Microsoft Solutions

Ok this is very simple and quick, first is the error that gets displayed and next is the solutions that shows you how to edit the registry for the ports to be open:

ERROR THAT YOU NORMALLY GET
Testing the Name Service Provider Interface (NSPI) on the Exchange Mailbox server.
An error occurred while testing the NSPI RPC endpoint.
Test Steps
Attempting to ping RPC endpoint 6004 (NSPI Proxy Interface) on server exch20.domain.com.
The attempt to ping the endpoint failed.
Tell me more about this issue and how to resolve it
Additional Details
The RPC_S_SERVER_UNAVAILABLE error (0x6ba) was thrown by the RPC Runtime process

SOLUTION
1. Open Registry Editor (start > run and type regedit then press enter)on the CAS server or on the front-end Server
2. Now verify that the ValidPorts registry value exist under
3. HKEY_LOCAL_MACHINE\Software\Microsoft\Rpc\Rpcproxy
4. Also confirm that the key includes the NetBIOS and FQDN for all mailbox servers and for each required port which is as you know 6001, 6002 and 6004