Controls whether or not the Mail-Followup-To header field is generated when sending mail. When set, Mutt will generate this field when you are replying to a known mailing list, specified with the ``subscribe'' or `` lists'' commands.
This field has two purposes. First, preventing you from receiving duplicate copies of replies to messages which you send to mailing lists, and second, ensuring that you do get a reply separately for any messages sent to known lists to which you are not subscribed. The header will contain only the list's address for subscribed lists, and both the list address and your own email address for unsubscribed lists. Without this header, a group reply to your message sent to a subscribed list will be sent to both the list and your address, resulting in two copies of the same email for you.