-----Original Message-----
From: ietf-bounces(_at_)ietf(_dot_)org
[mailto:ietf-bounces(_at_)ietf(_dot_)org] On Behalf Of Barry Leiba
Sent: Friday, March 02, 2012 6:27 AM
To: ietf(_at_)ietf(_dot_)org
Subject: Re: Last Call: <draft-melnikov-smtp-priority-07.txt> (Simple
Mail Transfer Protocol extension for Message Transfer Priorities) to
Proposed Standard
Does anyone's answer change if I point out that we already have at
least two protocols that have an MTA alter message header fields in
between message receipt and message relay?
An MTA that supports DKIM [RFC6376] might, under some conditions,
remove an existing signature and add its own.
That's a good example.
An MTA that implements the Authentication-Results header field
[RFC5451] might remove an existing header and add its own.
Unfortunately I don't think this one is, because the header changes it
encourages take place strictly within an ADMD (specifically, the receiving
one), not as a mechanism for tunneling transport metadata through
non-participating environments.
Arguments can be made that the former is more of a gateway function
than a relay function (and Ned has already pointed out that we're
pretty sanguine about this sort of manipulation in gateways), and that
the latter is behaving more or less as a trace field, which we also
allow some flexibility with.
RFC6376 also asks for trace header field treatment for DKIM-Signature.
That said, this does show that we've dabbled in this area of mushy boundaries,
and the results haven't exactly been disastrous.
-MSK
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