Thanks for your reply. I’ve checked MAIL_DOMAIN every way I can imagine, I’ve tried every combination of MAIL_DOMAIN value and destination email address. What value in the Discourse setup is it checked against e.g. DISCOURSE_HOSTNAME, DISCOURSE_SMTP_DOMAIN, something else?
I’m a bit confused by your second suggestion given this line in the OP:
Shouldn’t rejections show up even before Discourse is set up to do anything with them? Bounces aren’t showing up either, I’ve tested using the method recommended here: Configure VERP to handle bouncing e-mails. No trace of anything on admin/email.
Is there a log in either container that shows (or could be configured to show) more information about the interaction between mail-receiver and app are interacting?