set and honor mail-followup-to header

What do you think about this implementation of honoring and setting the Mail-Followup-To header as an extension of the group reply?

The rationale is:

When replying to a mailinglist post you either want to reply to the author only (normal reply) or to the list itself and everybody else (group reply). The patch allow to honor the list of recipients of any followups the original sender specified.

In any case, when one of the recipients is a mailinglist, set the header so that anybody who replies to that message will not reply to you directly.