The line in the default replcomps, which goes like this:
/usr/lib/mh/replcomps:%(lit)%(formataddr
%<{reply-to}%?{from}%?{sender}%?{return-path}%>)\
should have "%?{sender}" removed per RFC 822:
o The "Sender" field mailbox should NEVER be used
automatically, in a recipient's reply message.
unless a nearby paragraph is very loosely interpreted:
This recommendation is intended only for automated use of
originator-fields and is not intended to suggest that replies
may not also be sent to other recipients of messages. It is
up to the respective mail-handling programs to decide what
additional facilities will be provided.
The uppercase NEVER is rather emphatic, however.
Notice that the sender field is only used if both the reply-to and from
field are blank. What's a mailer agent supposed to do? I think falling
back on the from field is completely reasonable, as it's a next-best
option sort of thing.
Should it skip sender and go directly to return-path? I think not,
because the return-path not only is supposed to go back to the
_sender_ (and thus we've gained nothing), but it is a 'trace' field:
4.3. TRACE FIELDS
Trace information is used to provide an audit trail of mes-
sage handling. In addition, it indicates a route back to the
sender of the message.
...
4.3.1. RETURN-PATH
This field is added by the final transport system that
delivers the message to its recipient. The field is intended
to contain definitive information about the address and route
back to the message's originator.
Note: The "Reply-To" field is added by the originator and
serves to direct replies, whereas the "Return-Path"
field is used to identify a path back to the origina-
tor.
...
It thus is intended for auditing, not for use by the user. That would
seem to doubly disqualify return-path. If the reply-to, from and sender
fields were all missing, this would be my last straw. (If it's missing
as well, you need to not only shoot the person who sent it (if you can
determine that), but also shoot your local MTA, as it should have
created the reply-to header (probably from the envelope from).)
Philip Guenther
guenther(_at_)stolaf(_dot_)edu