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