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You can use our connector with Delegation Permission: Microsoft Exchange Online - Microsoft 365 (Delegation Permission)
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In Outlook, select File >Account Settings >Account Settings. You will see a list of your email accounts.
Select the Microsoft Exchange account that you use to manage someone else's calendar and choose Change....
Choose More Settings, followed by the Advanced tab.
Select Uncheck the checkbox next to: Turn on shared calendar improvements., see the printscreen below:
Restart Outlook and try again to add the calendar.
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