In the Active Directory, Exchange recipients are stored as one of the following native Windows objects in the Domain container:
Existing Exchange mailboxes are replicated into the Active Directory as users or contacts.
Before enabling e-mail attributes for any Active Directory objects, you must first create a Connection Agreement in the appropriate organizational unit. Then replicate the selected object(s) into the Exchange directory. All organizational units that are under an organizational unit with a Connection Agreement will also be replicated. If you are joining an existing Exchange server that already contains recipients, you must replicate those recipients to the new Active Directory using a Connection Agreement in ADC.