Add the mailboxes that you want to back up to the Exchange Online app.
Procedure
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From the navigation pane, go to Protect > Applications > Office 365.
The Office 365 apps page appears.
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In the Name column, click the app.
The app page appears.
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Click Add, and then click Add Mailbox.
The Add Mailbox dialog box appears.
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From the Office 365 plan list, select the Office 365 plan.
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From the Select mailboxes list, select the mailboxes to add.
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Click Save.