To enable cross organization/premise Free/Busy lookup for users located on Exchange 2007 servers when there is an organization relationship in place with the business instance of the Microsoft Federation Gateway (default instance for Exchange 2010 SP1) it is necessary to add the availability address space for the remote organization and have at least one Exchange 2010 SP1 CAS. This is to provide the Exchange 2007 server with a valid path to route the remote availability requests. Because there is only an Organization relationship in place between the organizations then there is no way for the Exchange 2007 servers to utilize this connection without proper configuration. The following is the process to allow the 2007 servers to proxy the request to the servers that understands and can handle the federated Free/busy requests.
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