Okay to clarify, the destination SMTP domain server which receives the bounced email doesn’t recognize the + addressing, so it only forwards emails based on the To field to the gmail address which is picked up by discourse via POP. That if the To field includes the reply_key then it won’t forward that email to the gmail account used by discourse.
So I can’t put the reply_key in the email address, can it put somewhere else? In the subject field or body or somewhere where discourse may be able to parse it?