IIRC, adding the header made some different mail software behave correctly. The history should be on Meta somewhere.
I’ve had a quick look. It seems that this is the genesis of the List-Post header: Emails are not threaded in Outlook 2013 - #5 by codinghorror
I’m not sure that it was fully thought through or that it was relevant to the problem being solved there (but again I’m still learning here myself).
Looks correct to me. What’s incorrect is mail software that ignores the
Reply-To address … it’d be like you introducing yourself to me as Jonathan, but no, I insist on calling you Edward instead.
It sounds like Thunderbird uses the List-Post header for “Reply to List”, and the From header for “Reply” (ignoring Reply-To, which here is the same as List-Post). This makes some sense on some potential settings of a mailing list (e.g. the Reply-To might have been set by the mailing list rather than the original sender) as it would allow “Reply to List” to go to the list and “Reply” to go to the original sender (where otherwise it would also go to the list).
The absence of List-Post should remove the “Reply to List” button (Thunderbird 3 for users - Mozilla | MDN).
I guess that without the “Reply to List” option, the “Reply” button would send email to the address in the Reply-To header as expected.
I’ve got two related questions:
- Why does Discourse use the List-Post header (as you only ever want an email to go to the Reply-To address)?
- Does Thunderbird respect Reply-To when List-Post is not set?