Subject: Re: [ietf-smtp] DSNs

From: Viktor Dukhovni <[email-masked]>
Date: Tue, 5 May 2020 12:48:40 -0400

On Sun, Apr 26, 2020 at 05:02:04PM -0400, Sam Varshavchik wrote:

> Incoming mail is addressed to a valid mailbox. Your MTA accepts it. As such,  
> the sender already knows it is a valid recipient.

When I set edge systems in environments with multiple internal mail
stores, with external mail arriving to [email-masked] for internal
redistribution to wherever that user's email is actually delivered, the
envelope recipient address is rewritten to help the mail along to its
finan destination, but this is not "forwarding", it is still "relay".

The internal mailstore topology is none of the sender's concern, but
also I do not wish to be a source of backscatter if the envelope sender
address is fake.

> I don't follow what information is getting leaked, if a second later a  
> success DSN gets sent to the sender. The sender already knows it's a valid  
> mailbox, by the virtue of the fact that the mail was accepted.

The success DSN would originate deeper inside the ADMD, from whence I
prefer to neither send such DSNs, nor promise their delivery.  Instead
the sender's ADMD sends the SUCCESS notice when I accept the message
without advertising DSN support.

-- 
    Viktor.

---

From: Sam Varshavchik <[email-masked]>
Date: Tue, 05 May 2020 20:04:48 -0400

> also I do not wish to be a source of backscatter if the envelope sender

Suppressing DSNs won't entirely prevent that. For whatever reason the  
internal mail server may end up bouncing it. The edge server, having  
accepted it and rewritten the recipient address has no guarantees that the  
rewritten recipient's address is deliverable. So it can still bounce and can  
generate backscatter.