Our routing table has become corrupt to the point that any messages we send to a particular routing group are returned as undeliverable, even though the target server is online. Restarting the servers doesn't seem to help. How can we clear the routing table?

As long as any one server is running the routing service, the routing data will persist. The solution is to stop all the routing services on all the Exchange routing group masters at the same time—the first service that you shut down must remain off until you stop all the services. You don't have to stop any other services or reboot the servers. After you stop and restart all the routing services on all your servers, the routing engine will automatically rebuild the routing table.