Have you tried the resolution steps in this thread and the other related ones?
Add a line to your app.yml:
DISCOURSE_SMTP_DOMAIN: [your server's fqdn]
Then in /var/discourse, ./launcher rebuild app
discourse-doctor may still report failures, but test emails from within the admin console should work, and normal email flow should resume.
If that doesn’t work, please report back, because then there’s some other aspect that hasn’t been discovered yet.