Should I set my IP address' PTR record to the domain (
mydomain.com) or the subdomain where the mail server is (
mail.mydomain.com)? It's a little confusing.
To expand on my comment, here is an example of proper forward and reverse DNS for mail (methods can vary if you will have multiple mail servers, here is an example where three are load balanced):
All mail servers and priority:
Forward DNS on mail servers:
RDNS for mail IP: