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OWA will not work for mailbox in non-internet facing AD site

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Last Modified: 2012-05-09
We have Exchange 2010 servers in 2 sites (one internet facing (site A) and the other not (site B)).  If I use the main OWA (site A) to open a mailbox from site A, it works, but if the mailbox is from site B it doesn't work giving the message "Outlook Web App isn't available".  The event log on the CAS server in Site A has an error that says "The Outlook Web App proxy failed because no Outlook Web App virtual directory configured for Kerberos authentication could be found in the site."

If, internally, I go directly to a CAS OWA in site B and try to open the same mailbox from site B it works fine.  If you try to connect to a mailbox in site A from here, you get the normal message "Use the following link to open this mailbox with the best performance" with a link to the proper OWA in the internet facing site. (normal)

I have checked to make sure that the msExchInternalHostName (Internal URL) attribute of the Outlook Web App virtual directory exists and it matches the fully qualified domain name (FQDN) of the Client Access server.  I have also even tried to go into IIS on the CAS server in site B and set the Integrated Windows authentication enabled on the Outlook Web App virtual directory.  I'm stuck.

The users in Site B will need to be able to get OWA access from the internet (hence going through the CAS server in the internet facing site A) and get to their mailboxes hosted on the mailbox server in site B.  Thanks in advance for all available help.
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