If you are having problems sending emails to a certain email address, you can use WHM to try to figure out what the problem might be. Click Mail Troubleshooter.
Enter the email address you are having problems with.
Click Do It.
This screen will show you all of the remote smtp connections that your server is making to deliver the email. If you get any errors this will let you know.
Looking at the legend there are no problems sending to this email address.