From Birnam Designs Wiki

To set your server to always use an external mail server for a domain:

If you haven't already modified the MX record:

• In WHM > DNS Fucntions > Edit a DNS Zone
• chose the domain and where it says mail | 14400 | cname | domainname.com
• change to: mail | 14400 | A | 111.111.111.111 (the IP of the mail server)

Then:

remove the domain from the list in /etc/localdomains

vim /etc/localdomains

and add it to the list in /etc/remotedomains

vim /etc/remotedomains