I've moved several users to our new Exchange 2010 server. They are able to send email everywhere except to users still on the 2003 server. They just sit in the queue and the senders get a delayed delivery warning, then eventually an ndr. 2003 users can send to 2010 users fine.
I've recreated routing group connectors and send connectors multiple times with no change. I've enable logging on the on the send connector, but the emails to users on the 2003 server do not even show up in the log. Not sure where to go next.
The company does use a smart host, but I don't think that's causing the issue because I've configured the connectors not to use it with no change in the results.