I was getting the following error on an Exchange 2007 SP1 server when trying to run the https://www.testexchangeconnectivity.com/ for outlook anywhere.

ExRCA is hard the Name Service Provider Interface (NSPI) on the Exchange Mailbox server.
An error occurred while hard the NSPI RPC endpoint.
Test Steps
Attempting to ping RPC Endpoint 6004 (NSPI Proxy Interface) on server WATCEXCH.waturf.local
The attempt to ping the endpoint failed.
Tell me more about this issue and how to resolve it
Additional Details
RPC_S_SERVER_UNAVAILABLE error (0x6ba) was thrown by the RPC Runtime

The problem turned out to be Windows Server 2008 has made TCP/IPv6 the defaulting communication protocol stack over which relations are made by clients connecting to the server that is running Microsoft Exchange. The RPCProxy component tries to connect to the DSProxy component through port 6004 over TCP/IPv6. But, the DSProxy component does not listen on the TCP/IPv6 stack, which causes connection requests from the RPCProxy component to fail.

Perform the following procedure:

1.Under Network Relations, select the network adapter, and then click Properties.

2.In the properties window, click to clear the check box for Internet Protocol Version 6 (IPv6).

Note:
Clearing this check box causes the RPCProxy component on the Client Access server to use TCP/IPv4 to talk to the DSProxy component on the Mailbox server.
3.Click Start, and then click Run.

4.Type regedit in the Open box.

5.By Registry Editor, locate the following registry key:

HKEY_Local_Machine\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters

6.Right-click the Parameters key, click New, and then click DWORD (32-bit) Value. For the key, add the following values:

Name: DisabledComponents

Data: 0xFFFFFFFF

7.Restart the Client Access server.

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