Outlook Anywhere RPC over HTTP

Symptoms:

Attempting to ping RPC endpoint 6001 (Exchange Information Store) on server server.contoso.com.
The attempt to ping the endpoint failed.
Additional Details An RPC error was thrown by the RPC Runtime process.
Error 1818 CallCancelled

Source

On the Mailbox servers: a DWORD entry needs to be created on each Mailbox server named "Do Not Refer HTTP to DSProxy" at HKLM\System\CCS\Services\MSExchangeSA\Parameters\ and the value set to "1".
Next, as indicated earlier, the RPCProxy will block access to the DC servers unless there servers are included in the ValidPorts regkey.
So, set the following on the Client Access Servers:
1.The ValidPorts setting at HKLM\Software\Microsoft\RPC\RPCProxy needs setting so that the entries referring to 6004 point to DC servers in addition to the mailbox server.
2.The PeriodicPollingMinutes key at HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeServiceHost\RpcHttpConfigurator\ needs setting to zero to prevent RpcHttpConfigurator from updating the Valid Ports key automatically.
Finally, you need to make sure that the DCs are listening on port 6004:
On the Global Catalog servers: a REG_MULTI_SZ entry needs to be created on each GC named NSPI interface protocol sequences at HKLM\System\CCS\Services\NTDS\Parameters\ and the value set to ncacn_http:6004.
There is 1 last thing to deal with in this SSL-ID load balanced configuration.
Outlook profile creation hard codes a call to DSProxy on 6004.
Which means that we can get split connectivity during profile creation.
To deal with this minimal volume of traffic, there is 1 final regkey that should be set on the mailbox servers:
On the Mailbox Servers - set the HKLM\System\CCS\Services\MSExchangeSA \Parameters key "NSPI Target Server" to the FQDN of the DC that profile creation should use.
By using only 1 DC for profile creation, all DSProxy calls will be proxied into that single DC, once again avoiding split connectivity.

Source