Are the users members of the same security groups?
What Active Directory groups are considered Protected Groups?
The following Active Directory groups are considered Protected Groups:
Administrators
Account Operators
Backup Operators
Cert Publishers
Domain Admins
Enterprise Admins
Print Operators
Schema Admins
Server Operators
Note: The Administrator and Krbtgt Active Directory user accounts are also considered protected.
What if my BlackBerry device user must remain in a Protected Group?
It is possible to modify Active Directory permissions to allow BlackBerry device users who are members of protected groups to send messages from their BlackBerry devices without creating secondary email accounts. Microsoft Support Knowledge Base article 817433 outlines a procedure for modifying the permissions associated with the AdminSDHolder Active Directory object that were modified by the recent Microsoft Exchange update. However, this procedure is not recommended by Microsoft or by Research In Motion (RIM).
http://us.blackberry.com/support/software/sendasfaq.jsp