SMTPサービスに何が問題なのかわからない

[satonotdead]

セキュリティオプションはあまり正確ではないため、無効にする必要があります（セキュリティを損なうことなく）。

Setting up SMTP

If you want to disable TLS, you need to uncomment this line, and set it to false: #DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true) However, there’s no relationship between POODLE and newer TLS protocol versions, nor is there any relationship between SMTP and POP3. Mailgun does support TLS on SMTP connections, they just don’t support it correctly.