New SP1 KB: http://support.microsoft.com/kb/947168
Microsoft Exchange Server 2007 Service Pack 1 (SP1) was applied to the Internet-facing Client Access Server (CAS). However, Exchange 2007 SP1 was not applied to internal-facing CASs. When a user tries to log on to Microsoft Office Outlook Web Access through a proxy, the user receives the following error message:
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.