Exchange 2010 On Premesis accessing Office 365 Mailboxes

Posted by robd on December 04, 2019
Office 2010, Office 365, Outlook 2010

Had a very weird issue where users on prem couldnt access mailboxes for users online even though the permissions were correct.

The error was:

Had a good google and the recommendations were to do the following which didnt work for me:

Connect to 365 and run:

So what I did to fix this was to add this for

  1. Exit Outlook.
  2. Start Registry Editor. To do this, use one of the following procedures, as appropriate for your version of Windows:
  3. In Registry Editor, locate and then click the following registry subkey:
    HKEY_CURRENT_USER\Software\Microsoft\Exchange
  4. On the Edit menu, point to New, and then click DWORD Value.
  5. Type AlwaysUseMSOAuthForAutoDiscover, and then press Enter.
  6. Right-click AlwaysUseMSOAuthForAutoDiscover, and then click Modify.
  7. In the Value data box, type 1, and then click OK.
  8. Exit Registry Editor.

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