Troubleshooting E-mail in WHM
From Lunarpages Web Hosting Wiki
This is how you would troubleshoot e-mail problems using Web Host Manager (WHM). WHM makes for a very handy tool to have to help you solve some simple e-mail problems. The first thing you would need to do is login to WHM. From the front page, click on "Email" and then "Mail Troubleshooter". This will take you to the Visual Mail Troubleshooter page.
In the empty text box, enter the e-mail address you wish to troubleshoot, then hit the "Do it" button. If no errors are shown and your e-mail account is still having problems, it could be that it was setup incorrectly via the e-mail client or via cPanel.