I have 2 Exchange client access/hub transport/mailbox servers. One is 2007 and one is 2010 (new setup). Both are operating fine. I have only one send connector. The Exchange 2007 sends all outbound mail and receives all inbound mail (for the exchange 2007) mailboxes but fails to deliver (and collects in queue) all mail going to Exchange 2010 server.
The Exchange 2010 cannot send or receive any mail and on the queue I saw the error "451 5.7.3 Cannot achieve Exchange Server authentication"