With the Followup-To, it's almost
certainly going to have to be hand-edited by the sender:
http://pobox.com/~djb/replyto.html says how an MUA can automatically
create this field.
The only prerequisite is that your MUA know what mailing lists you're
subscribed to. (The fundamental problem is obviously insoluble without
this information.)
It seems to me that Dont-Reply-To would be a bit easier to automate.
It would require substantially more code for the common case. I haven't
seen any examples where the extra flexibility would be useful. KISS.
---Dan
Let your users manage their own mailing lists. http://pobox.com/~djb/qmail.html