Charles Lindsey writes:
OTOH, it would make good sense for the list-owner to fill it in it not
provided by the user (because the list-owner shouls know whether that
user is on the list).
Your assumption is wrong, your logic is wrong, and your conclusion is
wrong.
The list distributor does _not_ know whether the sender is on the list:
consider sublists, HTTP subscriptions, etc. Even if the distributor
_does_ know that the sender is on the list, the distributor shouldn't
add Mail-Followup-To; there are several issues here, the most obvious
being that some senders _do_ want copies of followups.
Didn't you learn anything from the Reply-To mess? If you want to add
information to a message in transit, stick to the envelope: Return-Path,
Received, List-*, etc. Don't touch the sender's header fields.
---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago