The Exchange 2007 mailbox role provides and uses the following:

1. Holds Exchange databases
2. Hold mailboxes and public folders
3. Part of an Active Directory Domain
4. Requires plenty of space in Active Directory
5. Allows a maximum of 50 mail storage groups (in Exchange 2007 enterprise)
6. Allows maximum of 50 mail databases across all storage groups (in Exchange 2007 enterprise)