[Mailman-Users] DMARC and Gmail

Alain Williams addw at phcomp.co.uk
Wed Apr 16 18:55:12 CEST 2014

On Thu, Apr 17, 2014 at 01:27:23AM +0900, Stephen J. Turnbull wrote:

> There are several possibilities.  One is that DMARC doesn't define the
> semantics of "reject".  (Why doesn't that surprise me?)  Here's what
> they say:
>    15.4.  Rejecting Messages
>    This proposal calls for rejection of a message during the SMTP
>    session under certain circumstances.  This is typically done in one
>    of two ways:
>    o  Full rejection, wherein the SMTP server issues a 5xy reply code
>       as an indication to the SMTP client that the transaction failed;
>       the SMTP client is then responsible for generating notification
>       that delivery failed (see Section 4.2.5 of [SMTP]).
>    o  A "silent discard", wherein the SMTP server returns a 2xy reply
>       code implying to the client that delivery (or, at least, relay)
>       was successfully completed, but then simply discarding the
>       message with no further action.
>    Each of these has a cost.  For instance, a silent discard may
>    prevent "backscatter" (the annoying generation of delivery failure
>    reports, which go back to the RFC5321.MailFrom address, about
>    messages that were fraudulently generated), but effectively means
>    the SMTP server has to be programmed to give a false result, which
>    can confound external debugging efforts.

They should have allowed/defined a new 2xy code that could be returned, eg 253
which means ''Mail accepted but will be discarded''. So a simple sending MTA could
just look at the initial '2' and think 'job done', a more complex one could note
that the receipt wasn't quite right.

However: it still means that some people on mail lists occasionally don't get
stuff - this will cause confusion at best or could be dangerous (if the mail
list has a critical function).

Alain Williams
Linux/GNU Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256  http://www.phcomp.co.uk/
Parliament Hill Computers Ltd. Registration Information: http://www.phcomp.co.uk/contact.php
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