Many a times, Exchange 2010 misses creating an interop routing group connector between the legacy Exchange platform & 2010. The 2010 setup will prompt for a bridgehead server to configure the mail flow, but I have been in customer sites where it doesn’t get created at all.
Running Get-RoutingGroupConnector in the 2010 Shell will show the connectors you have.
If you don’t have one, create the interop routing group connector from the shell by running the command below.
New-RoutingGroupConnector –Name “Interop RGC” –SourceTransportServers “exchange 2010 servers” –TargetTransportServers “exchange 2003 servers” –Cost 10 –Bidirectional $true –PublicFolderReferralsEnabled $true
You can always give a different name for the connector and can add more than one source/target servers, listing them separated by commas. Running the command above will create the connector both ways, from legacy Exchange to 2010 & vice versa.