With respect to mailing lists, I'll partially disagree with Keith. For
most mailing lists, the list agent is best thought of as a user, in terms
of MTA behavior.
treating lists as either "users" or "MTA" inevitably leads to confusion.
it's more realistic to say that they are somewhere in between, acting
as users at some times and as MTAs in others.
My posting to the list terminates at the list agent, as
far as the transport service is concerned. As far as the user-level,
"human" model, of course the list agent is just an intermediary between the
author and all the list members. The members must receive mail that
appears to be from the author, not from the list.
The problem is that the reply behavior violates this, seeming instead to be
like forwarding: replies should go to the list.
replies should go to where the person sending the reply wants them to go.
this might be any subset of: the author of the subject message, the
reply recipients named by the author of the subject message, the recipients
named in the header of the subject message, or the list from which the
message was received.
At any rate it might have helped distinguish mailing lists postings from
individual postings.
there are better ways to do this, such as the List-* fields which are
unambiguously for use by lists.
Keith