I just installed an Exchange 2010 mbx server and a 2010 hub/cas server in an Exchange 2007 enviromnent. I moved a test mailbox over to the 2010 mbx server and it can receive email but it can't send to mailboxes on the Exchange 2007 server. they just sit in the 2010 queue. and there are inbound messages piling up in the Exchange 2010 queue. not sure where to start with this one. hidden connectors should be installed by default.