Subject: Re: Editing changes/status 821bis, draft 10

From: Graham Klyne <[email-masked]>
Date: Tue, 02 Mar 1999 12:36:40 +0000

At 16:22 02/03/99 +1100, Robert Elz wrote:

>For SMTP, MX should be mandatory.   For cients talking to their ISP's
>mail relay, MX should be avoided in general (and so should calling what
they're
>using SMTP).

I'm puzzled by that last sentence.  I'd have thought that even for locating
an outgoing relay, MX could have advantages for an ISP.  If they advertise
the outgoing relay as (say):

    relay.isp.com

that could be MX'ed to whatever particular host is providing relay service
at any particular time.  Seems to me that could have a number of advantages:
  - can switch to another machine in case of failure of service upgrade
  - can advertise multiple outgoing relays
  - etc., etc.

#g

------------
Graham Klyne
([email-masked])

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From: Robert Elz <[email-masked]>
Date: Wed, 03 Mar 1999 00:22:24 +1100

Date:        Tue, 02 Mar 1999 12:36:40 +0000
    From:        Graham Klyne <[email-masked]>
    Message-ID:  <[email-masked]>

  | that could be MX'ed to whatever particular host is providing relay service

An A record (or set of A records, possibly which represent different servers)
can be changed just as easily as an MX record.   Personally I prefer to
connect to such servers via their A record (explicitly) and give them a MX
record to a 521 type server so anyone who just sees the name somewhere and
decides they may as well try sending mail to it will have it fail miserably.

kre

---

From: Philip Hazel <[email-masked]>
Date: Tue, 2 Mar 1999 13:36:47 +0000 (BST)

>   - can advertise multiple outgoing relays

All that can be done just as well by using multiple A records for
relay.isp.com. For example

relay1.isp.com.        A     1.2.3.4
relay2.isp.com.        A     1.2.3.5
4.3.2.1.in-addr.arpa.  PTR   relay1.isp.com.
5.3.2.1.in-addr.arps.  PTR   relay2.isp.com.
relay.isp.com          A     1.2.3.4
                       A     1.2.3.5

IMHO, the names of MX records should be mail domains, that is, domains 
that people are expected to send mail to, whereas the names of A records 
are hosts. In this case, the client is trying to find specific hosts. 
Therefore A records are the way to go.

-- 
Philip Hazel            University of Cambridge Computing Service,
[email-masked]      Cambridge, England. Phone: +44 1223 334714.