Can't see the Calendar folder of an online archive mailbox by using Microsoft Outlook Web App
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Original KB number: 4341449
Assume that you have a cloud mailbox, and Exchange Online Archiving is enabled for the mailbox. In Outlook Web App (Microsoft Outlook Web App), you can't see the Calendar folder of the online-archive mailbox.
The behavior is by design.
Use the Outlook client to access calendar items in the online-archive mailbox.