Re: [Asrg] reject and DSN, was What are the IPs

John Levine <johnl@taugh.com> Wed, 01 July 2009 12:45 UTC

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Date: 1 Jul 2009 12:30:54 -0000
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From: John Levine <johnl@taugh.com>
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Subject: Re: [Asrg] reject and DSN, was What are the IPs
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>Backscatter is a problem, but bounce messages do have advantages over 5xx 
>error codes when it comes to communicating with the sender. For example, 
>you can't know what the sending MTA is going to do with a 5xx error code - 
>they might just drop it. DSNs were invented for a reason, and it's a shame 
>to lose them entirely - even when you have reason to believe that the 
>return-path (or at least the return-path domain) isn't forged.

I don't think it's a very good idea to try to guess how other people's
MTAs might be broken.  It's true that some don't pass back 5xx codes
properly, but it's equally true that some mangle or discard DSNs.  The
reason that DSNs were invented is that SMTP is still store and
forward, and sometimes you can't tell during the SMTP session whether
a delivery will subsequently fail.  But if you can tell, which these
days is the vast majority of the time, 5xx is cheaper and less likely
to cause collateral damage.

R's,
John