Is it possible to configure Mailman to set the Reply-To header of a
message depending on whether the sender is subscribed to the mailing
list?

Rationale:

For an open mailing list set up to receive bug reports from users, I
would like to set

• Reply-To = sender, mailing-list if the sender is not a subscriber
• Reply-To = mailing-list if the sender is a subscriber

I've had a complaint from one of my collaborators that he couldn't
filter messages correctly when they were sent directly to him rather
than just to the list. That's okay, and I like to keep most traffic
on the list rather than hide it in order to publicly record the
conversation. At the same time, replying just to the list when I
can't be sure that the sender will get my reply is not the right
strategy, and since switching on the Reply-To setting, I've made that
mistake many times.

Hence the desire to have the above option.

Thanks for your consideration.

PS.: The true problem happens on the client side, where replies that
go to the list and the user aren't filtered correctly. I would
consider telling people to fix that rather than to set Reply-To if the
above can't be done.