For Mailgun, on an fresh install i had to do:
DISCOURSE_SMTP_FORCE_TLS: true
DISCOURSE_SMTP_ENABLE_START_TLS: false
./discourse-doctor
was reporting:
======================================== ERROR ========================================
UNEXPECTED ERROR
Net::ReadTimeout
the whole troubleshoot document Troubleshoot email on a new Discourse install - documentation / sysadmin - Discourse Meta didn’t solve my issue.
Found this thread about amending it that would have helped me: Cannot send email - problem with port 465 - installation - Discourse Meta
Then found this Yet Another thread you posted that showed some flips for me to do.