Thunderbird:Help Documentation:Mail-Followup-To and Mail-Reply-To

Mail-Followup-To (MFT) and Mail-Reply-To (MRT) are email header fields that tells a mail user agent (MUA) like Thunderbird how to reply to messages which is especially helpful using mailing lists.

If you already know how MFT and MRT work, you may just skip to Configure Thunderbird

How MUAs reply to email messages

MUAs have (at least) two reply functions: reply-to-author and reply-to-all. In Thunderbird they are just called "Reply" and "Reply all".

If you reply-to-author, your MUA creates a new message and sets the To header to the original message's From header. If the original message contains a Reply-To header, your MUA uses its content instead of From. This way the original message's author can tell you to reply to another address if he wants to get replies there.

If you reply-to-all, your MUA creates a new message and sets the To header like above. Additionally it sets the new message's Cc header to the list of all recipients (To+Cc) of the original message, probably excluding your own address.

All these things work pretty well so far, but problems arise with mailing lists:

Reply problems with mailing lists

If you get a mail via a mailing list and you reply-to-all, then the author will get two copies of your message if he's subscribed to the list. One directly because he's in his message's From header and one via the mailing list.

As a widespread workaround, some mailing lists set the Reply-To header of all messages to the mailing list. If users then use reply-to-author instead of reply-to-all, the author will never get duplicates. However, this has some disadvantages: It breaks the original meaning of reply-to-author and if the author is not subscribed, he will not receive replies to his messages.

The solution

This part is still missing, please read Dan Bernstein's Mail-Followup-To proposal in the meantime.

Configure Thunderbird

Fortunately, Thunderbird can do all these things automatically for you:

• If you reply-to-author and Mail-Reply-To is set in the original message, Thunderbird uses it instead of Reply-To or From. This works since Thunderbird-1.5.
• If you reply-to-all and Mail-Followup-To is set in the original message, Thunderbird uses it instead of (MRT or Reply-To or From)+To+Cc. This works since Thunderbird-1.5 with a minor bug fixed in Thunderbird-3.0-beta1.
• If you tell Thunderbird which mailing lists you are subscribed to, it will automatically set Mail-Followup-To when writing to one of these lists. This works since Thunderbird-3.0-beta1.
• If you tell Thunderbird which mailing lists set Reply-To, it will automatically set Mail-Reply-To when writing to one of these lists. This works since Thunderbird-3.0-beta1.

The latter two features must be configured via hidden preferences:

• Set mail.identity.default.subscribed_mailing_lists to be a comma separated list of mailing lists that you are subscribed to.
• Set mail.identity.default.replyto_mangling_mailing_lists to be a comma separated list of mailing lists that you write messages to and that set the Reply-To header. If this value's first character is an asterisk (*), Thunderbird will add MRT to all outgoing messages.
• To be able to set MFT and MRT manually in the composer, set mail.compose.other.header to Mail-Followup-To,Mail-Reply-To (comma separated list, no spaces!).
• To show MFT and MRT in the Normal Headers view, set mailnews.headers.extraExpandedHeaders to Mail-Followup-To Mail-Reply-To (space separated list).

See Editing Configuration Files for details about how to set these preferences.