Keith Moore <moore(_at_)cs(_dot_)utk(_dot_)edu> writes:
Ideally duplicate suppression would be implemented in the recipient's
MUA (NOT the MUA composing replies) or in the recipient's message store.
One reason I believe this is that there are several other sources of
duplicate messages than mailing lists. Another reason I believe this is
that I think MUAs should be able to chase down threads that cross folder
boundaries (for instance between INBOX, Sent, and a list folder) - and
if they do that, they're within epsilon of being able to suppress
duplicates anyway.
Suppressing duplicates in the MUA is extremely difficult to do properly,
to the degree that I would contend it's impossible to do with the current
e-mail protocol in a way that would make sense to the casual user. It's
necessary to suppress the personal copy, since the list copy is the one
that will sort correctly using the List-Id header and may have additional
information or modifications made in the course of processing by the
mailing list management software. However, the personal copy almost
always arrives first, so doing this right requires temporary quarantine
and possible later release of mail, something that adds a tremendous
amount of complexity to mail handling.
--
Russ Allbery (rra(_at_)stanford(_dot_)edu)
<http://www.eyrie.org/~eagle/>