https://tools.ietf.org/html/rfc8314#section-3.3
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, to maximize the use of encryption for
submission, it is desirable to support both mechanisms for Message
Submission over TLS for a transition period of several years. As a
result, clients and servers SHOULD implement both STARTTLS on
port 587 and Implicit TLS on port 465 for this transition period.
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.
It remains to be seen whether the new RFC actually changes practices in
the field, but there is now some "official" support for the born-again
port 465 "submissions" service.
--
Viktor.
