Hi,
I have 2 suggestions / questions:
1. How does the smart host authenticate your mail server. Some consortium only only 1 IP address (from each School) to relay through it and will drop all other connections.
Unfortunatley the only way of checking this is to temporarily swap the IP addresses.
2. Gateways. Does the new server have gateways setup or one strange thing we found when we had Exch2003 and 2007 running at the same time was we had to put the IP address details for the smart host in the host file and then deleted after the 2003 box had been retired.
Alan