Is port 465 deprecated?

For a guide on how to setup Exim4 with Gmail and implicit TLS, see How To Secure A Linux Server.

No. Some sources like Debian’s guide on Gmail and Exim4 and the StackOverflow question What is the difference between ports 465 and 587? claim that port 465 is deprecated. RFC 8314 entitled Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access recommends that you use port 465 with implicit TLS instead of STARTTLS on port 587:

In brief, this memo now recommends that:

  • TLS version 1.2 or greater be used for all traffic between MUAs and Mail Submission Servers, and also between MUAs and Mail Access Servers.
  • MUAs and Mail Service Providers (MSPs) (a) discourage the use of cleartext protocols for mail access and mail submission and (b) deprecate the use of cleartext protocols for these purposes as soon as practicable.
  • Connections to Mail Submission Servers and Mail Access Servers be made using “Implicit TLS” (as defined below), in preference to connecting to the “cleartext” port and negotiating TLS using the STARTTLS command or a similar command.

More specifically:

The STARTTLS mechanism on port 587 is relatively widely deployed due to the situation with port 465 (discussed in Section 7.3). This differs from IMAP and POP services where Implicit TLS is more widely deployed on servers than STARTTLS. It is desirable to migrate core protocols used by MUA software to Implicit TLS over time, for consistency as well as for the additional reasons discussed in Appendix A.

However, some have conflated SMTPS with implicit TLS on port 465, which is not the same thing. In particular, section 7.3 of RFC 8314 explains that SMTPS was briefly assigned to port 465 and subsequently revoked:

Unfortunately, some widely deployed mail software interpreted “smtps” as “submissions” [RFC6409] and used that port for email submission by default when an end user requested security during account setup.

Although STARTTLS on port 587 has been deployed, it has not replaced the deployed use of Implicit TLS submission on port 465.

To reiterate, “Implicit TLS submission” which is defined in section 3 is not the same as SMTPS and you should use port 465 over port 587 if possible. Another point of confusion is the use of SSL on port 465. As a result, you’ll find many resources on the Internet claiming not to use port 465. It is true that you should prefer TLS over SSL, but port 465 is not deprecated.

The RFC also states:
Note that there is no significant difference between the security properties of STARTTLS on port 587 and Implicit TLS on port 465 if the implementations are correct and if both the client and the server are configured to require successful negotiation of TLS prior to Message Submission.

The key phrase here being “require successful negotation”. If STARTTLS is not configured this way, then it is less secure than Implicit TLS.