Keith Moore writes:
There's a conflict between using a field like this to suppress
duplicate copies and using the same field to indicate whether the
author thinks replies should go to somewhere besides "everyone who
received this message"
Mail-Followup-To specifies an address list for followups, overriding the
usual (Reply-To||From)+To+Cc.
There are many reasons for a sender to set Mail-Followup-To. Maybe the
sender wants to eliminate duplicates. Maybe the sender wants to redirect
discussion somewhere else. Maybe the sender has some other purpose. In
every case, Mail-Followup-To communicates the bottom line: an address
list for followups.
Where did you get the idea that Mail-Followup-To communicates anything
other than this? It doesn't. It says nothing about the sender's reasons.
It simply specifies an address list for followups. End of story.
If you do want to communicate something else, use a new header field.
---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago