I just setup free busy between two forests of exchange (2007 and 2010) and it is working fine for all outlook 2007/10 users. I have a scenario, my exchange 2010 forest have some outlook 2003 user who are not able to see free busy of exchange 2007 users.
please note that I have just configured the availability address and nothing else.
1. An Exchange 2010 user using Outlook 2003 request free/busy information for an Exchange user in the Exchange 2007 forest.
2. Outlook 2003 connects to the “SCHEDULE+ FREE BUSY” system folder in a public folder database in the Exchange 2010 forest.
3. The RPC Client Access service on an Exchange 2010 server intercepts the free/busy request and sends it to the Availability service on an Exchange 2007 Client Access server in the Exchange 2007 forest.
4. The Exchange 2007 CAS server retrieves the free/busy information in the respective mailbox and returns it to the RPC Client Access service in the Exchange 2010 forest.
That makes sense since Outlook 2003 doesn't use the availability service, it gets its free-busy data from the system public folders. I recommend that you upgrade Outlook, or consider synchronizing the free-busy public folders between the organizations
using something like the InterOrg Replication Tool. I believe Quest Software also has a tool, at least they did a few years ago.
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