Using ssl certificate for Postfix and Dovecot in multiple domains.

[apinto]

If you only have one server, yes, all users mail servers should use the same DNS Record (mx.domain.com).

This is important if you use SSL, however if you do not use SSL/TLS you can create a DNS mx/mail subdomain (for ex mx.domain1.com, mail.domain2.com, mx.domain3.com, etc.) pointing to the same IP as the original mx; this should work without any kind of problem.

Anyways, and like I said, you can use different DNS records as long as they point to the same IP but you will have a drawback:

The ONLY single drawback is that the client needs to accept the security certificate on the FIRST time they connect, all the emails are sent correctly encrypted and received without issue. Anyone who send an email to those domains will not get any warning, they will not even know what is going on.