Offline address book is used by Office Outlook so that users can locate contact information even when they are not connected to the server.
Offline address book works when the Outlook client is configured in cache mode or when it is in offline mode.
The offline address-book (OAB) of Exchange is available when the user is not connected, Since Exchange 2007, it is updated via a Web-based distribution point. The offline address-book (OAB) is a copy of the Exchange address book that the user can download via their Outlook client.
The Offline Address Book (OAB) for Exchange users is generated immediately when your account is created.Web-based distribution supports more clients and consumes less bandwidth.The OAB is downloaded via a virtual directory on the client access server.Then, the offline address book is rolled out by OAB virtual directory.So make sure the internal and external URL for OAB and Autodiscover virtual directory is configured properly.