Outlook doesn’t connect to mailbox in Exchange Online after hybrid migration.

I have been working on a Hybrid Migration where several users weren’t able to have their Outlook client connect to the mailbox after it migrated to Exchange Online. The mailbox move completes successfully the Outlook client gives a popup that an Administrator has made a change and to close and re-open Outlook. Unfortunately when the user opens Outlook it is stuck trying to connect to the On-Premise Exchange server and doesn’t update. The Outlook version is the ProPlus (Click to Run) Microsoft 365 version that comes with their Office 365 E3 license.

If the user creates a new Outlook profile it is able to connect with the mailbox in Exchange Online. I then asked to switch back to the original Outlook profile and it was successful in connecting to the mailbox in Exchange Online.

Something in the Outlook client was getting updated when a new profile was created that then allows the old profile to work. After testing on a few different machines I found adding the following registry key meant the original Outlook profile would connect without creating a new profile.

  1. Open Registry Editor: Press Windows Key + R to open a Rundialog box. Type “regedit” and then press Enter.
  2. In Registry Editor, locate HKEY_CURRENT_USER\Software\Microsoft\Exchange
  3. Create a new DWORD Value “AlwaysUseMSOAuthForAutoDiscover”
  4. Set the value to 1
  5. Exit Registry Editor
  6. Start Outlook

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