You can exclude a mailbox from backups of an Exchange Online app.
Excluding a mailbox does not delete the mailbox from the app, but the mailbox is not backed up.
Procedure
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From the navigation pane, go to Protect > Office 365.
The Office 365 Overview page appears.
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On the Apps tab, click the app.
The Overview page of the app appears.
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In the row of the mailbox that you want to exclude, click the action button
, and then select Manage > Exclude from backup.A confirmation dialog box appears.
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Click Yes.
Tip
To see excluded mailboxes, on the Mailboxes tab, click the gear button 
, and then select Clear all filters. In the table heading row, click any Column Settings button 
, and then select Columns > Status. The Status column is added to the table. Excluded mailboxes have a status of Do Not Backup.