If this occurs, please disable and re-enable email for your domain. 

1. Login to CWP
2. Click on domains, then choose the domain where the problem is occurring.
3. Choose Mail Accounts, then the Mail Settings button.
4. Uncheck the "Activate mail service on domain" box, click OK, then re-check the box and click OK.

 If the problem continues, please give it at least 60 minutes for the configuration updates to apply to all instances of our mail server. If 60 minutes has passed and the problem continues, please open a support ticket that includes the error and your domain name.