You have two Active Directory sites on a slow network in a Microsoft Exchange Server 2010 organization.
You have an Exchange Server 2010 Client Access server and an Exchange Server 2010 Mailbox server in one Active Directory site.
You have an Exchange Server 2010 Client Access server and add an Office Outlook user in the other Active Directory site.
The user whose mailbox is located in the different Active Directory site tries to download the Exchange Offline Address Book (OAB).
In this scenario, it takes a long time to download the OAB.
This issue occurs because the Autodiscover service uses the UserSite information to verify the Client Access server for OAB. However the UserSite information is set to the Mailbox server site. This behavior causes the Autodiscover service to refer the Outlook user to the Client Access server with the Mailbox server instead of the closest Client Access server.
To resolve this issue, install the following update rollup:
2645995 Description of Update Rollup 1 for Exchange Server 2010 Service Pack 2
Note After you apply the update rollup, the Autodiscover service refers the Outlook user to the closest (local) Client Access server. However, if the user never retrieves a service connection point (SCP) object to the local Client Access server, or the local Client Access server does not have a copy of the user's OAB, the user downloads the OAB from the Client Access server in the same site where the Mailbox server is located. Additionally, the user can potentially download the OAB from a much farther site. Therefore, make sure that the appropriate SCP objects are set up, and that Active Directory sites are designated appropriately.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the OAB, visit the following Microsoft website: