In this post - Using subject line for reply-by-email identifier the user mentioned a previously solved post answering the same question.
https://meta.discourse.org/t/reply-to-email-key-in-subject-instead-of-email-address/7916
I cannot access this page but i am looking to do the same thing, set the reply key in the subject instead of the reply to address.
Does anyone know what happened to that post or what the outcome was?