root(_at_)online(_dot_)pcd(_dot_)usr(_dot_)com writes:
Usually messages come in with the same email address in the
From
line as in the
Return-Path
line. This is usually the case, but there are times when these lines
have different email addresses. For example I saw a message that had this
as the from line:
From: mac(_at_)pilotmail(_dot_)net
Bzzt. That's a "From:" line, not a "From " line. The former is a
standard rfc822 header. The later is the semi-standard mailbox message
delimiter used on many (but not all) UNIX systems. Many MUAs hide the
"From " line from the user, so you may need to examine your mailbox in
some other fashion (using a pager like "less" or maybe an editor (don't
save any changes!)) in order to see it.
However when I looked at the full header I saw this in addition to the
aforementioned from line:
Return-Path: root(_at_)bilbo(_dot_)satelnet(_dot_)org
This is rather common with mailing lists. The Return-Path: header and
"From " pseudo-header both contain the 'envelope' sender address -- the
address of the entity that sent that very copy of the message -- while
the From: header contains the header sender address -- the address of
the entity which originated that message text. When a mailing list
forwards a message, it leaves the From: header alone (or it *should*),
while it changes the envelope sender because it is sourcing a new copy
of the message.
What's the differance in usage between the envelope and header
address? Error messages are sent to the envelope sender, because that
where this copy came from. The header sender may have nothing to do
with it being forwarded to someone else, and therefore can often do
nothing about it. In the mailing list context, this lets errors
messages caused by bad addresses on the list go to the list software
instead of the person who sent the original message. Meanwhile, the
header address is who actually wrote the message, and (in the absence
of a Reply-To: header) where replies should be sent.
Philip Guenther