Nick Ing-Simmons wrote:
One of the USENET concepts which has translated quite well to mail is
the use of the References: header to thread mails.
The References field was defined in RFC 724 (possibly earlier) which
predated Usenet by years. It was not adopted by Usenet until after
RFC 724 had been superseded by RFCs 733 and 822.
If there was a clearly defined header that said "you got this mail from
the list <whatever(_at_)somewhere>"
You mean something like
Return-Path: <owner-ietf-822(_at_)mail(_dot_)imc(_dot_)org>
or
Sender: owner-ietf-822(_at_)mail(_dot_)imc(_dot_)org
it would be trivial to add a
"Reply to list" button to a MUA.
Not necessarily. Sometimes there are multiple list expansions.