Office 365 Migration  - Internal Outlook clients are pointing to On-premise exchange server

The migration was completed successfully, all the MX and auto-discover records are set to office 365 in our DNS portal, We tested that the Outlook clients are connecting from outside the organization are able to auto-discover the mailbox settings and able to connect to their Office365 mailbox.

Now the problem started, when the on-premise users are trying configure their outlook client, it is routing to their On-premise exchange server.

What to do ? just two steps to go , to make sure your on-premise clients connect to their Office 365 mailbox.

  1. Create a CName record for Auto Discover in your internal DNS pointing to the office 365 auto discovery server created in your Public DNS portal.
    From outside your on-premise network ping ( Note: should be replaced with your organization namespace), note down the IP address it is pointing , then come back to your local DNS and create a A record pointing to the IP which you noted down. ( should point to the IP which was resolved during ping operation from outside network).
  2. Once one of the above record is created in your network, try to ping from your internal network and check if it is pointing to autodiscover server.
  3. Now you need to make your AutoDiscoverServiceInternalURI to null on your CAS server, use below command to make it null.

    Set-ClientAccessServer –Identity “your CAS servername” –AutoDiscoverServiceInternalUri $NULL
Noe go back and try creating the outlook profile again , now it should discover the Office365 mailbox server settings with out any issues.


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