In Exchange Server 2010 we can set the default Offline Address Book (OAB) that users will use with their mail client.
To set the default OAB in Exchange server 2010 follow the steps below:
Open EMC -> Organization Configuration -> Mailbox -> Database Management TAB.
Double click on the mailbox database.
Go to the Client settigns tab and click on Browse next to the Offline address book.
Select the address book you would like your users to get by default.
Click OK and Apply.