Confusion between Reply To and Reply List in email responses

[zogstrip (Régis Hanol)]

@nbianca investigated and removed it in this commit. She replied in

Reply-key variable doesn't get expanded in email custom headers

Hi there, while trying to fix the problem that Thunderbird users cannot reply to Discourse notification emails I discovered this matter. Details at Confusion between Reply To and Reply List in email responses - #40 by Flominator Can we fix this? Is there maybe a patch or something? Thanks in advance Flo