Some Office 365 Exchange jobs may complete with warnings or fail because one or more mailboxes fail to backup. Looking at the job log we can see a generic error.
Error 2020/07/17 00:02:07 73 Data Plugin NVSERVER Error from library - Check in More Info
email@domain.com : Exception occured - Reason:Object reference not set to an instance of an object
Error 2020/07/17 00:02:07 73 Data Plugin NVSERVER Failed to backup message for email@domain.com
Error 2020/07/17 02:03:03 73 Data Plugin NVSERVER VM [email@domain.com]: Backup failed
Enabling trace will produce the exact error for a particular mailbox
07/17/2020 08:58:18 email@domain.com : Caught Exception not specific: System.AggregateException:
AsyncHelpers.Run method threw an exception. ---> Microsoft.Graph.ServiceException: Code: MailboxNotEnabledForRESTAPI
Message: REST API is not yet supported for this mailbox.
