What is the difference between port 465 and 587? Should I setup SSL for both of them or is one or the other enough?

My email server uses port 587 and it supports STARTTLS. While you are setting up SSL for your email server, you may also want to ensure that it supports STARTTLS and some other stuff like domain keys.

You can enable both if you like. Port 465 is not really for just SMTP and I would recommend to be sure to support port 587 along with STARTTLS as Alexis mentioned.

In December of 1998, R. Gellens and J. Klensin submitted RFC 2476 in support of adding a new specification for internet email communications. The RFC proposed a split of the traditional message submission and message relay concept. The RFC defined that message submission should occur over port 587 to ensure new policy and security requirements don't interfere with the traditional relay traffic over message relay port 25.