Make sure they're consistent with the same rollup pack because OWA proxying won't work! Background: I want to connect to a single OWA CAS server no matter where the mailbox is located in the organization...IE, I want proxying to work.

Started with this error:

Outlook Web Access is not available. If the problem continues, contact technical support for your organization and tell them the following: There is no Microsoft Exchange Client Access server that has the necessary configuration in the Active Directory site where the mailbox is stored.

This was because the secondary site CAS server didn't have the OWA authentication set to "Integrated Windows Authentication". Then I got this error:

Outlook Web Access is not currently available for the user mailbox that you are trying to access. If the problem continues, contact technical support for your organization and tell them the following: The Microsoft Exchange Client Access server that is proxying the Outlook Web Access requests is running an older version of Microsoft Exchange than the Client Access server in the mailbox Active Directory site.

This was due to the fact that the main CAS (used for proxying) was running rollup 8 for SP1 and CAS at the site with it's mailbox was running rollup 7. Downgraded the main to 7 and bing, we're working.

Wonder what else wouldn't have worked...

DON'T FORGET TO REBOOT THE SERVERS AFTER MAKING THESE CHANGES!!!! I forced AD replication, restarted IIS...I had to actually reboot the servers to see the new changes. Farted around with that for over an hour this morning!

ALSO MAKE SURE YOU HAVE A CERT INSTALLED FOR THE CONFIGURED InternalURL ADDRESS.