CAS role accepts the OAB download request from clients and proxies to a Mailbox role server. As always, your best friend in troubleshooting is none other as “Test Email Auto Configuration” tool in Outlook.

CAS maintains a log of each request it handles in the log files, present in %Exchange Install Path%\Logging\Http Proxy\OAB\. Take a look at the video @ 31.31 minute and understand the Outlook connection process and then jump to using Test email auto configuration with Outlook.

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By default, Cached Exchange Mode creates and uses an Offline Folder file (.ost) and then downloads and maintains a synchronized copy of the items in all folders in your mailbox.

You work with the information on your computer, and Outlook synchronizes the information with the server.

In Exchange 2013, OAB is processed by mailbox server role which owns the copy of the database of the Arbitration system mailbox with OAB Persisted Capabilities.

OAB resides on the mailbox server in the folder “C:\Program Files\Microsoft\Exchange Server\V15\Client Access\OAB”.

When your connection to the Exchange computer is interrupted, you can continue to work with your data.

When a connection is restored, changes are automatically synchronized, and the folders and items on the server and on your computer are identical again.

You can also specify that updates to the Address Book be downloaded during synchronization.

If you use an Exchange Server email account (like all UW-Madison Office 365 users), we recommend that you use Cached Exchange Mode.

The database homing the System mailbox should be active and mounted.