Was also struggling with this behavior and found out, that they might actually have put some thinking into it, when implementing it in Thunderbird: Thunderbird:Help Documentation:Mail-Followup-To and Mail-Reply-To - MozillaWiki
- 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.
The difference I found explained at https://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Headers.html :
‘Mail-Reply-To’: This field takes precedence over ‘Reply-To’. It is used because some mailing lists set the ‘Reply-To’ field for their own purposes (a somewhat controversial practice).
Now I tried injecting this header via “email custom headers”, but unfortunately reply_key does not get expanded properly. Am I doing something wrong?
Mail-Reply-To: sender-name+%{reply_key}@domain.net