Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.mail.misc
Subject: Re: Procedure for adding headers?
Message-ID: <1990Nov21.084347.25984@Think.COM>
Date: 21 Nov 90 08:43:47 GMT
References: <16502@csli.Stanford.EDU> <1990Nov21.003144.4755@Think.COM> <3235@exodus.Eng.Sun.COM>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 44

In article <3235@exodus.Eng.Sun.COM> argv@turnpike.Eng.Sun.COM (Dan Heller) writes:
>In article barmar@think.com (Barry Margolin) writes:
...
>RFC822 (I think) specifies the Precedence: header that can be set to
>things like "junk" to indicate that if the mail can't be delivered,
>drop it on the floor (don't bother sending a mailer daemon).  There
>are other values that are used to indicate high-volume mailing lists
>and things, but these are rarely used (in fact, all I've ever seen
>is "junk" :-).  Admittedly, I should know more about this <gulp>.

I can't find the Precedence field in RFC822.

>> Mail transfer
>> programs generally are not required to look at the header of the messages
>> they are transfering.
>That's rather sweeping, but I know what you mean.  MTAs -are- required
>to look at and even change certain headers under certain circumstances.

Those situations are primarily exceptional circumstances outside the scope
of the Internet mail standards.  For instance, when gatewaying between a
network that uses domain addresses and one that uses old-style UUCP
addresses (foo!bar!baz), the gateway must tranform addresses in the header;
in this case, the application-level gateway is effectively a pseudo-user
agent (consider the far-out extension to a gateway sitting in the middle of
the Chunnel that translates the message between English and French).  Or if
a site wants to transform addresses of the form User@host.think.com to
User@think.com (so that non-MX mailers can reply, and so that host names
may be changed without causing problems when trying to reply to mail from
those hosts) on the way out to the real network, it may add such a
transformation.  This is a convenience feature, not part of the protocol
specification.

Actually, SMTP MTAs are required to look at the first line of a message in
order to update the Return-Path line, and must insert a Received line after
it.  These are actually considered to be the SMTP envelope.  The
Return-Path line is required to be first so that the MTA doesn't have to
parse the entire header looking for it.  Other than this, the only other
mention of message format that I think RFC 821 (the SMTP specification)
makes is that the message must conform to RFC 822.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar