Answer: The Offline Address Book (OAB) is an offline copy of all the email addresses found in the Exchange Organization.
Client using Outlook can download the OAB and store it on their computer or laptop so when they are offline or the server is offline they can still access the address book.
As an Exchange administrator you can create a new OAB and decide which addresses or information from the Organization is available to users when they or the server are offline.
To create a new Offline Address Book use the steps below:
Open EMC- >Organization Configuration -> Mailbox -> Offline Address Book tab.
Right click on a blank area and Select New Offline Address Book
Name the OAB and select which address list you would like to make available to your users.
Select the Distribution point
Note: If you have Outlook 2003 clients select Enable public folder distribution
Click on Next and Finish.
If you would like to make the new OAB the default OAB simply right click on it and select Set as Default.