If you wish to use an external Mailserver, a few changes will need to be made.
1) Go to User Panel -> domain.com -> Email -> MX Records
There should be an option for "Local Mail Server". Uncheck that option.
2) You'll need to change the MX dns record to point to the external mailserver. Remove the "mail" MX record and add "other.domain.com." as the new MX record where that is the domain of the other server.