A Record / Mail error

Hi, I'm wondering if anybody can help me with an issue i'm having with the DNS and mail records? So normally I would just change the A record www and non www record to the IP address to point to a new site. However for this site, the mail is tied into the domain. i.e. the smtp and mail records are CNAME and point to mydomain.com. So when I change the A record the mail breaks. I"ve tried setting up a new mailserver.mydomain.com record and pointing the smtp and mail records to this new record. It sends out mail but doesn"t receive any. Here are the details: mydomain.com. 900 IN A (ip address for current site) mail.mydomain.com. 900 IN CNAME mydomain.com smtp.mydomain.com. 900 IN CNAME mydomain.com www.mydomain.com. 900 IN CNAME mydomain.com Cheers