One important thing to keep in mind for those who run their own mail systems is that you'll want the A and PTR RRs to match up or you increase the chances of your messages being considered spam.

In addition, you'll want the name used by your users (e.g. "mail.example.com") to match what's on your certificate or (at least some) mail clients will throw up errors (just as browsers do).

Multiple mail servers used by different users? nginx can proxy POP3, IMAP, and SMTP as well, so everyone can use "mail.example.com" and get routed to the correct backend server.