Getting to know the legacy Routing Group Connector in Exchange 2007:
"For those of you running bigger organizations, you may need to have multiple legacy RGCs. There are several things to consider before you do this, and as usual, the Exchange team blog tells you all you need to know. Before you go run off and read that link, though, here's a summary of the highlights:
Add the legacy bridgehead servers to the "ExchangeLegacyInterop" group so they have the proper permissions to authenticate to the Exchange 2007 HT servers
