看起来DO正在他们的Discourse托管计划中禁用SMTP

[supermathie (Michael Brown)]

Digital Ocean 默认阻止出站端口 25 已经实行了多年。这并不是什么新鲜事。

Matt 在这里解释得很清楚：

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…