The mailbox database is temporarily unavailable., Cannot open mailbox…”with details like: .Server = SN6PR06MB4621.namprd06.prod.outlook.com .mdbGuid = ...

.mailboxGuid = ...

It means that Exchange Online temporarily locked or offloaded the mailbox database associated with that mailbox. This usually happens due to throttling, load balancing, or too many concurrent connections hitting the mailbox.

This is NOT a permanent failure. It is a temporary block.

Causes:

1. Exchange Online is throttling your requests:

“Too many concurrent connections opened.”

Throttling when processing 5–7 items.

2. Too many parallel EWS requests

Resolution:

Wait and Retry: Wait a few minutes before trying again.