Only reseller admin has the right to delete a mail domain.
To add a new mail domain:
Login into control panel as host admin.
Click on the Mail Manager option, present in left frame, new contents will be loaded into right frame.
Select the domain name which you want to delete. Click Delete button and click Ok on confirmation dialog box. As the result following actions take place
Domain will be deleted from the specific mail server.
All the users of the mail domain will be deleted