I have introduced an Exchange 2007 server into our Exchange 2000 organisation. Did everything by the book. Mail will flow from the 2000 server to the 2007 server but not from the 2007 server to the 2000 server. I have checked, deleted created and check
ed again the Routing Group Connector but it still will not work. The 2007 Exchange mail queue reports the following error "451 4.4.0 Primary target IP address responded with: 535 5.7.3 Authentication unsuccessful." Can anyone help!