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Re: [taugh.com-standards] Re: Gen-ART and OPS-Dir review of draft-ietf-appsawg-nullmx-06

wietse@porcupine.org (Wietse Venema) Thu, 04 September 2014 15:34 UTC

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Subject: Re: [taugh.com-standards] Re: Gen-ART and OPS-Dir review of draft-ietf-appsawg-nullmx-06
In-Reply-To: <alpine.BSF.2.11.1409021427000.24698@joyce.lan>
To: John R Levine <johnl@taugh.com>
Date: Thu, 04 Sep 2014 11:34:45 -0400
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From: wietse@porcupine.org
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John R Levine:
> No, 550 to reject the MAIL FROM is correct.  See RFC 5321, sec 3.3.  It's 

My concern is with sending 521 after RCPT TO.  Sorry for coming
with this late in the process. I wasn't aware of the nullmx revival.

>From my point of view, it is desirable that 4XY and 5XY reply codes
have consistent semantics, i.e. they differ only in whether or not
the sender is supposed to retry a rejected request.

Until now the Postfix community has relied on this property as part
of a temporary safety net (to avoid missing email, replace 5XY with
4XY; after watching the logs for some time, remove the safety net).

Generally, if 4XY and 5XY reply codes have different semantics
besides do/don't retry, then I expect that will increase the
likelihood of mistakes by MTA operators.

Specifically, if 421 (and 221) terminate a session but 521 does
not, then that complicates life for implementors and MTA operators.

	Wietse

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