This article applies to 123-reg servers running cPanel.
This article will explain how you can turn off the local email service for a domain on your server using cPanel.
Login to cPanel WHM interface.
From the cPanel WHM interface home screen, click on the DNS Functions icon.
Click on the Edit DNS Zone icon.
Select your domain from the list shown in the Choose a zone to Edit section, then press the Edit button.
Scroll to the bottom of the page to the Email Routing section and select the option Remote Mail Exchanger, press the Save button when you are finished.
This will prevent any issues of emails trying to be sent internally for a domain setup on your server running cPanel when that domain uses an external email service.