Using Coexistence Manager for Notes (CMN) the Directory Connector syncs the Mail-In Database to Active Directory as a Contact, even though the setting 'Allow foreign directory synchronization' is set to No.
Currently Mail-in Database, when set to 'Allow foreign directory sync' is set to No CMN doesn't adhere to the setting.
This is a defect and will be fixed in a future release of CMN.
WORKAROUND
Use the following steps to exclude the name of the Domino Mail-In database within the CMN Directory Connector.
Running the Directory Connector again for the Mail-In databases, the excluded objects should no longer appear in the AD.