As an Administrator, If during your Exchange 2010 install, when you migrated that single test mailbox from the old Exchange 2003 server into the nice shiny new Exchange 2010 mail server on that new MS Server 2008 64bit you have up and running. You found you couldn’t send mail internally or receive mail internally or in fact receive mail from an external source either, your not alone.

Reviewing the “Queue Viewer” on your Exchange 2010 bi you see there are mails in the queue trying to send with the following error

there is currently no route to the mailbox database

And you also see mails in the inbound queue on your exchange 2003 box.

Fear not, for there is a simple fix. You must Create A Routing Group Connector Between Exchange 2003 and Exchange 2010

According to Microsoft when you installed the new Exchange server 2010 despite the compatibility that exchange 2003 and exchange 2010 can coexist on your domain, they don’t quite let you know that the routing may not properly be configured, my guess is because there are too many permutations of network configurations you might have. So if your like me, you have a single Exchange 2003 server that you want to talk to your Exchange 2010 server then the solution is pretty simple.

Make sure you login to the exchange 2010 box with an account that has “GOD” privileges on your domain
1. Click start
2. in the search box type “Shell”
3. Right click and run “Exchange Management Shell”
4. Copy and paste the following line into notepad

New-RoutingGroupConnector -Name “Interop RGC” -SourceTransportServers “exchange2010FQDN” -TargetTransportServers “Exchange2003FQDN” -Cost 10 -Bidirectional $true -PublicFolderReferralsEnabled $true

Adjust the names accordingly and be sure the quotes are quotes and not some funky character that looks like quotes as is what sometimes happens when copying from the internet.
5. Paste the line into the Exchange Management Shell (EMS) and you should see something not too different from this.

6. Now you can verify the creation by running “Get-RoutingGroupConnector”
7. On your exchange 2003 box restart “Simple Mail Transport Protocol” Service
8. On your Exchange 2010 box restart “Mail Exchange Transport” Service

Hope this helped you all

References I used
Message Rerouting and the Unreachable Queue
http://technet.microsoft.com/en-us/library/bb232161.aspx

Routing group connector between an Exchange 2010 organization and Exchange 2003 organization doesn’t exist
A routing group connector between the Exchange 2010 routing group and Exchange 2003 routing groups hasn’t been configured, or the last routing group connector between the Exchange 2010 routing group and Exchange 2003 routing groups has been removed. No routing group connector exists to provide a routing path to the Exchange 2003 recipients. To resolve this problem, first verify that the routing group connector is missing. If that’s the case, you can create a routing group connector. For more information, see Create Additional Routing Group Connectors from Exchange 2010 to Exchange 2003. If a routing group connector does exist, the message is in the Unreachable queue for some other reason. Check the configuration of the routing group connector

Create Additional Routing Group Connectors from Exchange 2010 to Exchange 2003
http://technet.microsoft.com/en-us/library/aa997292.aspx

New-RoutingGroupConnector -Name “Interop RGC” -SourceTransportServers “Ex2010Hub1.contoso.com” -TargetTransportServers “Ex2003BH1.contoso.com” -Cost 10 -Bidirectional $true -PublicFolderReferralsEnabled $true

Upgrade from Exchange 2003 Transport
http://technet.microsoft.com/en-us/library/dd638103.aspx
Exchange Management Shell in Exchange 2010
http://technet.microsoft.com/en-us/library/dd795097.aspx