Confusion between Reply To and Reply List in email responses

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 Mail Headers (GNU Emacs Manual) :

‘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

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