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

The error was:

Cannot expand the folder. The set of folders cannot be opened.

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

Connect to 365 and run:

#remove permissions
Remove-MailboxPermission -Identity Manager@bohemiangrove.co.uk -User User1@bohemiangrove.co.uk -AccessRights FullAccess

#Add permissions
Add-MailboxPermission -Identity Manager@bohemiangrove.co.uk -User User1@bohemiangrove.co.uk -AccessRights FullAccess -InheritanceType All -AutoMapping:$false

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.