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.
The main issue is that Exchange 2007 will first look locally for the free busy requests, when there is an external recipient that needs to be looked up it will then look to the availability address space that has been configured to see if there is a domain match. Exchange 2010 will perform the local lookup first just as the 2007 server but then will do the Organization Relationship lookup to see if there is a relationship in place with the External recipients SMTP domain.
The solution is to run the Add-AvailabilityAddressSpace cmdlet and the proxyUrl parameter to allow the relay to occur from the Exchange 2007 to the Exchange 2010 sp1 servers. The ProxyUrl parameter that is included with Exchange 2007 SP2 specifies whether to direct a Microsoft Exchange Server 2007 Client Access server to proxy its free/busy requests through an Exchange Server 2010 SP1 CAS when requesting federated free/busy data for a user in another organization for which you have an organization relationship in place. The proxyUrl should point to the local Exchange 2010 servers availability service (EWS URL).
After this is configured correctly, the flow of information will go as follows:
Exchange 2007 Mailbox in Organization 1 requests Free/busy for a user in Organization 2
The Exchange 2007 server submits the request to the Exchange 2010 server in Organization 1 because of the new Add-AvailabilityAddressSpace with the internal Relay option
The Exchange 2010 SP1 server in Organization 1 submits the request on behalf of the 2007 mailbox
The Exchange 2010 server in Organization 2 returns the requested data
Steps to Add-Availability Address Space for the 2007 Mailboxes
Open the Exchange Management Shell on the Exchange 2007 CAS server and run the following (the Fabrikam.com should be the external domain name you are trying to view the Free/Busy data for) (The Proxy URL should reflect the 2010 sp1 CAS URL that in the same organization as the 2007 mailboxes requesting the data). The Proxy URL can be a load balanced address or a single CAS.