It is not clear how alias-forwarders are supposed to handle messages with
multiple recipients. When handling such a message, Exim and some other
MTAs will determine the final alias destination(s) of all addresses in the
recipient list before delivery starts, so that only one copy of the
message needs to be sent to each destination host.
Say for example a message is sent to two addresses at an alumni forwarding
service: fanf2(_at_)alumni(_dot_)cam(_dot_)ac(_dot_)uk,
abc123(_at_)alumni(_dot_)cam(_dot_)ac(_dot_)uk(_dot_) If both of these
addresses happen to forward to the same place, say dot(_at_)dotat(_dot_)at and
abc123(_at_)dotat(_dot_)at, then the alumni.cam.ac.uk system will want to send
one
copy of the message to the dotat.at system with multiple recipients. What
address should it use in the Resent-From: header? It can't use both
addresses, because then it would have to add a Resent-Sender: header and
we get back to the original question (and anyway the draft doesn't allow
for alias-forwarders to add a Resent-Sender: header).
Perhaps the forwarder should just use
mailer-daemon(_at_)alumni(_dot_)cam(_dot_)ac(_dot_)uk
as the Resent-From: address on all messages it handles?
The problem is more tricky of the forwarder is dealing with multiple
virtual domains. The central email relay of the University of Cambridge
(known as ppsw.cam.ac.uk) handles over 150 virtual mail domains. Which
domain should it use in the Resent-From: address when a message was
originally sent to addresses in two different domains which alias-forward
to the same domain? Since ppsw.cam.ac.uk is a valid mail domain used only
for operational purposes, perhaps it should just add Resent-From:
mailer-daemon(_at_)ppsw(_dot_)cam(_dot_)ac(_dot_)uk to all messages it handles?
Note that this would not do anything to protect the mail domains that are
actually used by our users!
Tony.
--
f.a.n.finch <dot(_at_)dotat(_dot_)at> http://dotat.at/
BISCAY: EAST OR NORTHEAST 5 OR 6, OCCASIONALLY 7 IN NORTH. THUNDERY SHOWERS.
MODERATE OR GOOD.