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.
If you have set up Exchange Server 2007 Client Access Servers in a CAS-CAS Proxy scenario, where the CAS server in the main site is exposed to the internet and the CAS servers in other remote locations depend on the internet-exposed CAS to proxy requests to them, users in the remote site may get the above error when they try to access their mailboxes via Outlook Web Access.
The cause is very simple. The Client Access Server in the remote site may have the latest Update Rollup for Exchange 2007 installed on it, while Client Access Server in the main site is still having an older Update Rollup.
I noticed this problem when the Client Access Server in the main site is running with Update Rollup 3, while the remote site has already got Update Rollup 4 installed. A quick install of the latest Update Rollup on all servers solved the problem.