If the domain for our outbound emails is mail.example.com and the MX records (mail servers) for this domain are:
mail1.example.com and mail2.example.com what's the prpoer PTR records in this case:
192.168.1.1 - mail.example.com 192.168.1.2 - mail.example.com
192.168.1.1 - mail1.example.com 192.168.1.2 - mail2.example.com
It doesn't really have anything to do with your mx records, outgoing mail and incoming mail can be routed through separate servers.
PTR should always match the hostname of the server. So if you have two outbound servers mail1.example.com and mail2.example.com then correct PTRs would be
mail1.example.com for the IP of mail1.example.com mail2.example.com for the IP of mail2.example.com