Hi,
We have a dedicated sever with Plesk, from the panel that gave us our server farm we set 3 rDNS:
main-server.mymaindomain.com with ip 100.136.213.102
main-server2.mymaindomain.com with ip 200.157.203.115
main-server3.mymaindomain.com with ip 155.121.234.100
In Postfix main.cf there this...