Process of Generating and Distributing the Offline Address Book

          The process of generating and distributing the offline address book consists of the following components:
• Offline address book generation process. To create and update the offline address book, the Offline Address Book (OABGen) service runs on the Mailbox server that hosts the Organizational mailbox. The OABGen service identifies all recipients that should be members of the offline address book, and then creates the offline address book files in the C:\Program Files\Microsoft\Exchange Server \V15\ClientAccess\OAB folder.
• OAB virtual directory. The OAB virtual directory is the distribution point Microsoft Office Outlook 2007 and newer clients use to download the offline address book. When you install Exchange Server 2013, the OAB virtual directory is created under the Default Web Site on the Client Access server, and under the Exchange Back End website on Mailbox servers. By default, the OAB virtual directory is configured with an internal URL. If Outlook clients from outside the organization are accessing the Exchange environment, you also should configure an external URL.
• Autodiscover service. Autodiscover service was introduced in Exchange Server 2007 as a feature that enabled Office® Outlook 2007 or newer clients, as well as some mobile devices, to configure their profile to access Exchange Server automatically. This service provides the correct OAB URL for Outlook clients.
• OAB distribution. When clients need to download the offline address book, the client sends a request to the Client Access server configured through Autodiscover. The Client Access server then proxies the request to the Mailbox server that is hosting the OAB files. The OAB files are then distributed directly from the Mailbox server to the client.