Re: [exim] Running our own email server on GCP

For submission (since you were talking of 465 and 587) they
are essentially equivalent - so long as you don't transfer
messages on an in-clear channel. Enforcing that is trivial.

For MTA-MTA traffic on 25 you need to support in-clear, or
you'll fail to receive quite a large percentage of mail.
Obviously you should support STARTTLS also on 25, and there
is no standardised TLS-on-connect MTA-MTA method.
--
Cheers,
Jeremy