Today I learned that in 2018 with RFC8314 a new mail submission service was standardized by the IETF and the IANA. It’s called
submissons and re-allocates the original well-known port of
smtps which was introduced in 1997 and revoked again in 1998 after the standardization of STARTTLS for SMTP and submission.
submissions standardizes the quite common but not standard-conform usage of the
smtps port 465 as a port for mail submission by clients (MUAs) instead of MTAs, which should have used the submission port 587 instead when following the IETF and IANA standards at the time. RFC8314 tries to fix that but might causes just more chaos as people like me are, or better were, still convinced that 465 was not to be used as it was revoked as well-known port in 1998. Well, now we can say, not any more.
I got into the whole topic after a discussion in my matrix room “Random talks with Sheo” and being quite sure that 465 was deprecated. But here I am, learning that it’s deprecated for SMTPS but has a new use as TLS-encapsulated port for submission. As this might not be clear from the latest version of the RFC, I recommend you to have a look at version 5 of the draft as well as the email by “Sabrina Tanamal” from 2017-10-12. And further more clearance brought the Wikipedia page to SMTPS which has two sections about the whole problem. Amazing what one can learn every day…
PS: If you wonder, yes, now it’s recommended to do mail submissions over 465 as “implicit TLS” port instead of using port 587 with STARTTLS.