Incoming mail getting rejected as 550 unknown user

A lot of our mail was getting bounced as spam and added to the CBL blacklist. After numerous removals and re-adds to the list I contacted the CBL and they said the reason why our mails looked suspicious was because the server was identifying itself through HELO as xx.xx.xxx.xx.cust.carohosting.com. They suggested I change the server name to reflect something less generic, like 'mydomain.com'. So I changed the hostname for the box at the command prompt from xx.xx.xxx.xx.cust.carohosting.com' to 'mydomain.com'. Now all emails I send have the correct HELO identifier. However, I have caused another problem... I now cannot RECEIVE any emails to any of my users. All incoming emails get rejected as 550 unknown user. I looked at my control panel and they're all there. And they're all in the /usr/local/apache/wwwroot/mydomain.com/popaccts/mydomain.com/ folder. I can even send email successfully. I just can't receive any. The other weird thing is that on my other domain (on the same machine and IP) mydomain2.com, all email works perfectly. The only thing I changed was the hostname from 'xx.xx.xxx.xx.cust.carohosting.com' to 'mydomain.com' on the server, /etc/hosts AND /etc/sysconfig/network files. What could be causing this problem? It's so odd, especially due to the fact that all mydomain2.com mail is coming in perfectly? Are there any useful command prompt commands that I can use to diagnose the problem, like listing all current mail accounts and servers, etc.? Thanks for your help.