You can add all mailboxes to the backup content for Exchange Online so that they are automatically discovered when new mailboxes are added.
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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On the Content tab, in the upper-right area of the page click Add.
The Configure Content page appears.
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Select Add content to backup, and then click Next.
The Add Content tool appears.
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Under Standard, select All mailboxes, and then click Next.
The Content tab appears.
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Click Next.
All mailboxes that are discovered will be added to the content. When the discovery job is complete, the Office 365 Plan tab appears.
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From the Office 365 plan list, select the plan to use for all mailboxes, and then click Next.
The Summary tab appears.
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Click Submit.