Problem description
Admins would like to back up only the Exchange Online In-Place Archive Mailbox instead of both - Primary and In-Place Archive mailboxes.
Admins may have a hybrid set up between Exchange Online and Exchange On-Premises, where they have their Primary Mailbox on the On-Premises and the Archive Mailbox in the Exchange Online In-Place Archive; and they wish to back up just the Exchange Online In-place Archive.
In such a scenario, the backup of Exchange Online User fails with: MailboxNotEnabledForRESTAPI
Traceback
The following error can be seen in the User backup log, which can be downloaded from the inSync Admin Console -> click on Users -> select the user -> click on ellipsis -> download Debug Logs -> unzip the downloaded logs and check the logs for Exchange Online
SyncError: Unable to read file. Permission Denied Response Code: 404 Text:{"error":{"code":"MailboxNotEnabledForRESTAPI","message":"The mailbox is either inactive, soft-deleted, or is hosted on-premise."}} (#100000046)
Resolution
This scenario is Unsupported.
It is NOT possible to back up only the Exchange Online In-Place Archive Mailbox without backing up the Primary Mailbox.
This is because, the In-Place Archive Mailbox is linked to the Primary Mailbox in Exchange Online and is not returned alone, in Rest APIs.
Similarly, in a hybrid setup; Exchange Primary Mailbox residing on the On-Premises cannot be returned by Microsoft in the Rest API calls. Thus, the backup of only the Exchange Online In-Plae Archive too, would fail.
This information is also shared by Microsoft in one of their documents here: Use the Outlook mail REST API
📝 Note
The API does not support accessing in-place archive mailboxes, not on Exchange Online nor on Exchange Server.