Microsoft Bookings double-booking staff despite busy calendar entries

This issue often happens in hybrid Exchange setups because Bookings can’t always read the “busy” status correctly from on-prem mailboxes, even if you’ve set everything up to respect availability. Bookings works best when the staff mailbox is fully in Exchange Online, so if you test with a cloud-only user you’ll likely see it behave properly. In a hybrid setup, free/busy sharing can be unreliable, which is why double bookings slip through. If moving mailboxes online isn’t possible right now, a common workaround is to use a cloud resource calendar for Bookings and then forward those events to the on-prem mailbox. That way, Bookings always sees the right availability, and your staff still get their appointments.

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