[Mailman-Users] Delivered-For: header.. msg_header options

Note that the $u macro will be defined if and only if there is
one and only one recipient for the current message. This will not be
the case for mailing list mail being sent by the server, unless you
have turned on list customization. In that case, mailman will send
out one copy of the message for each and every recipient, and will
put their e-mail address in the footer.

Also, in the case of a single recipient per message, most MTAs
are already configured to put this kind of information into the
headers, usually the "Received:" headers.