I am in the process of migrating a client to Exchange 2007. I have a legacy Exchange 2003 server which has 150+ mailboxes on it. I moved one test mailbox to the new Exchange 2007 server. When I send messages from the Exchange 2007-hosted mailbox to INTERNAL recipients, they receive the messages with no problem. Mail sent to EXTERNAL addresses outside the organization gets stuck in the "Unreachable Domain" queue on the 2007 server and ultimately generates a NDR.
Users on the 2003 server function normally both ways. I created a Send Connector on the Organization Hub Transport tab, telling the 2007 server to use the 2003 server as a smart host. Then the 2007 user could send out to the Internet, but suddenly nobody on the 2003 server could because the servers started complaining that I'd created a loop situation. I've changed it back to the way that it was and the mail for the 2003 mailboxes is working normally now.
I need to do this migration slowly, so I really need these servers to coexist peacefully for a while. Can anyone help?