The mailbox database is temporarily unavailable.
This error message indicates that the mailbox database is offline, corrupt, shutting down, or exhibiting other conditions that make the mailbox unavailable. Confirm that the database where the mailbox resides is online and available.
Note: When the endpoint reporting the error is for Office 365, this error is usually transient. When experiencing this error, we suggest a five-minute wait, and then retry depending on the user count of the tenant.