There are 2 Types of mail users

1)  Users - these are individual users who can access and administer their own mailboxes

2)  Administrators - these are users who can access and administer their own mailboxes, as well as be able to manage the settings of other users and the domain mail account.  As a Domain Administrator, you can add and remove additional Domain Administrators.

NOTE:  A single user account is setup as the 'Primary Domain Administrator' when the mail domain account is originally setup.  This 'Primary Domain Administrator' is considered the Account OWNER, and therefore you can’t remove this account or its administrator priveledges.  You also cannot change the 'Primary Domain Administrator' account. The System Administrator would have to do anything related to the Primary Domain Administator.