Consider setting Mail-Followup-To header when Reply-To is set to original sender

When REPLY-TO is set to the original sender, users have complained that when they hit "Reply-All" (Reply-To-All) in their clients, they expect to send a mail to the original sender AND the list (FROM(list)+REPLY-TO(original sender)+TO(recipient)+CC). Instead, the TO header is set to REPLY-TO(original sender)+TO(recipient)+CC and thus not including the list.

I investigated this and it seems that most mail clients follow this procedure for "Reply All":

if MAIL-FOLLOWUP-TO in received mail then use MAIL-FOLLOWUP-TO as TO
else if MAIL-REPLY-TO in received mail then use MAIL-REPLY-TO + TO + CC as TO
else if REPLY-TO in received mail then use  REPLY-TO + TO + CC as TO # this
else use FROM + TO + CC as TO

This means, when schleuder is configured to set REPLY-TO to the original sender, the clients will set TO to REPLY-TO (original sender) + TO (recipient) + CC, which does NOT include the mailing list.

Thus I would suggest to add a MAIL-FOLLOWUP-TO header when REPLY-TO is set to the original sender. The MAIL-FOLLOWUP-TO header then includes:

• list
• original sender
• CC

If you don't see any problems, I would implement this and open a MR. Please let me know :)