@Machismo with the caveat that I haven’t run a mail server in most of a decade, I think the best practice is to pick a hostname for the mail server, make sure its A and PTR records match, then use that host’s A name as the MX for everything else. I think it’s okay to use CNAMEs if you want the MX records to be “pretty”.

domain1.com A 1.2.3.4
1.2.3.4 PTR domain1.com
domain2.com MX domain1.com