Re: What I've been wondering about the DMARC problem

ned+ietf@mauve.mrochek.com Sat, 19 April 2014 17:12 UTC

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From: ned+ietf@mauve.mrochek.com
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Date: Sat, 19 Apr 2014 08:31:52 -0700 (PDT)
Subject: Re: What I've been wondering about the DMARC problem
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References: <534ED376.8060303@bluepopcorn.net> <20140418013433.2763.qmail@joyce.lan> <CAL0qLwY4xVrPwABRhv90JSRF8wta0P5OCw_UWzVYOyUZk2-W4Q@mail.gmail.com>
To: "Murray S. Kucherawy" <superuser@gmail.com>
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Cc: Jim Fenton <fenton@bluepopcorn.net>, John Levine <johnl@taugh.com>, ietf <ietf@ietf.org>
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> On Thu, Apr 17, 2014 at 6:34 PM, John Levine <johnl@taugh.com> wrote:

> > >"If the RFC5322.From domain does not exist in the DNS, Mail Receivers
> > >SHOULD direct the receiving SMTP server to reject the message."
> >
> > As far as I can tell, that bit of poor advice hasn't been implemented.

> Why is that poor advice?  It's not uncommon for an MTA receiving mail to
> confirm that the message is replyable, at least insofar as an A and MX are
> available for whatever comes after the "@".

It's outrageously poor advice, for the simple reason that there's all kinds of
legitimate email that's sent for all kinds of different reasons that you don't
want people to be able to reply to. And the sooner they get a failure when they
try and reply, the better.

As such, the ability to reply to the RFC5322.From tells you almost nothing
about its legitimacy.

It's yet another case where a failure to consider the multiple semamtics
field like RFC5322.From has, and designing to a subset of those designs,
ends up screwing things up.

				Ned