MAPI Referral Service enpoint RPC_S_SERVER_UNAVAILABLE error

Trying to perform a cutover migration from Exchange 2003 to Exchange Online. Have run testconnectivity.microsoft.com many times, read through tons (more than a few times), gone through lots of troubleshooting but still ending with the same failure. Interestingly, Outlook 2007 does work using the RPC/HTTP configuration without issue. https://mail.exchsvr.com/rpc/rpcproxy.dll returns blank screen. Hoping that someone can provide some insight. Feeling like it is some small detail that is missing, but it has completely eluded me. Below is the latest result from testconnectivity:

 	Testing Outlook connectivity.
 	The Outlook connectivity test failed.
 	 	Additional Details

 	 	Test Steps
 	 	Testing RPC over HTTP connectivity to server mail.exchsvr.com
 	RPC over HTTP connectivity failed.
 	 	Additional Details

 	 	Test Steps
 	 	Attempting to resolve the host name mail.exchsvr.com in DNS.
 	The host name resolved successfully.
 	 	Additional Details

 	Testing TCP port 443 on host mail.exchsvr.com to ensure it's listening and open.
 	The port was opened successfully.
 	 	Additional Details

 	Testing the SSL certificate to make sure it's valid.
 	The certificate passed all validation requirements.
 	 	Additional Details

 	 	Test Steps

 	Checking the IIS configuration for client certificate authentication.
 	Client certificate authentication wasn't detected.
 	 	Additional Details

 	Testing HTTP Authentication Methods for URL https://mail.exchsvr.com/rpc/rpcproxy.dll?mail.exchsvr.local:6002.
 	The HTTP authentication methods are correct.
 	 	Additional Details

 	Attempting to ping RPC proxy mail.exchsvr.com.
 	RPC Proxy was pinged successfully.
 	 	Additional Details

 	Attempting to ping the MAPI Mail Store endpoint with identity: mail.exchsvr.local:6001.
 	The endpoint was pinged successfully.
 	 	Additional Details

 	Testing the MAPI Address Book endpoint on the Exchange server.
 	The address book endpoint was tested successfully.
 	 	Additional Details

 	 	Test Steps

 	Testing the MAPI Referral service on the Exchange Server.
 	An error occurred while the Referral service was being tested.
 	 	Additional Details

 	 	Test Steps
 	 	Attempting to ping the MAPI Referral Service endpoint with identity: mail.exchsvr.local:6002.
 	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. 
Elapsed Time: 1022 ms. 

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Thanks for any assistance!