Reverse DNS is not something that should programmed automatically.  It also Should only point to FQDN (Ex: server.yourserver.com).  If you are setting up rDNS to the users domain name, it's legal, however in reality, all of you /24 should point to your servers hostname.   When MX lookups do a PTR lookup for domain xyz.com and see xyz.com as the server name, but the mail header reads from server.yourserver.com, it may very well fail the message.  JS