On our cPanel servers, you have a few choices with how the server will handle your email. You can edit these by clicking on the MX Entry icon:
Local Mail Exchanger
If the lowest number mail exchanger points to an IP on this server the server will be configured to accept mail locally and from outside the server.
Backup Mail Exchanger
If a mail exchanger other than the lowest points to an IP on this server, the server will be configured to act as a backup mail exchanger.
Remote Mail Exchanger
If there are no mail exchange that point to an IP on this server the server will be configured to not accept mail locally and send mail to the lowest MX record.
Note: Automatic detection of MX configuration is not possible if MX entries do not resolve (ie: you mistype a domain name or enter one that does not exist). If your MX configuration is set to auto and you add or edit an MX record that does not resolve you will see a warning and MX configuration will default back the last known setting.