VPOP3 has two 'types' of administrator.
Users who can administer VPOP3, and log in and change settings etc. These are normal users, but have the Administrator box checked on the User → General settings for that user. These user have a red key icon in the users list.
The user who receives error messages from VPOP3. This is called the Main Administrator and is chosen on the Settings → Admin Settings page in the VPOP3 settings.