I will describe below some points that you should validate in your Exchange Server 2003/2000 to allow the message flow, here are they:
- Validate which IP address Exchange Server 2007 resolves from the Exchange 2000/2003 server, try to connect in the port 25 of that server
- Validate if the SMTP Virtual Server in Exchange 2000/2003 is listening in the correct IP Address (same IP that Exchange Server 2007 is resolving)
- Validate if there is any connection filter blocking connection from Exchange Server 2007
- If you are still having problems, try to stop your Antivirus software and validate if it works
- If you have more than one Exchange 2003/2000 you can manage them in the Routing Group Connector, you can do that through Set-RoutingGroupConnector cmdlet in Exchange Server 2007
Do you have another point to be considerating during the troubleshooting, please post a comment and I will be adding to this current list.