When you install VPOP3 it will ask you to create an initial user. This counts towards your user count and is created as your initial administrator account. More administrator accounts can be created later if you wish, and you can remove the initial user if you wish.
The default username & password for the initial user is:
but obviously these can be changed to whatever you wish.
If you have a user who you wish to be the administrator, it can be easier to set that particular user as the initial user.
Make sure you remember the details of the initial user you have created as you will need these details to be able to log into the VPOP3 settings later!
When VPOP3 is first installed, the initial user is where things like error messages are sent (because there's only one user, so it has to be that user who receives these messages). You can change who receives error messages by changing the Main Administrator setting.