Mail flow from Exchange 2003 to Exchange 2007

Scenario:
You have added your first Exchange 2007 server into your Exchange 2003 network. You followed all of the steps but still mail will not flow.

Reason:
The original connectors don't always work but I haven't installed at enough locations yet to work out a pattern.

Solution:
Firstly check for link state suppression on the Exchange 2003 server:
Regedit (usual caveats about using regedit)
Drill down to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\RESvc\Parameters.
Right-click Parameters and select New | DWORD value.
Name the new DWORD value SuppressStateChanges.
Double-click SuppressStateChanges.
In the Value data field, enter 1.

Restart the SMTP and Exchange Routing Engine services on the Exchange 2003 server.

Then:
In EMC on Exchange 2007, delete the receive connectors on the HT (there will probably be 2 of them), one called Client (port 587) and one called Default (port 25).

In ESM on Exchange 2003, delete the one routing group connector from the E2003 routing group that specifies the E2007 name.

In ESM on Exchange 2003, delete the one routing group connector from the E2007 routing group that specifies the E2003 name.

Then in EMS on Exchange 2007, type the following:
New-RoutingGroupConnector -Name "2003 to 2007 conduit" -SourceTransportServers "E2007 server name" -TargetTransportServers "E2003 server name" -Cost 1 -Bidirectional $true -PublicFolderReferralsEnabled $true

There are no services that require stopping / restarting.

Mail will start to flow.