Exchange 2003 to new Exchange 2007 server sending failure

After introducing first Exchange Server 2007 server into an existing Exchange Server 2003 organization, you may encounter an mail flow issue between the two servers.

Symptoms:
- Emails sent from Exchange 2007 to Exchange 2003
- Emails not sent from Exchange 2003 to Exchange 2007 and might be seen in the queue of the Exchange 2003 server.

Solutions:
Remove the smart host from the Default SMTP Virtual Server and instead configure a new SMTP Connector for the Routing Group to send internet email (the namespace of “*”) to the smart host.