Creo que tienes razón.
Y resulta que no eres el primero… al buscar DISCOURSE_SMTP_PORT 465 encontré un montón de problemas similares.
- Not able to send email Net::ReadTimeout - #18 by Stranik
- Email not working but i can access smtp server from container
- DigitalOcean + Siteground Email via port 465 won’t work (2525 will work)
- Problem with SMTP configuration
- Post-install, Curl can send email, but Discourse cannot. Help to fix?
- Mail problem on discourse with reverse proxy
- Can't setup SMTP with SSL/TLS
- SMTP email issue Net::Timeout - #3 by neounix
El último contiene un enlace a la (posible) solución.
Con este parche, es posible configurar
DISCOURSE_SMTP_FORCE_TLS=true para usar TLS implícito en el puerto 465