We have a co-existence of Exchange 2003 and Exchange 2007. Currently there is just a test mailbox on the Exchange 2007 Mailbox Server.
Exchange 2003 is in a Cluster
Exchange 2007 is a CCR.
Through testing I have been able to create a user directly on the 2003 Exchange server and send and receive just fine.
I am able to move that user over to the Exchange 2007 box and it transfers fine.
After that point I am able to receive email internally and externally while the box is on the Exchange 2007 Server.
I am able to send email externally but NOT internally to the Exchange 2003 server.
Any help on this would be greatly appreciated! Thanks!