I’m ignoring all the DNS stuff because that has nothing to do with connecting to a SMTP server hosted by someone else.
I think setting DISCOURSE_SMTP_ENABLE_START_TLS to false will solve the problem here.
Generally port 465 is TLS (otherwise known as SSL) - it doesn’t require the START_TLS as it’s already TLS.
You would set START_TLS to true if you were using port 26 (yes 26 perhaps 25 as well) with Namecheap’s privateemail.com service.
Also I believe Namecheap’s SMTP server is responding poorly here - it should be bailing out early and causing an error rather than holding the connection open and not responding - causing a “timeout”.
You would see a no peer certificate available or Secure Renegotiation IS NOT supported error, if both the server handled this correctly and there more verbose error logging here.