One of the most common problems with co-existing Exchange Servers (an organisation consisting of an Exchange 2003 server and an Exchange 2007 or 2010 server) is mailflow between the servers.
When Exchange 2007 or 2010 is installed in an existing Exchange 2003 organisation a Routing Group Connector (RGC) is configured during the installation of the Exchange 2007 / 2010 Exchange server to allow mailflow between the legacy server and the new one.
The way Exchange 2003 servers communicate with other Exchange 2003 servers or Exchange 2007 and 2010 servers is by utilising the Default SMTP Virtual Server.
One of the most common causes of disrupted mailflow is that the Default SMTP Virtual Server has been modified. Either the default port has been changed from the port 25 or a smarthost has been added (Delivery Tab / Advanced Button).
The smarthost configuration in Exchange 2003 should be performed using an SMTP Connector rather than modifying the Default Virtual SMTP Server. The general rule of thumb is to create a new Virtual Server rather than modify the existing one if a different port is required to send to a smarthost or a SPAM/Virus Appliance.
All of these actions can be performed using the Send Connectors in Exchange 2007/2010 so these settings should all be returned to default. This means that the Default SMTP Virtual Server should be using port 25 and should not have a smarthost configured.
Once you have done this mailflow should resume between the different versions of Exchange.