wayward mail

[David Hogan dhog@cs.su.oz.au]

> From:	Scott Schwartz <schwartz@groucho.cse.psu.edu>
> 
> | not quite... upas uses the SMTP information, not the message contents,
> | as i think it should.
> 
> Well, ok, but the local postmasters tell me that the internet
> conventions are otherwise.

Indeed they are.  On the internet, all mail messages are supposed
to be formatted according to rfc-822, with the headers indicating
who the actual sender and recipient(s) are.  The SMTP information
(aka envelope) is part of the delivery mechanism, usually derived
from those headers, but not necessarily equivalent to them.  In
particular, when a mailing list expander receives some mail, say to
9fans@cse.psu.edu, it sends out an essentially identical piece of
mail (same rfc-822 From and To headers) but with a different SMTP
envelope: the envelope sender becomes an alias for the maintainer
of the list (9fans-request@cse.psu.edu), and the recipients are
the members of the list.  This is good, because bounces go back
to the list maintainer (who is most likely to be able to do
something about them) but replies go to the list and/or the original
sender, depending on the behaviour of the user agent (and the user
driving it).  The difference between the rfc-822 headers and the
envelope is also used to ensure that a bounce doesn't generate
another bounce, which could lead to loops, by having the mail software
use a null envelope sender when it sends the bounce message.

I understand that at Bell Labs the mail conventions are quite
different (and in fact much simpler).  The contents of a mail
message are unformatted (ie no rfc-822 or anything) and are
deposited in your mailbox with a single From line prepended,
which will be the SMTP envelope sender if the mail arrived via SMTP.
The mail user agent (upas/edmail) replies to the address in the From
line (it doesn't have much choice :-).

Unfortunately, these two sets of mail conventions aren't always
compatible, and in particular they break down when internet
mailing lists are involved.  I have to say that I prefer rfc-822,
since (amongst other things) it lets you know who the _other_
recipients of the message were (assuming the sender wanted you
to know this :-)