Похоже, что DO отключает SMTP в своих тарифах хостинга Discourse

[supermathie (Michael Brown)]

Digital Ocean blocking outbound port 25 by default has been in place for years. This is nothing new.

Matt explains it well here:

Proposed changes to discourse-setup because Digital Ocean blocks outbound smtp

OK then, let me give you the full spiel, then: Port 25 outbound is an obvious candidate for filtering, because if you can hit the Internet on port 25, you can deliver e-mail to practically anyone. If that e-mail is unwanted, the only useful “identity” information that the recipient has to try and prevent further abuse is the sending IP address. That identity is tied, in general, to a hosting provider, not an individual subscriber, so any reputation hit for spamming goes, in the first instanc…