What’s the content of DISCOURSE_SMTP_ADDRESS? Is it the actual hostname of the SMTP server, or can Discourse perform an MX lookup? I do need Discourse to do the MX lookup, and couldn’t find any actual info in docs, even though I tracked this down to http://api.rubyonrails.org/classes/ActionMailer/Base.html. Docs mostly just talk of SMTP credentials. (For comparison, Postfix gives control over this behavior, while MSMTP just doesn’t support MX lookup on the relay host).
Unfortunately I can’t even try this out for now on my staging Discourse instance — it runs on the same physical server (our only one), and I can’t touch this because Docker/AUFS are causing not-yet-diagnosed instabilities: stopping my production Discourse instance fails and leaves it unusable until rebooting the host. (No, not my lucky day — and it was a bad idea to trust Docker to actually be robust).