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[JammyDodger]

ポート 465 については、Troubleshooting email on a new Discourse install で触れられていますが、参考になりますでしょうか？

@Discourse

Email smtp port selection (Using 465?)

The ability to be able to AUTH using ‘telnet’ is extremely important in your first steps of email troubleshooting.

Port 465 (SMTP over SSL) is largely deprecated in favor of STARTTLS on 25. You may need to try alternate ports such as port 2525 or port 587 (Mail Submission) when things do not seem to work as expected.