If you wish to just point it to another domain do the following:
1.Log into your Cpanel.
3.Click "Modify Mail Exchanger (MX Entry)."
4.Put in mail.thedomainname.com of the domain you wish for it to go to.
If you wish to point it to another server using the same domain:
1.Log into your WHM (if you are not a reseller you will need to e-mail firstname.lastname@example.org to do it for you).
2.Click edit DNS zone.
3.Pick the domain you wish to edit.
4.Change the MX from domain.com to mail.domain.com.
5.mail.domain.com should have the A entry of the external mail server IP address.
Note: If you ever send mail from any sites that's on the same server where your site is hosted your external mail server will not receive the mail. The reason for this is that a server will always try to send locally before it tries what the actual DNS zone points to. The way to fix this is to e-mail email@example.com saying "Please remove my domain (whatever.com) from /etc/localdomains and add it to /etc/remotedomains". Only we are able to make this change for you. After the change is completed, this problem should be fixed.