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Debug the RPC-Over-HTTP issue with Office 365 Migrations

You’re attempting an Exchange cut-over migration or staged migration and Office 365 is unable to connect to your Exchange on-premises servers. This article will cover your RPCoverHTTP settings.

This article assumes the following:  your Client Access Server is already configured, your Client Access Server has a valid 3rd Party SSL Certificate assigned, HTTPS traffic is being properly directed to your Client Access Server.


This document modifies the registry… ensure you take precautions. BACKUP!!

  1. On your outward facing Client Access Server, launch the registry editor and navigate to the following key
  2. Right click the ValidPorts key and click Modify
  3. Remove the entries that are already in place
  4. You will need to create a single string defining port 6001,6002, and 6004 for the NetBIOS name, internal FQDN, and external FQDN of your CAS server
    1. NetBIOS Name: CAS-1:6001-6002;CAS-1:6004;
    2. Internal FQDN: CAS-1.domain.local:6001-6002;CAS-1.domain.local:6004;
    3. External FQDN: mail.domain.com:6001-6002;mail.domain.com:6004;
    4. Add it all together:
  5. Click OK

For more information for RPCoverHTTP, please see this article:


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