GFI MailEssentials uses 3 ways to count and retrieve users: Exchange 2000/2003, Active Directory or an internal user database.

Exchange 2000/2003
Starting from MailEssentials version 9, when GFI MailEssentials is installed with Microsoft Exchange 2000/2003 (on the same machine), MailEssentials can count all Active Directory users that have a mailboxes on the Exchange Information Store. For previous versions, when MailEssentials is installed with Exchange 2000, users are retrieved from Active Directory (check the next section)

Active Directory
When GFI MailEssentials is installed on a machine that is part of Active Directory, but does not have Exchange server installed, Active Directory can be used. GFI MailEssentials will only count mail-enabled users and can only retrieve these from the Active Directory domain of which the machine where GFI MailEssentials is installed. Active Directory is queried using the LDAP protocol

Note: The System mailbox is not counted by MailEssentials

Internal user database An internal user database is used when the users are not retrieved from Active Directory. This is the only option if MailEssentials is installed on a machine that is not part of Active Directory. However, if you have Active Directory, you can choose this option also. In this case, the users will have to be entered manually through the MailEssentials configuration and be used for user-based content filtering. Email addresses will also be retreived from outgoing emails and saved in the user database.

Reference: KBID001646