This warning may be logged when using the Server-Side Synchronization feature in Dynamics 365 to integrate with Microsoft Exchange. Dynamics 365 communicates with Microsoft Exchange using Exchange web services (EWS). If Dynamics 365 attempts to retrieve information for a mailbox, Microsoft Exchange may respond with an ErrorMailboxStoreUnavailable. As documented in the following Microsoft Exchange documentation, this response indicates Exchange could not process the request because the mailbox was not available at the time:
https://docs.microsoft.com/en-us/exchange/client-developer/web-service-reference/responsecode
"This error indicates that one of the following error conditions occurred:
- The mailbox store is corrupt.
- The mailbox store is being stopped.
- The mailbox store is offline.
- A network error occurred when an attempt was made to access the mailbox store.
- The mailbox store is overloaded and cannot accept any more connections.
- The mailbox store has been paused. "