Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!helios.ee.lbl.gov!ucsd!brian
From: brian@ucsd.EDU (Brian Kantor)
Newsgroups: comp.mail.misc
Subject: Re: Explanation on why reply may not work needed
Message-ID: <1697@ucsd.EDU>
Date: 8 May 89 21:40:17 GMT
References: <1017@marvin.Solbourne.COM>
Reply-To: brian@ucsd.edu (Brian Kantor)
Organization: The Avant-Garde of the Now, Ltd.
Lines: 47

The question of why a From: and a From_ line might differ on received
mail comes up about annually; I guess it's about time again.

In a pure RFC822 compliant world, it would never be necessary to update
the From: line when mail passed through your site.  If the from address
reaching you ALWAYS looks like   user@host.dom.ain  then you are living
in  the right world and don't have to worry about this.  This is because
such addresses can be considered to be "absolute" - they are complete in
themselves and do not need any references to be used.

UUCP bangist addresses, on the other hand, are NOT "absolute", they are
relative.  A uucp "address" of the form  sitea!user  is not
a complete address, but rather is a path, either complete or incomplete.
When such a path is found on a from line in a message entering your
machine, it is valid only insofar as it is relative to your machine,
which means that you must update it if you relay the message onward.
If you pass the message on via uucp, you MUST prepend your uucp
sitename! to the front of the path.  If you relay it onto an RFC822-
compliant network, you must indicate that relative addressing from your
site by appending the '@host.dom.ain' of your relay machine to the
address.

Thus the answer that you received from Pyramid that "one must never
alter a From: line" is incorrect and reflects an incomplete
understanding of the current multi-network mail world.  You do NOT
update a From: line if it is a valid RFC822-style address, but if
it is a bangist-only line, you MUST prepend   yourhostname!  to
the front of it.  Note that a!b!c!u@e.f.g   IS a valid RFC822-style
address, and you would correctly LEAVE IT ALONE if you received it
in a from line, since it is already an "absolute" address.

Summary:
	u@host.dom.ain				- don't touch
	a!b!u@host.dom.ain			- don't touch
	a!b!u					- prepend us!
			- or -			- append @us.dom.ain

BTW, some sites currently passing mail are using the original
mongo-stupid Unix mailer that doesn't even know what a From: line IS,
much less how to update it.  Mail passing through these sites is
fated to be terminally mangled and probably can't be replied to.  All
you can really do about this is warn your users and avoid those sites.

	Brian Kantor	UCSD Postmaster
		UCSD Office of Academic Computing	(619) 534-6865
		UCSD C-010, La Jolla, CA 92093 USA	fax: 619 534 7018
		brian@ucsd.edu	BRIAN@UCSD ucsd!brian