I tested the domain “postalfree.net” and I can’t see any immediate problems with it. The name and “mail.postalfree.net” resolves to the IP 80.241.212.154, and that IP answers properly on ports 25, 110 and 143.
So your client needs to tell you precisely what the issue is, what errors he gets, what hostnames he uses to connect to your server, what ports he uses, if he uses SSL, and so on. And you’d need to check the logs in /var/log for problems at the time when he tries to access mail. Can’t say much more really at this point…