This is using a single MS Exchange server 2003, in MS Windows 2003 R2 AD domain. I tried to setup rpc over https. After that, I browse to testconnectivity.microsoft.com to check, and it failed with NSPI port 6004 with error message:
Testing the Name Service Provider Interface (NSPI) on exchange mailbox server
An error occurred while testing the NSPI RPC endpoint
Attempting to ping NSPI RPC Endpoint 6004 (NSPI proxy interface) on server exch1.internal.local. The attempt to ping the endpoint failed.
The RPC_S_SERVER_UNAVAILABLE error (0x6ba) was thrown by RPC runtime process.