どうやらDOは彼らのDiscourseホスティングプランでSmtpを無効にしているようです

[supermathie (Michael Brown)]

Digital Oceanはデフォルトでアウトバウンドポート25をブロックしていますが、これは長年続いています。新しいことではありません。

Mattがここでうまく説明しています。

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…