If you are hosting a domain's web traffic but using another service to manage its mail, contact forms on the website will not work correctly unless you change tell the server that the domain is using a remote mail exchanger. This can be done in WHM for any cPanel account you control.
- Log in to WHM
- On the left hand side of the page, click "Edit DNS Zone" underneath the "DNS Functions" heading.
- Select the domain you wish to configure from the list, then click "Edit"
- Scroll down to the bottom of the page and click the radio button next to "Remote Mail Exchanger".
- Click "Save".
If you wish to start managing mail through the domain again, you'll just need to change this setting back to "Local Mail Exchanger" and edit the MX records as necessary.