Let me lay it all out. We are running an Exchange 2007 environment with 2-CAS, 2-HT's and 2-MBX's clustered. We started a migration to Exchange 2010 and after starting the migration it was decided that we would instead got to Exchange 2013. In Exchange 2010 we have 3-CAS, 2-HT's and 2-MBX's. We have moved our external connections over to Exchange 2010 and only have test mailboxes on 2010 and that's where we stopped.
We installed Exchange 2013 3-CAS, 2-MBX's. We are testing out 2013 installation. We can access 2013 Web App with the 2013 test accounts. I can access 2013 test mailbox with outlook 2010 by configuring outlook for rpc/https. I am not getting free/busy form 2007 or 2013 when logged into my 2013 test mailbox. My virtual directories are configured as the following:
Exchange 2007 - mail2007.domain.com\
Exchange 2010 - mail.domain.com\
Exchange 2013 - mail2013.domain.com\