may be aborted by the RSET (or
a new EHLO) command.  There may be zero or more transactions in a session.
MAIL (or SEND, SOML, or SAML) MUST NOT be sent if a mail transaction is
already open, i.e., it should be sent only if no mail transaction had been
started in the session, or it the previous one successfully concluded with
a successful DATA command, or if the previous one was aborted with a RSET.

If the transaction beginning command argument is not acceptable, a 501
failure reply MUST be returned and the SMTP server MUST stay in the same
state.  If the commands in a transaction are out of order to the degree
that they cannot be processed by the server, a 503 failure reply MUST be
returned and the SMTP server MUST stay in the same state.

The last command in a session MUST be the QUIT command.  The QUIT command
cannot be used at any other time in a session, but SHOULD be used by the
client SMTP to request connection closure, even when no session opening
command was sent and accepted.

4.1.5 Private-use Commands

As specified in section 2.2.2, commands starting in "X" may be used by
bilateral agreement between the client (sending) and server (receiving)
SMTP agents.  An SMTP server that does not recognize such a command is
expected to reply with "500 Command not recognized".  An extended SMTP
server MAY list the feature names associated with these private commands in
the response to the EHLO command.

Commands sent or accepted by SMTP systems that do not start with "X" MUST
conform to the requirements of section 2.2.2.

4.2 SMTP Replies

Replies to SMTP commands serve to ensure the synchronization of requests
and actions in the process of mail transfer and to guarantee that the SMTP
client always knows the state of the SMTP server. Every command MUST
generate exactly one reply.

The details of the command-reply sequence are described in section 4.3.

An SMTP reply consists of a three digit number (transmitted as three
alphanumeric characters) followed by some text unless specified otherwise
in this document.  The number is for use by automata to determine what
state to enter next; the text is for the human user.  The three digits
contain enough encoded information that the SMTP client need not examine
the text and may either discard it or pass it on to the user, as
appropriate. Exceptions are as noted elsewhere in this document.  In
particular, the 220, 221, 251, 421, and 551 reply codes are associated
with message text that must be parsed and interpreted by machines.  In the
general case, the text may be receiver dependent and context dependent, so
there are likely to be varying texts for each reply code.  A discussion of
the theory of reply codes is given in section 4.2.1.  Formally, a reply is
defined to be the sequence: a three-digit code, <SP>, one line of text,
and <CRLF>, or a multiline reply (as defined in section 4.2.1).  Since, in
violation of this specification, the text is sometimes not sent, clients
which do not receive it SHOULD be prepared to process the code alone (with
or without a trailing space character).  Only the EHLO, EXPN, and HELP
commands are expected to result in multiline replies in normal
circumstances, however, multiline replies are allowed for any command.

In ABNF, server responses are:

  Greeting = "220 " Domain [ SP text ] CRLF
  Reply-line = Reply-code [ SP text ] CRLF 

where "Greeting" appears only in the 220 response that announces that the
server is opening its part of the connection.

An SMTP server SHOULD send only the reply codes listed in this document.
An SMTP server SHOULD use the text shown in the examples whenever
appropriate.

An SMTP client MUST determine its actions only by the reply code, not by
the text (except for 251 and 551 and, if necessary, 220, 221, and 421
replies); in the general case, any text, including no text at all (although
senders SHOULD NOT send bare codes), MUST be acceptable. The space (blank)
following the reply code is considered part of the text.  Whenever
possible, a receiver-SMTP SHOULD test the first digit (severity indication)
of the reply code.

The list of codes that appears below must not be construed as permanent.
While the addition of new codes should be a rare and significant activity,
with supplemental information in the textual part of the response being
preferred, new codes may be added as the result of new Standards or
Standards-track specifications. Consequently, a sender-SMTP MUST be
prepared to handle codes not specified in this document and MUST do so by
interpreting the first digit only.

4.2.1 Reply Code Severities and Theory

The three digits of the reply each have a special significance.  The first
digit denotes whether the response is good, bad or incomplete. An
unsophisticated SMTP client, or one that receives an unexpected code, will
be able to determine its next action (proceed as planned, redo, retrench,
etc.) by examining this first digit.  An SMTP client that wants to know
approximately what kind of error occurred (e.g., mail system error, command
syntax error) may examine the second digit.  The third digit and any
supplemental information that may be present is reserved for the finest
gradation of information.

There are five values for the first digit of the reply code:

1yz   Positive Preliminary reply
   The command has been accepted, but the requested action is being held in
   abeyance, pending confirmation of the information in this reply.  The
   SMTP client should send another command specifying whether to continue
   or abort the action. Note: unextended SMTP does not have any commands
   that allow this type of reply, and so does not have continue or abort
   commands.

2yz   Positive Completion reply
   The requested action has been successfully completed.  A new request may
   be initiated.

3yz   Positive Intermediate reply
   The command has been accepted, but the requested action is being held in
   abeyance, pending receipt of further information.  The SMTP client
   should send another command specifying this information.  This reply is
   used in command sequence groups (i.e., in DATA).

4yz   Transient Negative Completion reply
   The command was not accepted, and the requested action did not occur.
   However, the error condition is temporary and the action may be
   requested again.  The sender should return to the beginning of the
   command sequence (if any). It is difficult to assign a meaning to
   "transient" when two different sites (receiver- and sender- SMTP agents)
   must agree on the interpretation. Each reply in this category might have
   a different time value, but the SMTP client is encouraged to try again.
   A rule of thumb to determine whether a reply fits into the 4yz or the
   5yz category (see below) is that replies are 4yz if they can be
   successful if repeated without any change in command form or in
   properties of the sender or receiver.  (that is, the command is repeated
   identically and the receiver does not put up a new implementation.)

5yz   Permanent Negative Completion reply
   The command was not accepted and the requested action did not occur.
   The SMTP client is discouraged from repeating the exact request (in the
   same sequence).  Even some "permanent" error conditions can be
   corrected, so the human user may want to direct the SMTP client to
   reinitiate the command sequence by direct action at some point in the
   future (e.g., after the spelling has been changed, or the user has
   altered the account status).

The second digit encodes responses in specific categories:

x0z   Syntax: These replies refer to syntax errors, syntactically correct
   commands that don't fit any functional category, and unimplemented or
   superfluous commands.

x1z   Information:  These are replies to requests for information, such as
   status or help.

x2z   Connections: These are replies referring to the transmission channel.

x3z   Unspecified.

x4z   Unspecified.

x5z   Mail system: These replies indicate the status of the receiver mail
   system vis-a-vis the requested transfer or other mail system action.

The third digit gives a finer gradation of meaning in each category
specified by the second digit.  The list of replies illustrates this. Each
reply text is recommended rather than mandatory, and may even change
according to the command with which it is associated.  On the other hand,
the reply codes must strictly follow the specifications in this section.
Receiver implementations should not invent new codes for slightly different
situations from the ones described here, but rather adapt codes already
defined.

For example, a command such as NOOP, whose successful execution does not
offer the SMTP client any new information, will return a 250 reply.  The
reply is 502 when the command requests an unimplemented non-site-specific
action.  A refinement of that is the 504 reply for a command that is
implemented, but that requests an unimplemented parameter.

The reply text may be longer than a single line; in these cases the
complete text must be marked so the SMTP client knows when it can stop
reading the reply.  This requires a special format to indicate a multiple
line reply.

The format for multiline replies requires that every line, except the last,
begin with the reply code, followed immediately by a hyphen, "-" (also
known as minus), followed by text.  The last line will begin with the reply
code, followed immediately by <SP>, optionally some text, and <CRLF>.  As
noted above, servers SHOULD send the <SP> if subsequent text is not sent,
but clients MUST be prepared for it to be omitted.

For example:
   123-First line
   123-Second line
   123-234 text beginning with numbers
   123 The last line

In many cases the SMTP client then simply needs to search for the reply
code followed by <SP> at the beginning of a line, and ignore all preceding
lines.  In a few cases, there is important data for the client in the
reply "text".  The client will be able to identify these cases from the
current context.

4.2.2 Reply Codes by Function Groups

   500 Syntax error, command unrecognized
      (This may include errors such as command line too long)
   501 Syntax error in parameters or arguments
   502 Command not implemented  (see section 4.2.4)
   503 Bad sequence of commands
   504 Command parameter not implemented

   211 System status, or system help reply
   214 Help message
      (Information on how to use the receiver or the meaning of a
      particular non-standard command; this reply is useful only
      to the human user)

   220 <domain> Service ready
   221 <domain> Service closing transmission channel
   421 <domain> Service not available, closing transmission channel
      (This may be a reply to any command if the service knows it
      must shut down)

   250 Requested mail action okay, completed
   251 User not local; will forward to <forward-path>
      (See section 3.4)
   252 Cannot VRFY user, but will accept message and attempt 
       delivery
      (See section 3.5.3)
   450 Requested mail action not taken: mailbox unavailable
      (e.g., mailbox busy)
   550 Requested action not taken: mailbox unavailable
      (e.g., mailbox not found, no access, or command rejected
      for policy reasons)
   451 Requested action aborted: error in processing
   551 User not local; please try <forward-path>
      (See section 3.4)
   452 Requested action not taken: insufficient system storage
   552 Requested mail action aborted: exceeded storage allocation
   553 Requested action not taken: mailbox name not allowed
      (e.g., mailbox syntax incorrect)  
   354 Start mail input; end with <CRLF>.<CRLF>
   554 Transaction failed (Or, in the case of a connection-opening
       response, "No SMTP service here")

4.2.3  Reply Codes in Numeric Order

   211 System status, or system help reply
   214 Help message
      (Information on how to use the receiver or the meaning of a
      particular non-standard command; this reply is useful only
      to the human user)
   220 <domain> Service ready
   221 <domain> Service closing transmission channel
   250 Requested mail action okay, completed
   251 User not local; will forward to <forward-path>
      (See section 3.4)
   252 Cannot VRFY user, but will accept message and attempt 
      delivery
      (See section 3.5.3)

   354 Start mail input; end with <CRLF>.<CRLF>

   421 <domain> Service not available, closing transmission channel
      (This may be a reply to any command if the service knows it
      must shut down)
   450 Requested mail action not taken: mailbox unavailable
      (e.g., mailbox busy)
   451 Requested action aborted: local error in processing
   452 Requested action not taken: insufficient system storage

   500 Syntax error, command unrecognized
      (This may include errors such as command line too long)
   501 Syntax error in parameters or arguments
   502 Command not implemented (see section 4.2.4)
   503 Bad sequence of commands
   504 Command parameter not implemented
   550 Requested action not taken: mailbox unavailable
      (e.g., mailbox not found, no access, or command rejected
      for policy reasons)
   551 User not local; please try <forward-path>
      (See section 3.4)
   552 Requested mail action aborted: exceeded storage allocation
   553 Requested action not taken: mailbox name not allowed
      (e.g., mailbox syntax incorrect)
   554 Transaction failed  (Or, in the case of a connection-opening
       response, "No SMTP service here")

4.2.4 Reply Code 502

Questions have been raised as to when reply code 502 (Command not
implemented) SHOULD be returned in preference to other codes.  502 SHOULD
be used when the command is actually recognized by the SMTP server, but not
implemented.  If the command is not recognized, code 500 SHOULD be
returned.  Extended SMTP systems MUST NOT list capabilities in response to
EHLO for which they will return 502 (or 500) replies.

4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>

When an SMTP server returns a positive completion status (2yz code) after
the DATA command is completed with <CRLF>.<CRLF>, it accepts responsibility
for:

 - delivering the message (if the recipient mailbox exists), or

 - if attempts to deliver the message fail due to transient conditions,
   retrying delivery some reasonable number of times at intervals as
   specified in section 4.5.4.

 - if attempts to deliver the message fail due to permanent conditions, or
   if repeated attempts to deliver the message fail due to transient
   conditions, returning appropriate notification to the sender of the
   original message (using the address in the SMTP MAIL FROM command).

When an SMTP server returns a transient error completion status (4yz) code
after the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
any further attempt to deliver that message.  The SMTP client retains
responsibility for delivery of that message and may either return it to the
user or requeue it for a subsequent attempt (see section 4.5.4.1).  The
sending user SHOULD be able to interpret the return of a transient or
permanent failure status as a non-delivery indication.

4.3 Sequencing of Commands and Replies

4.3.1 Sequencing Overview

The communication between the sender and receiver is an alternating
dialogue, controlled by the sender.  As such, the sender issues a command
and the receiver responds with a reply.  Unless other arrangements are
negotiated through service extensions, the sender MUST wait for this
response before sending further commands.

One important reply is the connection greeting.  Normally, a receiver will
send a 220 "Service ready" reply when the connection is completed.  The
sender SHOULD wait for this greeting message before sending any commands.

Note: all the greeting-type replies have the official name (the
fully-qualified primary domain name) of the server host as the first word
following the reply code.  Sometimes the host will have no meaningful name.
See 4.1.3 for a discussion of alternatives in these situations.

For example,
   220 ISIF.USC.EDU Service ready
or
   220 mail.foo.com SuperSMTP v 6.1.2 Service ready
or
   220 [10.0.0.1] Clueless host service ready

The table below lists alternative success and failure replies for each
command.  These SHOULD be strictly adhered to: a receiver may substitute
text in the replies, but the meaning and action implied by the code numbers
and by the specific command reply sequence cannot be altered.

4.3.2 Command-Reply Sequences

Each command is listed with its usual possible replies.  The prefixes used
before the possible replies are "I" for intermediate, "S" for success, and
"E" for error.  Since some servers may generate other replies under special
circumstances, and to allow for future extension, SMTP clients SHOULD, when
possible, interpret only the first digit of the reply and MUST be prepared
to deal with unrecognized reply codes by interpreting the first digit only.
Unless extended using the mechanisms described in section 2.2, SMTP servers
MUST NOT transmit reply codes to an SMTP client that are other than three
digits or that do not start in a digit between 2 and 5 inclusive.

These sequencing rules and, in principle, the codes themselves, can be
extended or modified by SMTP extensions offered by the server and accepted
(requested) by the client.

In addition to the codes listed below, any SMTP command can return any of
the following codes if the corresponding unusual circumstances are
encountered:

500  For the "command line too long" case or if the command name was not
   recognized.  Note that producing a "command not recognized" error in
   response to the required subset of these commands is a violation of this
   specification.

501  Syntax error in command or arguments.  In order to provide for future
   extensions, commands that are specified in this document as not
   accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 message if
   arguments are supplied in the absence of EHLO-advertised extensions.

421  Service shutting down and closing transmission channel

Specific sequences are:

CONNECTION ESTABLISHMENT
   S: 220
   E: 554
EHLO or HELO
   S: 250
   E: 504, 550
MAIL
   S: 250
   E: 552, 451, 452, 550, 553, 503
RCPT
   S: 250, 251 (but see section 3.4 for discussion of 251)
   E: 550, 551, 552, 553, 450, 451, 452, 503, 550
DATA
   I: 354 -> data -> S: 250
                     E: 552, 554, 451, 452
   E: 451, 554, 503
RSET
   S: 250
VRFY
   S: 250, 251, 252
   E: 550, 551, 553, 502, 504
EXPN
   S: 250, 252
   E: 550, 500, 502, 504
HELP
   S: 211, 214
   E: 502, 504
NOOP
   S: 250
QUIT
   S: 221

4.4 Trace Information

When an SMTP server receives a message for delivery or further processing,
it MUST insert trace ("time stamp" or "Received") information at the
beginning of the message content, as discussed in section 4.1.1.4.

This line MUST be structured as follows:

 - The FROM field, which MUST be supplied in an SMTP environment, SHOULD
   contain both (1) the name of the source host as presented in the EHLO
   command and (2) an address literal containing the IP address of the
   source, determined from the TCP connection.

 - The ID field MAY contain an "@" as suggested in RFC-822, but this is not
   required.

 - The FOR field MAY contain a list of <path> entries when multiple RCPT
   commands have been given.  This may raise some security issues and is
   usually not desirable; see section 7.2.

An Internet mail program MUST NOT change a Received: line that was
previously added to the message header.  SMTP servers MUST prepend Received
lines to messages; they MUST NOT change the order of existing lines or
insert Received lines in any other location.

As the Internet grows, comparability of Received fields is important for
detecting problems, especially slow relays.  SMTP servers that create
Received fields SHOULD use explicit offsets in the dates (e.g., -0800),
rather than time zone names of any type.  Local time (with an offset) is
preferred to UT when feasible.  This formulation allows slightly more
information about local circumstances to be specified.  If UT is needed,
the receiver need merely do some simple arithmetic to convert the values.
Use of UT loses information about the time zone-location of the server.  If
a time zone name is used, it SHOULD be included in a comment.

When the delivery SMTP server makes the "final delivery" of a message, it
inserts a return-path line at the beginning of the mail data.  This use of
return-path is required; mail systems MUST support it.  The return-path
line preserves the information in the <reverse-path> from the MAIL command.
Here, final delivery means the message has left the SMTP world.  Normally,
this would mean it had been delivered to the destination user or an
associated mail drop, but in some cases it may be further processed and
transmitted by another mail system.

It is possible for the mailbox in the return path to be different from the
actual sender's mailbox, for example, if error responses are to be
delivered to a special error handling mailbox rather than to the message
sender.  When mailing lists are involved, this arrangement is common and
useful as a means of directing errors to the list maintainer rather than
the message originator.

The text above implies that the final mail data will begin with a return
path line, followed by one or more time stamp lines.  These lines will be
followed by the mail data headers and body [MSGFMT].

It is sometimes difficult for an SMTP server to determine whether or not it
is making final delivery since forwarding or other operations may occur
after the message is accepted for delivery.  Consequently, any further
(forwarding, gateway, or relay) systems MAY remove the return path and
rebuild the MAIL FROM command as needed to ensure that exactly one such
line appears in a delivered message.

A message-originating SMTP system SHOULD NOT send a message that already
contains a Return-path header.  SMTP servers performing a relay function
MUST NOT inspect the message data, and especially not to the extent needed
to determine if Return-path headers are present. SMTP servers making final
delivery MAY remove Return-path headers before adding their own.

The primary purpose of the Return-path is to designate the address to which
messages indicating non-delivery or other mail system failures are to be
sent.  For this to be unambiguous, exactly one return path SHOULD be
present when the message is delivered.  Systems using RFC 822 syntax with
non-SMTP transports SHOULD designate an unambiguous address, associated
with the transport envelope, to which error reports (e.g., non-delivery
messages) should be sent.

Historical note: Text in RFC 822 that appears to contradict the use of the
Return-path header (or the envelope MAIL FROM address) as the destination
for error messages is not applicable on the Internet. The MAIL FROM address
(as copied into the Return-path) MUST be used as the target of any mail
containing delivery error messages.

In particular:

 - a gateway from SMTP->elsewhere SHOULD insert a return-path header,
   unless it is known that the "elsewhere" transport also uses Internet
   domain addresses and maintains the envelope sender address separately.

 - a gateway from elsewhere->SMTP SHOULD delete any return-path header
   present in the message, and either copy that information to the SMTP
   envelope or combine it with information present in the envelope of the
   other transport system to construct the MAIL FROM part of the SMTP
   envelope.

The server must give special treatment to cases in which the processing
following the end of mail data indication is only partially successful.
This could happen if, after accepting several recipients and the mail data,
the SMTP server finds that the mail data could be successfully delivered to
some, but not all, of the recipients.  In such cases, the response to the
DATA command MUST be an OK reply.  However, the SMTP server MUST compose
and send an "undeliverable mail" notification message to the originator of
the message.

A single notification listing all of the failed recipients or separate
notification messages MUST be sent for each failed recipient.  For economy
of processing by the sender, the former is preferred when possible.  All
undeliverable mail notification messages are sent using the MAIL command
(even if they result from processing the obsolete SEND, SOML, or SAML
commands) and use a null return path as discussed in section 3.7.

The time stamp line and the return path line are formally defined as
follows:
 
   Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
   
   Time-stamp-line = "Received:" FWS Stamp <CRLF>
   
   Stamp = From-domain By-domain Opt-info ";"  FWS Daytime
   
   From-domain = "FROM" FWS Extended-Domain CFWS 
   
   By-domain = "BY" FWS Extended-Domain CFWS
   
   Extended-Domain = Domain / 
              ( Domain FWS "(" TCP-info ")" ) /
              ( Address-literal FWS "(" TCP-info ")"
    TCP-info = Address-literal / ( Domain FWS Address-literal )
                   ; Information derived by server from TCP connection, not client EHLO.

   Opt-info = [Via] [With] [ID] [For]
   
   Via = "VIA" FWS Link CFWS 
   
   With = "WITH" FWS Protocol CFWS 
   
   ID = "ID" FWS String / msg-id CFWS 
   
   For = "FOR" FWS 1*( Path / Mailbox ) CFWS 
   
   Link = <<>> / Addtl-Link
   Addtl-Link = Atom       ; Additional standard names for links are
                             registered with the Internet Assigned
                             Numbers Authority (IANA).
                             SMTP servers SHOULD NOT use unregistered
                             names. 
   Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
   Attdl-Protocol = Atom    ; Additional standard names for protocols
                              are registered with the Internet Assigned
                              Numbers Authority (IANA).  SMTP servers
                              SHOULD NOT use unregistered names.
   
   Daytime =  FWS [ day-of-week "," FWS ] Date FWS Time
   
   Date = DD FWS Mon FWS YYYY
          ; Note that the earlier form, which permits two-digit years, has
            been deprecated.  SMTP systems MUST use four-digit years.
   
   Time = HH ":" MM ":" SS  FWS  Zone
   
   DD = 1*2Digit           ; the one or two digit integer day of the
                             month in the range 1 to 31.
   
   Mon = "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
                "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
   
   YYYY = 4*4Digit         ; the four decimal integer year in the range
                             0000 to 9999. 
   
   HH = 2*2Digit           ; the two decimal digit hour of the day in
                             the range 00 to 24.
   
   MM = 2*2Digit            ; the two decimal digit integer minute of the hour
                              in the range 00 to 59.
   
   SS = 2*2Digit [ "." 1*Digit ]
                            ; the two decimal digit integer second of the
                              minute in the range 00 to 59, with
                              optional fractional seconds.
   
   Zone = ( "+" / "-" ) 4*4Digit [ <SP> "(" String ")" ]
                            ; A four digit, signed time zone offset,
                              such as -0500 for US Eastern Standard
                              Time.  This may be supplemented by a time
                              zone name in parentheses, e.g., "-0800
                              (PDT)". Note that there is no default;
                              time zone information is required and
                              MUST be supplied.
                               
4.5 Additional Implementation Issues

4.5.1 Minimum Implementation

In order to make SMTP workable, the following minimum implementation is
required for all receivers. The following commands MUST be supported to
conform to this specification:
   EHLO
   HELO
   MAIL
   RCPT
   DATA
   RSET
   NOOP
   QUIT

VRFY, required by RFC 1123 [RFC-1123], is no longer required by this
specification.   However, since it was required earlier, servers are
strongly encouraged to at least recognize the syntax and provide pro-forma
support.

Any system that includes an SMTP server supporting mail relaying or
delivery MUST support the reserved mailbox "postmaster" as a
case-insensitive local name. This postmaster address is not strictly
necessary if the server always returns 554 on connection opening (as
described in section 3.1).  The requirement to accept mail for postmaster
implies that RCPT TO commands which specify a mailbox for postmaster at any
of the domains for which the SMTP server provides mail service, as well as
the special case of "RCPT TO:<Postmaster>" (with no domain specification),
MUST be supported.  This requirement does not imply that SMTP systems must
deliver Postmaster mail in particular cases (e.g., problematic origin
addresses) in which they have substantive reasons for not doing so.

4.5.2 Transparency

Without some provision for data transparency, the character sequence
"<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.  In
general, users are not aware of such "forbidden" sequences.  To allow all
user composed text to be transmitted transparently, the following
procedures are used:

 - Before sending a line of mail text, the SMTP client checks the first
   character of the line.  If it is a period, one additional period is
   inserted at the beginning of the line.

 - When a line of mail text is received by the SMTP server, it checks the
   line.  If the line is composed of a single period, it is treated as the
   end of mail indicator.  If the first character is a period and there are
   other characters on the line, the first character is deleted.

The mail data may contain any of the 128 ASCII characters.  All characters
are to be delivered to the recipient's mailbox, including spaces, vertical
and horizontal tabs, and other control characters.  If the transmission
channel provides an 8-bit byte (octets) data stream, the 7-bit ASCII codes
are transmitted right justified in the octets, with the high order bits
cleared to zero.  See 3.7 for special treatment of these conditions in SMTP
systems serving a relay function.

In some systems it may be necessary to transform the data as it is received
and stored.  This may be necessary for hosts that use a different character
set than ASCII as their local character set or store data in records rather
than strings.  If such transformations are necessary, they MUST be
reversible, especially if such transformations are applied to mail being
relayed.

4.5.3 Sizes and Timeouts

There are several objects that have required minimum/maximum sizes. Every
implementation MUST be able to receive objects of at least these sizes.
Objects larger than these sizes SHOULD be avoided when possible.  However,
some Internet mail constructs such as encoded X.400 addresses [RFC-X400]
will often require larger objects: clients MAY attempt to transmit these,
but MUST be prepared for a server to reject them if they cannot be handled
by it.  To the maximum extent possible, implementation techniques which
impose no limits on the length of these objects should be used.

local-part
   The maximum total length of a user name or other local-part is 64
   characters.

domain
   The maximum total length of a domain name or number is 255 characters.

path
   The maximum total length of a reverse-path or forward-path is 256
   characters (including the punctuation and element separators).

command line
   The maximum total length of a command line including the command word
   and the <CRLF> is 512 characters.  SMTP extensions may be used to
   increase this limit.

reply line
   The maximum total length of a reply line including the reply code and
   the <CRLF> is 512 characters.  More information may be conveyed through
   multiple-line replies.

text line
   The maximum total length of a text line including the <CRLF> is 1000
   characters (not counting the leading dot duplicated for transparency).
   This number may be increased by the use of SMTP Service Extensions.

message content
   The maximum total length of a message content (including any message
   headers as well as the message body) MUST BE at least 64K octets. Since
   the introduction of multimedia mail [RFC-MIME], message lengths on the
   Internet have grown dramatically, and message size restrictions should
   be avoided if at all possible. SMTP server systems that must impose
   restrictions SHOULD implement the "SIZE" service extension ([RFC-SIZE]),
   and SMTP client systems that will send large messages SHOULD utilize it
   when possible.

recipients buffer
   The minimum total number of recipients that must be buffered is 100
   recipients. Rejection of messages (for excessive recipients) with fewer
   than 100 RCPT TO commands is a violation of this specification.  The
   general principle that relaying SMTP servers MUST NOT, and delivery SMTP
   servers SHOULD NOT, perform validation tests on message headers suggests
   that rejecting a message based on the total number of recipients shown
   in header fields is to be discouraged.  A server which imposes a limit
   on the number of recipients MUST behave in an orderly fashion,  such as
   to reject additional addresses over its limit rather than silently
   discarding addresses previously accepted.  A client that needs to
   deliver a message containing over 100 RCPT TO commands SHOULD be
   prepared to transmit in 100-recipient "chunks" if the server declines to
   accept more than 100 recipients in a single message.

Errors due to exceeding these limits may be reported by using the reply
codes. Some examples of reply codes are:

   500 Line too long.
or
   501 Path too long
or
   452 Too many recipients  (see below)
or
   552 Too much mail data.

[RFC-821] incorrectly listed the error where an SMTP server exhausts its
implementation limit on the number of RCPT TO commands ("too many
recipients") as having reply code 552.  The correct reply code for this
condition is 452. Clients SHOULD treat a 552 code in this case as a
temporary, rather than permanent failure so the logic below works.

When a conforming SMTP server encounters this condition, it has at least
100 successful RCPT commands in its recipients buffer.  If the server is
able to accept the message, then at least these 100 addresses will be
removed from the SMTP client's queue.  When the client attempts
retransmission of those addresses which received 452 responses, at least
100 of these will be able to fit in the SMTP server's recipients buffer.
Each retransmission attempt which is able to deliver anything will be able
to dispose of at least 100 of these recipients.

If an SMTP server has an implementation limit on the number of RCPT TO
commands and this limit is exhausted, it MUST use a response code of 452.
If the server has a configured site-policy limitation on the number of RCPT
TO commands, it MAY instead use a 5XX response code.

In order to interoperate with SMTP servers implementing an older version of
the protocol, SMTP clients MAY treat a 552 code obtained in response to an
RCPT command as if it were a 452 response code, especially after some RCPT
commands have already been accepted in the same mail transaction.

An SMTP client MUST provide a timeout mechanism.  It MUST use per-command
timeouts rather than somehow trying to time the entire mail transaction.
Timeouts SHOULD be easily reconfigurable, preferably without recompiling
the SMTP code.  To implement this, a timer is set for each SMTP command and
for each buffer of the data transfer.  The latter means that the overall
timeout is inherently proportional to the size of the message.

Based on extensive experience with busy mail-relay hosts, the minimum
per-command timeout values SHOULD be as follows:

Initial 220 Message: 5 minutes
   An SMTP client process needs to distinguish between a failed TCP
   connection and a delay in receiving the initial 220 greeting message.
   Many SMTP servers accept a TCP connection but delay delivery of the 220
   message until their system load permits more mail to be processed.

MAIL Command: 5 minutes

RCPT Command: 5 minutes
   A longer timeout is required if processing of mailing lists and aliases
   is not deferred until after the message was accepted.

DATA Initiation: 2 minutes
   This is while awaiting the "354 Start Input" reply to a DATA command.

Data Block: 3 minutes
   This is while awaiting the completion of each TCP SEND call transmitting
   a chunk of data.

DATA Termination: 10 minutes.
   This is while awaiting the "250 OK" reply. When the receiver gets the
   final period terminating the message data, it typically performs
   processing to deliver the message to a user mailbox.  A spurious timeout
   at this point would be very wasteful and would typically result in
   delivery of multiple copies of the message, since it has been
   successfully sent and the server has accepted responsibility for
   delivery.  See section 6.1 for additional discussion.

An SMTP server SHOULD have a timeout of at least 5 minutes while it is
awaiting the next command from the sender.

4.5.4 Queuing Strategies

The common structure of a host SMTP implementation includes user mailboxes,
one or more areas for queuing messages in transit, and one or more daemon
processes for sending and receiving mail.  The exact structure will vary
depending on the needs of the users on the host and the number and size of
mailing lists supported by the host. We describe several optimizations that
have proved helpful, particularly for mailers supporting high traffic
levels.

Any queuing strategy MUST include timeouts on all activities on a
per-command basis. A queuing strategy MUST NOT send error messages in
response to error messages under any circumstances.

4.5.4.1 Sending Strategy

The general model for an SMTP client is one or more processes that
periodically attempt to transmit outgoing mail.  In a typical system, the
program that composes a message has some method for requesting immediate
attention for a new piece of outgoing mail, while mail that cannot be
transmitted immediately MUST be queued and periodically retried by the
sender.  A mail queue entry will include not only the message itself but
also the envelope information.

The sender MUST delay retrying a particular destination after one attempt
has failed.  In general, the retry interval SHOULD be at least 30 minutes;
however, more sophisticated and variable strategies will be beneficial when
the SMTP client can determine the reason for non-delivery.

Retries continue until the message is transmitted or the sender gives up;
the give-up time generally needs to be at least 4-5 days.  The parameters
to the retry algorithm MUST be configurable.

A client SHOULD keep a list of hosts it cannot reach and corresponding
connection timeouts, rather than just retrying queued mail items.

Experience suggests that failures are typically transient (the target
system or its connection has crashed), favoring a policy of two connection
attempts in the first hour the message is in the queue, and then backing
off to one every two or three hours.

The SMTP client can shorten the queuing delay in cooperation with the SMTP
server.  For example, if mail is received from a particular address, it is
likely that mail queued for that host can now be sent. Application of this
principle may, in many cases, eliminate the requirement for an explicit
"send queues now" function such as that discussed in [RFC-ETRN].

The strategy may be further modified as a result of multiple addresses per
host (see below) to optimize delivery time vs. resource usage.

An SMTP client may have a large queue of messages for each unavailable
destination host.  If all of these messages were retried in every retry
cycle, there would be excessive Internet overhead and the sending system
would be blocked for a long period.  Note that an SMTP client can generally
determine that a delivery attempt has failed only after a timeout of
several minutes and even a one-minute timeout per connection will result in
a very large delay if retries are repeated for dozens, or even hundreds, of
queued messages to the same host.

At the same time, SMTP clients SHOULD use great care in caching negative
responses from servers.  In an extreme case, if EHLO is issued multiple
times during the same SMTP connection, different answers may be returned by
the server. More significantly, 5yz responses to MAIL FROM MUST NOT be
cached.

When a mail message is to be delivered to multiple recipients, and the SMTP
server to which a copy of the message is to be sent is the same for
multiple recipients, then only one copy of the message SHOULD be
transmitted.  That is, the SMTP client SHOULD use the command sequence:
MAIL, RCPT, RCPT,... RCPT, DATA instead of the sequence: MAIL, RCPT, DATA,
..., MAIL, RCPT, DATA.  However, if there are very many addresses, a limit
on the number of RCPT commands per MAIL command MAY be imposed.
Implementation of this efficiency feature is strongly encouraged.

Similarly, to achieve timely delivery, the SMTP client MAY support multiple
concurrent outgoing mail transactions.  However, some limit may be
appropriate to protect the host from devoting all its resources to mail.

4.5.4.2 Receiving Strategy

The SMTP server SHOULD attempt to keep a pending listen on the SMTP port at
all times.  This requires the support of multiple incoming TCP connections
for SMTP.  Some limit MAY be imposed.

As discussed above, when the SMTP server receives mail from a particular
host address, it could notify the SMTP client to retry any mail pending for
that host address.

4.5.5   Messages with a null reverse-path

There are several types of notification messages which are required by
existing and proposed standards to be sent with a null reverse path,
namely non-delivery notifications as discussed in section 3.7, other kinds
of Delivery Status Notifications (DSNs, see [RFC 1894]) and also Message
Disposition Notifications (MDNs, see [RFC 2298]). All of these kinds of
messages are notifications about a previous message, and they are sent to
the reverse-path of the previous mail message. (If the delivery of such a
notification message fails, that usually indicates a problem with the mail
system of the host to which the notification message is addressed. For
this reason, at some hosts the MTA is set up to forward such failed
notification messages to someone who is able to fix problems with the mail
system, e.g. via the postmaster alias.)

All other types of messages (i.e. any message which is not required by a
standards-track RFC to have a null reverse-path) SHOULD be sent with with
a valid, non-null reverse-path.

Implementors of automated email processors should be careful to make sure
that the various kinds of messages with null reverse-path are handled
correctly, in particular such systems SHOULD NOT reply to messages with
null reverse-path. 

5. Address Resolution and Mail Handling

Once an SMTP client lexically identifies a domain to which mail will be
delivered for processing (as described in sections 3.6 and 3.7), a DNS
lookup is performed to resolve the domain name (see [RFC-DNS]).  The names
are expected to be fully-qualified domain names (FQDNs): mechanisms for
inferring FQDNs from partial names or local aliases are outside of this
specification and, due to a history of problems, are generally discouraged.
The lookup first attempts to locate an MX record associated with the name.
If a CNAME record is found instead, the resulting name is processed as if
it were the initial name.  If no MX records are found, but an A RR is
found, the A RR is treated as if it was associated with an implicit MX RR,
with a preference of 0, pointing to that host.  If one or more MX RRs are
found for a given name, SMTP systems MUST NOT utilize any A RRs associated
with that name unless they are located using the MX RRs; the "implicit MX"
rule above applies only if there are no MX records present.  If MX records
are present, but none of them are usable, this situation MUST be reported
as an error.

When the lookup succeeds, the mapping can result in a list of alternative
delivery addresses rather than a single address, because of multiple MX
records, multihoming, or both.  To provide reliable mail transmission, the
SMTP client MUST be able to try (and retry) each of the relevant addresses
in this list in order, until a delivery attempt succeeds. However, there
MAY also be a configurable limit on the number of alternate addresses that
can be tried.  In any case, a host SHOULD try at least two addresses.

Two types of information is used to rank the host addresses: multiple MX
records, and multihomed hosts.

Multiple MX records contain a preference indication that SHOULD be used in
sorting (see below).  Lower numbers are more preferred than higher ones.
If there are multiple destinations with the same preference and there is no
clear reason to favor one (e.g., by recognition of an easily-reached
address), then the sender-SMTP MUST randomize them to spread the load
across multiple mail exchangers for a specific organization.

The destination host (perhaps taken from the preferred MX record) may be
multihomed, in which case the domain name resolver will return a list of
alternative IP addresses.  It is the responsibility of the domain name
resolver interface to have ordered this list by decreasing preference if
necessary, and SMTP MUST try them in the order presented.

Although the capability to try multiple alternative addresses is required,
specific installations may want to limit or disable the use of alternative
addresses.  The question of whether a sender should attempt retries using
the different addresses of a multihomed host has been controversial.  The
main argument for using the multiple addresses is that it maximizes the
probability of timely delivery, and indeed sometimes the probability of any
delivery; the counter-argument is that it may result in unnecessary
resource use. Note that resource use is also strongly determined by the
sending strategy discussed in section 4.5.4.1.

If a host receives a message with a destination for which it is a
designated Mail eXchanger, it MAY relay the message (potentially after
having rewritten the addresses), make final delivery of the message, or
hand it off using some mechanism outside the SMTP-provided transport
environment.

If it determines that it should relay the message without rewriting the
address, it MUST sort the MX records to determine candidates for delivery.
The records are first ordered by preference, with the lowest-numbered
records being most preferred.  The relay host MUST then inspect the list
for any of the names or addresses by which it might be known in mail
transactions.  If a matching record is found, all records at that
preference level and higher-numbered ones MUST be discarded from
consideration.  If there are no records left at that point, it is an error
condition, and the message MUST be returned as undeliverable.  If records
do remain, they SHOULD be tried, best preference first, as described above.

6. Problem Detection and Handling

6.1 Reliable Delivery and Replies by Email

When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
message in response to DATA), it is accepting responsibility for delivering
or relaying the message.  It must take this responsibility seriously. It
MUST NOT lose the message for frivolous reasons, such as because the host
later crashes or because of a predictable resource shortage.

If there is a delivery failure after acceptance of a message, the
receiver-SMTP MUST formulate and mail a notification message.  This
notification MUST be sent using a null ("<>") reverse path in the envelope.
The recipient of this notification SHOULD be the address from the envelope
return path (or the Return-Path: line).  However, if this address is null
("<>"), the receiver-SMTP MUST NOT send a notification.  Obviously, nothing
in this section can or should prohibit local decisions (i.e., as part of
the same system environment as the receiver-SMTP) to log or otherwise
transmit information about null address events locally if that is desired.
If the address is an explicit source route, it MUST be stripped down to its
final hop.

For example, suppose that an error notification must be sent for a message
that arrived with:
   MAIL FROM:<@a,@b:user@d>

The notification message SHOULD be sent using: 
   RCPT TO:<user@d>

Some delivery failures after the message is accepted by SMTP will be
unavoidable.  For example, it may be impossible for the receiving SMTP
server to validate all the delivery addresses in RCPT command(s) due to a
"soft" domain system error, because the target is a mailing list (see
earlier discussion of RCPT), or because the server is acting as a relay and
has no immediate access to the delivering system.

To avoid receiving duplicate messages as the result of timeouts, a
receiver-SMTP MUST seek to minimize the time required to respond to the
final <CRLF>.<CRLF> end of data indicator.  See RFC-1047 [RFC-1047] for a
discussion of this problem.

6.2 Loop Detection

Simple counting of the number of "Received:" headers in a message has
proven to be an effective, although rarely optimal, method of detecting
loops in mail systems.  SMTP servers using this technique SHOULD use a
large rejection threshold, normally at least 100 Received entries.
Whatever mechanisms are used, servers MUST contain provisions for detecting
and stopping trivial loops.

6.3 Compensating for Irregularities

Unfortunately, variations, creative interpretations, and outright
violations of Internet mail protocols do occur; some would suggest that
they occur quite frequently.  The debate as to whether a well-behaved SMTP
receiver or relay should reject a malformed message, attempt to pass it on
unchanged, or attempt to repair it to increase the odds of successful
delivery (or subsequent reply) began almost with the dawn of structured
network mail and shows no signs of abating.  Advocates of rejection claim
that attempted repairs are rarely completely adequate and that rejection of
bad messages is the only way to get the offending software repaired.
Advocates of "repair" or "deliver no matter what" argue that users prefer
that mail go through it if at all possible and that there are significant
market pressures in that direction.  In practice, these market pressures
may be more important to particular vendors than strict conformance to the
standards, regardless of the preference of the actual developers.

The problems associated with ill-formed messages were exacerbated by the
introduction of the split-UA mail reading protocols [RFC-POP2, RFC-POP3,
RFC-IMAP2, RFC-PCMAIL]. These protocols have encouraged the use of SMTP as
a posting protocol, and SMTP servers as relay systems for these client
hosts (which are often only intermittently connected to the Internet).
Historically, many of those client machines lacked some of the mechanisms
and information assumed by SMTP (and indeed, by the mail format protocol
[RFC-822]).  Some could not keep adequate track of time; others had no
concept of time zones; still others could not identify their own names or
addresses; and, of course, none could satisfy the assumptions that underlay
RFC-822's conception of authenticated addresses.

In response to these weak SMTP clients, many SMTP systems now complete
messages that are delivered to them in incomplete or incorrect form.  This
strategy is generally considered appropriate when the server can identify
or authenticate the client, and there are prior agreements between them.
By contrast, there is at best great concern about fixes applied by a relay
or delivery SMTP server that has little or no knowledge of the user or
client machine.

The following changes to a message being processed MAY be applied when
necessary by an originating SMTP server, or one used as the target of SMTP
as an initial posting protocol:

 - Addition of a message-id field when none appears

 - Addition of a date, time or time zone when none appears

 - Correction of addresses to proper FQDN format

The less information the server has about the client, the less likely these
changes are to be correct and the more caution and conservatism should be
applied when considering whether or not to perform fixes and how.  These
changes MUST NOT be applied by an SMTP server that provides an intermediate
relay function.

In all cases, properly-operating clients supplying correct information are
preferred to corrections by the SMTP server. In all cases, documentation of
actions performed by the servers (in trace fields and/or header comments)
is strongly encouraged.

7. Security Considerations

7.1 Mail Security and Spoofing

SMTP mail is inherently insecure in that it is feasible for even fairly
casual users to negotiate directly with receiving and relaying SMTP servers
and create messages that will trick a naive recipient into believing that
they came from somewhere else.  Constructing such a message so that the
"spoofed" behavior cannot be detected by an expert is somewhat more
difficult, but not sufficiently so as to be a deterrent to someone who is
determined and knowledgeable. Consequently, as knowledge of Internet mail
increases, so does the knowledge that SMTP mail inherently cannot be
authenticated, or integrity checks provided, at the transport level. Real
mail security lies only in end-to-end methods involving the message bodies,
such as those that can be provided in the MOSS framework [RFC-MOSS].

Various protocol extensions and configuration options that provide
authentication at the transport level (e.g., from an SMTP client to an SMTP
server) improve somewhat on the traditional situation described above.
However, unless they are accompanied by careful handoffs of responsibility
in a carefully-designed trust environment, they remain inherently weaker
than end-to-end mechanisms which use digitally signed messages rather than
depending on the integrity of the transport system.

Efforts to make it more difficult for users to set envelope MAIL FROM and
header "From" fields to point to valid addresses other than their own are
largely misguided: they frustrate legitimate applications in which mail is
sent by one user on behalf of another or in which error (or normal) replies
should be directed to a special address. (Systems that provide convenient
ways for users to alter these fields on a per-message basis should attempt
to establish a primary and permanent mailbox address for the user so that
Sender fields within the message data can be generated sensibly.)

This specification does not further address the authentication issues
associated with SMTP other than to advocate that useful functionality not
be disabled in the hope of providing some small margin of protection
against an ignorant user who is trying to fake mail.

7.2 "Blind" Copies

Addresses that do not appear in the message headers may appear in the RCPT
TO commands to an SMTP server for a number of reasons.  The two most common
involve the use of a mailing address as a "list exploder" (a single address
that resolves into multiple addresses) and the appearance of "blind
copies".  Especially when more than one RCPT command is present, and in
order to avoid defeating some of the purpose of these mechanisms, SMTP
clients and servers SHOULD NOT copy the full set of RCPT TO command
arguments into the headers, either as part of trace headers or as
informational or private-extension headers.  Since this rule is often
violated in practice, and cannot be enforced, sending SMTP systems that are
aware of "bcc" use MAY find it helpful to send each blind copy as a
separate message transaction containing only a single RCPT TO command.

There is no inherent relationship between either "reverse" (MAIL FROM, SAML
FROM, etc.) or "forward" (RCPT TO) addresses in the SMTP transaction
("envelope") and the addresses in the headers.  Receiving systems SHOULD
NOT attempt to deduce such relationships and use them to alter the headers
of the message for delivery.  The popular "Apparently-to" header is a
violation of this principle and SHOULD NOT be used.

7.3 VRFY, EXPN, and Security

As discussed in section 3.5, individual sites may want to disable one or
both VRFY or EXPN for security reasons.  As a corollary to the above,
implementations that permit this MUST NOT appear to have verified addresses
that are not, in fact, verified.  If a site disables these commands for
security reasons, the SMTP server MUST return a 252 response, rather than a
code that could be confused with successful or unsuccessful verification.

Returning a 250 reply code with the address listed in the VRFY command
after having checked it only for syntax violates this rule. Of course, an
implementation that "supports" VRFY by always returning 550 whether or not
the address is valid is equally not in conformance.

Within the last few years, the contents of mailing lists have become
popular as an address information source for so-called "spammers." The use
of EXPN to "harvest" addresses has increased as list administrators have
installed protections against inappropriate uses of the lists themselves.
Implementations SHOULD still provide support for EXPN, but sites SHOULD
carefully evaluate the tradeoffs. As authentication mechanisms are
introduced into SMTP, some sites may choose to make EXPN available only to
authenticated requestors.

7.4 Information Disclosure in Announcements

There has been an ongoing debate about the tradeoffs between the debugging
advantages of announcing server type and version (and, sometimes, even
server domain name) in the greeting response or in response to the HELP
command and the disadvantages of exposing useful information to potential
hostile attack.  The utility of the debugging information is beyond doubt.
Those who argue for making it available point out that it is far better to
actually secure an SMTP server rather than hope that trying to conceal
known vulnerabilities by hiding the server's precise identity will provide
more protection. Sites are encouraged to evaluate the tradeoff with that
issue in mind; implementations are strongly encouraged to minimally provide
for making type and version information available in some way to other
network hosts.

7.5 Information Disclosure in Trace Fields

In some circumstances, such as when mail originates from within a LAN whose
hosts are not directly from the public Internet, trace ("Received") fields
produced in conformance with this specification may disclose host names and
similar information that would not normally be available.  This ordinarily
does not pose a problem, but sites with special concerns about name
disclosure should be aware of it.  Also, the optional FOR clause should be
supplied with caution or not at all when multiple recipients are involved
lest it inadvertently disclose the identities of "blind copy" recipients to
others.

7.6 Scope of Operation of SMTP Servers

It is a well-established principle that an SMTP server may refuse to accept
mail for any operational or technical reason that makes sense to the site
providing the server. However, cooperation among sites and installations
makes the Internet possible.  If sites take excessive advantage of the
right to reject traffic, the ubiquity of email availability (one of the
strengths of the Internet) will be threatened; considerable care should be
taken and balance maintained if a site decides to be selective about the
traffic it will accept and process.

In recent years, use of the relay function through arbitrary sites has been
used as part of hostile efforts to hide the actual origins of mail.  Some
sites have decided to limit the use of the relay function to known or
identifiable sources, and implementations SHOULD provide the capability to
perform this type of filtering.  When mail is rejected for these or other
policy reasons, a 550 code SHOULD be used in response to EHLO, MAIL FROM,
or RCPT TO as appropriate.

8. IANA Considerations

IANA will maintain three registries in support of this specification.  The
first consists of SMTP service extensions with the associated keywords,
and, as needed, parameters and verbs.  As specified in section 2.2.2, no
entry may be made in this registry that starts in an "X".  Entries may be
made only for service extensions (and associated keywords, parameters, or
verbs) that are defined in standards-track or experimental RFCs
specifically approved by the IESG for this purpose.

The second registry consists of "tags" that identify forms of domain
literals other than those for IPv4 addresses (specified in RFC 821 and in
this document) and IPv6 addresses (specified in this document).  Additional
literal types require standardization before being used; none are
anticipated at this time.

The third, established by RFC 821 and renewed by this specification, is a
registry of link and protocol identifiers to be used with the "via" and
"with" subclauses of the time stamp ("Received: header") described in
section 4.4.  Link and protocol identifiers in addition to those specified
in this document may be registered only by standardization or by way of an
RFC-documented, IESG-approved, Experimental protocol extension.

9. References

[8BITMIME] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker, "SMTP
Service Extension for 8bit-MIMEtransport", RFC 1652, 07/18/1994.

[ABNF] Crocker, D., P. Overell, Eds., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[IPv6AddrString] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
Architecture", RFC 1884, December 1995.

[MSGFMT] P. Resnick, Work in progress, draft-ietf-drums-msg-fmt-05.txt,
August, 1998

[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC 822, Department of Electrical Engineering, University of
Delaware, August 1982.

[RFC-974] C. Partridge, "Mail routing and the domain system", RFC 974,
01/01/1986

[RFC-1047] C. Partridge, "Duplicate messages and SMTP", RFC 1047,
02/01/1988.

[RFC-1123] R. Braden, "Requirements for Internet hosts - application and
support", 10/01/1989

[RFC-BDAT] G. Vaudreuil, "SMTP Service Extensions for Transmission of Large
and Binary MIME Messages", RFC 1830, 08/16/1995.

[RFC-DNS] P. Mockapetris, "Domain names - implementation and
specification", RFC 1035 and P. Mockapetris, "Domain names - concepts and
facilities", RFC 1034. (STD 13)

[RFC-ETRN] J. De Winter, "SMTP Service Extension for Remote Message Queue
Starting", RFC 1985, 08/14/1996.

[RFC-IMAP2] M. Crispin, "Interactive Mail Access Protocol - Version 2", RFC
1176, 08/20/1990.

[RFC-IMAP4] M. Crispin, "Internet Message Access Protocol - Version 4", RFC
2060, 12/04/1996.

[RFC-INTLHDR] K. Moore, "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047, 12/02/1996.

[RFC-MIME] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC 2045, 12/02/1996.

[RFC-MOSS] S. Crocker, N. Freed, J. Galvin, S. Murphy, "MIME Object
Security Services", RFC 1848, 10/03/1995.

[RFC-NOTARY1] K. Moore, "SMTP Service Extension for Delivery Status
Notifications", RFC 1891, 01/15/1996.

[RFC-NOTARY2] K. Moore, G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, 01/15/1996.

[RFC-PCMAIL] M. Lambert, "PCMAIL: A distributed mail system for personal
computers", RFC 1056, 06/01/1988.

[RFC-PIPELINE] N. Freed, A. Cargille, "SMTP Service Extension for Command
Pipelining", RFC 1854, 10/04/1995.

[RFC-POP2] M. Butler, D. Chase, J. Goldberger, J. Postel, J. Reynolds,
"Post Office Protocol - version 2", RFC 937, 02/01/1985

[RFC-POP3] J. Myers, M. Rose, "Post Office Protocol - Version 3", RFC 1930,
5/14/96 (Std 53).

[RFC-REPLY] G. Vaudreuil, "Enhanced Mail System Status Codes", RFC 1893,
01/15/1996.

[RFC-SIZE] J. Klensin, N. Freed, K. Moore, "SMTP Service Extension for
Message Size Declaration", RFC 1870, 11/06/1995. (STD 10)

[RFC-X400] S. Hardcastle-Kille, "Mapping between X.400(1988) / ISO 10021
and RFC 822", RFC 1327, 05/18/1992.

[SMTPEX] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker, "SMTP
Service Extensions", RFC-1869, 11/06/1995. (STD 10)

[TCP] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981.

[US-ASCII] United States of America Standards Institute (now American
National Standards Institute), X3.4, 1968, "USA Code for Information
Interchange". ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains definitive for the
Internet.

10. Editor's Address

John C. Klensin
MCI Communications
800 Boylston St., 7th floor
Boston, MA 02199
USA
Email: [email-masked]
Phone: +1 617 960 1011
Fax:   +1 617 960 1009

11. Acknowledgments

Many people worked long and hard on the many iterations of this document.
There was wide-ranging debate on the mailing list about many technical
issues, and many contributors helped form the wording in this
specification. The hundreds of participants in the many discussions since
RFC 821 was produced are too numerous to mention, but they all helped this
document become what it is.

A. TCP Transport Service

The TCP connection supports the transmission of 8-bit bytes. The SMTP data
is 7-bit ASCII characters.  Each character is transmitted as an 8-bit byte
with the high-order bit cleared to zero.  Service extensions may modify
this rule to permit transmission of full 8-bit data bytes as part of the
message body, but not in SMTP commands or responses.

B. Generating SMTP Commands from RFC 822 Headers
 
Some systems use RFC 822 headers (only) in a mail submission protocol, or
otherwise generate SMTP commands from RFC 822 headers when such a message
is handed to an MTA from a UA.  While the MTA-UA protocol is a private
matter, not covered by any Internet Standard, there are problems with this
approach.  For example, there have been repeated problems with proper
handling of "bcc" copies and redistribution lists when information that
conceptually belongs to a mail envelopes is not separated early in
processing from header information (and kept separate).

It is recommended that the UA provide its initial MTA with an envelope
separate from the message itself.  However, if the envelope is not
supplied, SMTP commands SHOULD be generated as follows:

1. Each recipient address from a TO, CC, or BCC header field SHOULD be
   copied to a RCPT command (generating multiple message copies if that is
   required for queuing or delivery).  This includes any addresses listed
   in a RFC 822 "group".  Any BCC fields SHOULD then be removed from the
   headers.  Once this process is completed, the remaining headers SHOULD
   be checked to verify that at least one To:, Cc:, or Bcc: header remains.
   If none do, then a bcc: header with no additional information SHOULD be
   inserted as specified in [MSGFMT].

2. The return address in the MAIL command SHOULD, if possible, be derived
   from the system's identity for the submitting (local) user. And the From
   header field otherwise.  If there is a system identity available, it
   SHOULD also be copied to the Sender header field if it is different from
   the address in the From header field.  (Any Sender field that was
   already there SHOULD be removed.)  Systems may provide a way for
   submitters to override the envelope return address, but may want to
   restrict its use to privileged users.  This will not prevent mail
   forgery, but may lessen its incidence; see section 7.1.

When an MTA is being used in this way, it bears responsibility for ensuring
that the message being transmitted is valid.  The mechanisms for checking
that validity, and for handling (or returning) messages that are not valid
at the time of arrival, are part of the MUA-MTA interface and not covered
by this specification.

A submission protocol based on Standard RFC 822 information alone MUST NOT
be used to gateway a message from a foreign (non-SMTP) mail system into an
SMTP environment.  Additional information to construct an envelope must
come from some source in the other environment, whether supplemental
headers or the foreign system's envelope.

Attempts to gateway messages using only their header "to" and "cc" fields,
have repeatedly caused mail loops and other behavior adverse to the proper
functioning of the Internet mail environment.  These problems have been
especially common when the message originates from an Internet mailing list
and is distributed into the foreign environment using envelope information.
When these messages are then processed by a header-only remailer, loops
back to the Internet environment (and the mailing list) are almost
inevitable.

C. Source Routes

The <reverse-path> is a reverse source routing list of hosts and a source
mailbox.  The first host in the <reverse-path> SHOULD be the host sending
the MAIL FROM command.  Similarly, the <forward-path> may be a source
routing lists of hosts and a destination mailbox. However, in general, the
<forward-path> SHOULD contain only a mailbox and domain name, relying on
the domain name system to supply routing information if required.  The use
of source routes is deprecated; while servers MUST be prepared to receive
and handle them as discussed in section 3.3 and F.2, clients SHOULD NOT
transmit them.

For relay purposes, the forward-path may be a source route of the form
"@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE fully-qualified
domain names.  This form is used to emphasize the distinction between an
address and a route.  The mailbox is an absolute address, and the route is
information about how to get there.  The two concepts should not be
confused.

If source routes are used, RFC 821 and the text below should be consulted
for the mechanisms for constructing and updating the forward- and
reverse-paths.

The SMTP server transforms the command arguments by moving its own
identifier (its domain name or that of any domain for which it is acting as
a mail exchanger), if it appears, from the forward-path to the beginning of
the reverse-path.

Notice that the forward-path and reverse-path appear in the SMTP commands
and replies, but not necessarily in the message.  That is, there is no need
for these paths and especially this syntax to appear in the "To:" ,
"From:", "CC:", etc. fields of the message header. Conversely, SMTP servers
MUST NOT derive final message delivery information from message header
fields.

When the list of hosts is present, it is a "reverse" source route and
indicates that the mail was relayed through each host on the list (the
first host in the list was the most recent relay).  This list is used as a
source route to return non-delivery notices to the sender. As each relay
host adds itself to the beginning of the list, it MUST use its name as
known in the transport environment to which it is relaying the mail rather
than that of the transport environment from which the mail came (if they
are different).

D. Scenarios

This section presents complete scenarios of several types of SMTP sessions.
In the examples, "C:" indicates what is said by the SMTP client, and "S:"
indicates what is said by the SMTP server.

D.1 A Typical SMTP Transaction Scenario

This SMTP example shows mail sent by Smith at host bar.com, to Jones,
Green, and Brown at host foo.com.  Here we assume that host bar.com
contacts host foo.com directly.  The mail is accepted for Jones and Brown.
Green does not have a mailbox at host foo.com.

   S: 220 foo.com Simple Mail Transfer Service Ready
   C: EHLO bar.com
   S: 250-foo.com greets bar.com
   S: 250-8BITMIME
   S: 250-SIZE
   S: 250-DSN
   S: 250 HELP
   C: MAIL FROM:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 550 No such user here
   C: RCPT TO:<[email-masked]>
   S: 250 OK
   C: DATA
   S: 354 Start mail input; end with <CRLF>.<CRLF>
   C: Blah blah blah...
   C: ...etc. etc. etc.
   C: .
   S: 250 OK
   C: QUIT
   S: 221 foo.com Service closing transmission channel

D.2 Aborted SMTP Transaction Scenario

   S: 220 foo.com Simple Mail Transfer Service Ready
   C: EHLO bar.com
   S: 250-foo.com greets bar.com
   S: 250-8BITMIME
   S: 250-SIZE
   S: 250-DSN
   S: 250 HELP
   C: MAIL FROM:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 550 No such user here
   C: RSET
   S: 250 OK
   C: QUIT
   S: 221 foo.com Service closing transmission channel

D.3 Relayed Mail Scenario

Step 1  --  Source Host to Relay Host

   S: 220 foo.com Simple Mail Transfer Service Ready
   C: EHLO bar.com
   S: 250-foo.com greets bar.com
   S: 250-8BITMIME
   S: 250-SIZE
   S: 250-DSN
   S: 250 HELP
   C: MAIL FROM:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<@foo.com:[email-masked]>
   S: 250 OK
   C: DATA
   S: 354 Start mail input; end with <CRLF>.<CRLF>
   C: Date: Thu, 21 May 1998 05:33:29 -0700
   C: From: John Q. Public <[email-masked]>
   C: Subject:  The Next Meeting of the Board
   C: To: [email-masked]
   C:
   C: Bill:
   C: The next meeting of the board of directors will be
   C: on Tuesday.
   C:                         John.
   C: .
   S: 250 OK
   C: QUIT
   S: 221 foo.com Service closing transmission channel

Step 2  --  Relay Host to Destination Host

   S: 220 xyz.com Simple Mail Transfer Service Ready
   C: EHLO foo.com
   S: 250 xyz.com is on the air
   C: MAIL FROM:<@foo.com:[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 250 OK
   C: DATA
   S: 354 Start mail input; end with <CRLF>.<CRLF>
   C: Received: from bar.com by foo.com ; Thu, 21 May 1998 05:33:29 -0700
   C: Date: Thu, 21 May 1998 05:33:22 -0700
   C: From: John Q. Public <[email-masked]>
   C: Subject:  The Next Meeting of the Board
   C: To: [email-masked]
   C:
   C: Bill:
   C: The next meeting of the board of directors will be
   C: on Tuesday.
   C:                         John.
   C: .
   S: 250 OK

   C: QUIT
   S: 221 foo.com Service closing transmission channel

D.4 Verifying and Sending Scenario

   S: 220 foo.com Simple Mail Transfer Service Ready
   C: EHLO bar.com
   S: 250-foo.com greets bar.com
   S: 250-8BITMIME
   S: 250-SIZE
   S: 250-DSN
   S: 250 HELP
   C: VRFY Crispin
   S: 250 Mark Crispin <[email-masked]>
   C: SEND FROM:<[email-masked]>
   S: 250 OK
   C: RCPT TO:<[email-masked]>
   S: 250 OK
   C: DATA
   S: 354 Start mail input; end with <CRLF>.<CRLF>
   C: Blah blah blah...
   C: ...etc. etc. etc.
   C: .
   S: 250 OK
   C: QUIT
   S: 221 foo.com Service closing transmission channel

E. Other Gateway Issues

In general, gateways between the Internet and other mail systems SHOULD
attempt to preserve any layering semantics across the boundaries between
the two mail systems involved.  Gateway- translation approaches that
attempt to take shortcuts by mapping, (such as envelope information from
one system to the message headers or body of another) have generally proven
to be inadequate in important ways.  Systems translating between
environments that do not support both envelopes and headers and Internet
mail must be written with the understanding that some information loss is
almost inevitable.

F. Deprecated Features of RFC 821

A few features of RFC 821 have proven to be problematic and SHOULD NOT be
used in Internet mail.

F.1 TURN

This command, described in RFC 821, raises important security issues since,
in the absence of strong authentication of the host requesting that the
client and server switch roles, it can easily be used to divert mail from
its correct destination.  Its use is deprecated; SMTP systems SHOULD NOT
use it unless the server can authenticate the client.

F.2 Source Routing

RFC 821 utilized the concept of explicit source routing to get mail from
one host to another via a series of relays.  The requirement to utilize
source routes in regular mail traffic was eliminated by the introduction of
the domain name system "MX" record and the last significant justification
for them was eliminated by the introduction, in RFC 1123, of a clear
requirement that addresses following an "@" must all be fully-qualified
domain names. Consequently, the only remaining justifications for the use
of source routes are support for very old SMTP clients or MUAs and in mail
system debugging.  They can, however, still be useful in the latter
circumstance and for routing mail around serious, but temporary, problems
such as problems with the relevant DNS records.

SMTP servers MUST continue to accept source route syntax as specified in
the main body of this document and in RFC 1123.  They MAY, if necessary,
ignore the routes and utilize only the target domain in the address.  If
they do utilize the source route, the message MUST be sent to the first
domain shown in the address.  In particular, a server MUST NOT guess at
shortcuts within the source route.

Clients SHOULD NOT utilize explicit source routing except under unusual
circumstances, such as debugging or potentially relaying around firewall or
mail system configuration errors.

F.3 HELO

As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to HELO
when the server will accept the former.  Servers must continue to accept
and process HELO in order to support older clients.

F.4 #-literals

RFC 821 provided for specifying an Internet address as a decimal integer
host number prefixed by a pound sign, "#".  In practice, that form has been
obsolete since the introduction of TCP/IP.  It is deprecated and MUST NOT
be used.

F.5 Dates and Years

When dates are inserted into messages by SMTP clients or servers (e.g., in
trace fields), four-digit years MUST BE used.  Two-digit years are
deprecated; three-digit years were never permitted in the Internet mail
system.

F.6 Sending versus Mailing

In addition to specifying a mechanism for delivering messages to user's
mailboxes, RFC 821 provided additional, optional, commands to deliver
messages directly to the user's terminal screen.  These commands (SEND,
SAML, SOML) were rarely implemented, and changes in workstation technology
and the introduction of other protocols may have rendered them obsolete
even where they are implemented.

Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers MAY
implement them.  If they are implemented by servers, the implementation
model specified in RFC 821 MUST be used and the command names MUST be
published in the response to the EHLO command.

X. Change Summary and Loose Ends (Temporary)

X.1 Change summary

X.1.1 Substantive changes between draft-ietf-drums-smtpupd-00.txt and
draft-ietf-drums-smtpupd-01.txt

(i) Slightly clarified the discussions of rejection and failure of VRFY
requests and the associated response codes.

(ii) Slightly clarified the discussion of deferred address validation.

(iii) Removed the IPCE terminology and modified the text in section 4.1.1.2
to explicitly introduce the "mail gateway" terminology and to begin to
distinguish a mail gateway from a conventional relay.

(iv) Explicitly noted that SMTP clients for things like POP and IMAP may
send everything to a single relay for further processing, rather than
resolving final domain names.

(v) Tightened the RSET discussion.

(vi) Deprecation of 251 only for RCPT (still ok for VRFY)

X.1.2.  Substantive changes between draft-ietf-drums-smtpupd-01.txt and
draft-ietf-drums-smtpupd-02.txt.

Incorporated additional RFC 1123 material; reorganized several sections for
clarity.  Added definitions and other previous "loose end" material.

X.1.3.  Substantive changes between draft-ietf-drums-smtpupd-02.txt and
draft-ietf-drums-smtpupd-03.txt.

(i) Eliminated a number of placeholders and tightened some of the
definitions in section 2.  Added a few new placeholders for consistency
checking against other documents.

(ii) Removed the state diagrams, per direction at IETF Montreal.

(iii) Added new section 6.3, an attempt to summarize WG discussions on the
"posting" versus "delivery" versus "relay" functions of SMTP and on whether
"fixups" are appropriate in different cases.

(iv) Inserted section 6.1, a minor rewrite of section 5.3.3 of RFC1123.

(v) Added new text to 3.5.5 to discuss the spammer - EXPN relationship.

(vi) The "ASCII requirement" in 4.1.1.4 has been tightened somewhat.

(v) The remaining miscellaneous changes agreed to in Montreal have been
incorporated except as noted below.

X.1.4.  Substantive changes between draft-ietf-drums-smtpupd-03.txt and
draft-ietf-drums-smtpupd-04.txt.

Many small changes have been made between these two versions; the list that
follows is not exhaustive.

(i) To clarify some of the text, definitions have been introduced to
distinguish among originating, delivery, relay, and gateway SMTP systems.

(ii) The role of LF-terminated lines has been clarified.

(iii) Several changes have been made to clarify the principle that, no
matter what originating and final delivery systems might do, relay systems
are not permitted to tamper with message content, even to "fix" headers
that are determined to be invalid.  If they deem message content to be
seriously unacceptable, they are encouraged to reject the messages in
preference to trying to fix them up, but, in general, the theme is "don't
look/ don't tell".

(iv) A few more definitions have been added to the terminology section, and
the separate glossary has been eliminated.

(v) I have taken a shot at text to address some of the controversies that
have raged on the WG mailing list (e.g., sections 7.4 and 7.5). Since there
was no consensus on most of those topics, I expect that the inserted text
will satisfy no one except, perhaps, for agreement that saying nothing
would have been worse.  As a mechanism for moving forward, the text in
these controversial areas that now appears will be considered "base";
alterations will be made only if clear consensus emerges.

(vi) Per discussion in Los Angeles, source routes have been further
deprecated.

(vii) Some of the VRFY/EXPN materials have been moved to "security
considerations", where they appear to belong, some text has been added, and
the conformance statements adjusted to reflect what I perceive to be WG
consensus.

(viii) New MX resolution material has been added to section 5.  While most
of this material is from RFC974, the rules have been further tightened to
reflect current practice and experience (974 is written in a somewhat
speculative fashion for a standard).  In particular, the behavior of trying
the target host's A RR when MXs existed but all of them were eliminated is
now prohibited, which seems necessary if another of other ideas being
recommended or considered are to be feasible.

X.1.5.  Substantive changes between draft-ietf-drums-smtpupd-04.txt and
draft-ietf-drums-smtpupd-05.txt.

(i) All normative references to RFC 1123 have been removed from the main
body of the text (some still appear in the appendices where they will
remain).

(ii) Section 3.5 has been renamed slightly to distinguish between
"debugging of SMTP implementations" and "debugging of addresses". Better
terminology would be welcome.

(iii) Error conditions resulting from the DATA command have been clarified.

(iv) Section 4.2 (SMTP replies) has been revised and tightened to reflect
reality and recent discussion on the list.

(v) Appendix E has been revised a bit and moved into section 4.2.1. Given
the importance of the "check only first digit" rule, it has to be there.

(vi) Added new text for "no SMTP service supported" to sections 3.1, 4.2.2,
4.2.3, and 4.3.2.  As noted in 3.1, I'd rather add 521 (which would work
perfectly with the model) rather than overloading 554.

(vii) The Return-path language in section 4.4 has been cleaned up a bit.

(viii) Tightened the "postmaster" language in 4.5.1, requiring a small
change to 4.1.1.3.

(ix) I have unilaterally (with a little help from my friends), increased
some of the size limits.  64 was much too short for a domain name, and the
DNS limit of 255 (?) has now been inserted. That leaves the return path
much too short, but I haven't fixed it (maybe that will cause us to get rid
of them).  We still have a 64 character limit on the local-part, which is
also *much* too short. Votes for 128 or longer limits accepted.  See
X.1.6(I)

(x) The text on the "recipients buffer" has been rewritten so that (I hope)
it makes sense and gives some explicit guidance for how clients and servers
should proceed if limits are imposed.

X.1.6.  Substantive changes between draft-ietf-drums-smtpupd-05.txt and
draft-ietf-drums-smtpupd-06.txt.

Most of the changes in this revision have been editorial rather than
substantive.  Major substantive changes include:

(i)  The language about maximum sizes of SMTP command lines has been
reworked, per WG mailing list discussion.

(ii)  Several instances of  "Should" have been promoted to "Must" when the
reasons for the weaker rule seemed to have disappeared.  In particular, the
requirement that an SMTP implementation support timeouts has become a MUST.
Also, conformance to this specification requires support of EHLO.  Older
systems should claim conformance to the [to-be-historical] 821, not this
specification.

X.1.7.  Substantive changes between draft-ietf-drums-smtpupd-06.txt and
draft-ietf-drums-smtpupd-07.txt.

(i) Removed "implied RSET" text associated with QUIT, as specified at the
December 1997 IETF

(ii) Required that servers support EHLO, as specified at the December 1997
IETF

X.1.8. Substantive changes between draft-ietf-drums-smtpupd-07.txt and
draft-ietf-drums-smtpupd-08.txt.

This version involves mostly editorial work and cleanup of loose ends.

(i) New 7.5 added (old one renumbered) to discuss info disclosure through
Received fields.

(ii) Some character set and minor syntax issues clarified.

(iii) Material on code 571 added (thought this had been done long ago;
slipped through the cracks)

(iv) Many clarifications added as the result of list discussions and
suggestions.

(v) Error code presentation has been restructured.

(vi) ABNF conversion done

(vii) IPv6 address format inserted per RFC 1884, since we could not get
clear agreement on an alternative.

(viii) Trivial, silly, examples removed.  Others not yet renumbered.

(ix) 3.5.2 and 4.1.1 altered slightly per Eric Allman's notes.  Eric may
not like the way I've done either of these change very much: the first now
makes the distinction between returning an address and returning other
stuff (which was permitted by -06, but the text wasn't as clear as it
should have been): if it looks like an address, it needs to be an address.
Similarly, with 4.1.1, Eric wanted to explicitly permit/legitimize "DATA
<SP> <CRLF>".  I see several disadvantages to doing that, so have inserted
language that encourages receivers to tolerate trailing white space, which
may have the same practical effect.

X.1.9. Substantive changes between draft-ietf-drums-smtpupd-08.txt and
draft-ietf-drums-smtpupd-09.txt.

The first ten of these reflect, in order, minuted items from the Chicago
IETF (IETF 42).

(i) Clarification of "MUST", etc., in the context of this document
(section 2.3).

(ii) Altered VRFY text to make implementation a SHOULD (section 3.5.1) and
removed VRFY from the mandatory to implement list (section 4.5.1), per
42nd IETF (Chicago).

(iii) Clarified that exploders are expected to not purge sender addresses
from lists (section 3.10).  Note that the Chicago conclusion was that this
should be a "MUST".  I could not figure out how to do that without
absolutely prohibiting removing addresses to prevent loops, to guard
against spammers, or for similar legitimate purposes.  So I have written
this as a "SHOULD", with additional "strongly discouraged" words.   If
someone still wants a MUST, suggest text.

(iv) Altered text to permit clients that sometimes, or even always,
initiate sessions with HELO, rather than EHLO, to be fully-conforming
(section 3.2).  [[ Editor's note: I continue to believe that a client that
does not have any service extension support, even to the extent of being
able to send EHLO and parse the response without doing anything about it,
should not be considered fully-conforming to this spec (as distinct from
821).   Consequently, the new text in 3.2 stops well short of encouraging
clients that don't need service extensions from preferentially using HELO,
and the text in 2.2.1 (which specifies that the extension mechanisms must
be supported) has not been changed.

(v) Per Chicago discussions, the text requiring that QUIT be sent has not
been changed.  The text in 4.1.1.10 requiring that the server wait for
QUIT has been changed to a SHOULD.  However, the text in 4.1.1.5,
prohibiting close on receipt of RSET and that elsewhere prohibiting close
as a normal response, has not been changed.

(vi) Text has been inserted in 4.1.1 and the text in 4.3.2 altered
slightly to clarify the handling of parameters to RSET, DATA, and QUIT and
to 4.1.1.9 specify semantics for parameters to NOOP.  I have followed the
minutes on this although I personally agree with kre's mailing list
comments that the "servers SHOULD reject" decision leads to silly states.
I recommend that the WG review this.

(vii) Per discussion in Chicago, no substantive change has been made to
the specification about underscore characters in domain names (section
4.1.2).  However, the text has been altered to more accurately reflect
discussion on the mailing list and the source of the requirement.

(viii) Per discussion in Chicago, no change has been made to the
preference for local time in Received headers.

(ix) Per discussion in Chicago, code 571 has been removed and policy
rejection is now reflected i a 550 code (section 3.7 and the response code
lists).

(x)  Per discussion in Chicago, no change has been made to the
specification of use of raw CR or LF.

(xi) In section 4.3, the text has been changed, per comments from Dan
Bernstein and others, to require that clients be able to handle replies
that do not contain text strings. A few other places patched to match.

(xii) In sections 4.1.1.1 and 8, the placeholders have been removed.

(xiii) Per discussion on the mailing list (and specifically James
Berriman's concerns), the text has been clarified (sections 4.1.1.2 and
4.1.4) to prohibit MAIL unless no mail transaction is open.  This is a
MUST NOT prohibition -- SHOULD NOT makes no sense if this is the direction
we are going to go.  503 has also been added to the list of valid
responses for "MAIL" in 4.3.1 - it can't be issued before EHLO/HELO in any
event.  While it is clear that something should be said, this may not be
the desired outcome (I selected it because it was conservative and easy
given the text that was there already); the WG should check that the text
is as intended.

(xiv) Per discussion on the mailing list, a new section 4.5.5 has been
added to describe null return paths and their handling (forward pointer
from 3.7).  The text in 4.5.5 is substantially that suggested by Norbert
Bollow.   As with (xiii), there is now clear text, but it may not be what
the WG desires.  Please check.

[9b] Text lost in upload process restored.

(xv) "all addresses" substituted for "each...in turn" in 3.10.2.

(xvi) Requirement for "<" and ">" around paths clarified in section 3.3
(syntax productions were clear and correct, but not this overview
material).

X.2 Loose ends

<< All issues in this section are still open.>>

(i) The 821 BNF -> ABNF transition needs careful checking.

(ii) Trace field discussion should be checked carefully, particularly since
the syntax given is slightly different from 821. Provision has been made,
reflecting some current practice, for (optional) fractional seconds in time
stamps.

(iii) Remaining syntax differences and redundancies with respect to 822bis
(see note on section 4.1.2).  Note that the use of "Atom" for extension
parameter keywords is slightly different from the specification in RFC
1869.

(vi) We don't have a satisfactory definition for "link" as in "Received:..
via <link>" and, if there is a registry, I can't find it.  Needs to be
fixed somehow.

Z.  Full Copyright Statement

Copyright (C) The Internet Society (1998).  All Rights Reserved.

This document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist in
its implementation may be prepared, copied, published and distributed, in
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copyright notice and this paragraph are included on all such copies and
derivative works.  However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the Internet
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