Tenant admins can delete a mailbox from an Exchange Online app. (Tenant users do not have the ability to delete Exchange Online mailboxes, folders, or emails.)
If you added the Active Directory group to the Exchange Online app, and the mailbox is included in the Active Directory group, then the mailbox will be added back to the app when the next backup runs.
Deleting All Data for a User
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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 table, click the app that contains the mailbox.
The app page appears.
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Remove a mailbox from backups or delete data based on the following requirements:
- To remove a mailbox which is manually added to backup content from further backup operations, click the action button
in the row of the mailbox, and then select Manage > Remove from content. The status of these mailboxes is updated as Deleted and the discovery type remains unchanged.
Important
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Users who have mailboxes in the Deleted status do not consume an application user license.
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The back-end mailbox sync process that runs once a month, will mark all such mailboxes as soft deleted. After which, their retention timer starts based on the retention value specified in the O365 plan. Once the retention value is met, the emails are permanently pruned from backups.
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To permanently delete all the data that has been backed up, click the action button
in the row of the mailbox, and then click Manage > Delete backup data. The data will no longer be available for browsing and recovery.A confirmation dialog box appears.
- To remove a mailbox which is manually added to backup content from further backup operations, click the action button
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Click Yes.
Tip
To see deleted 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.
Deleting Mailboxes, Folders, and Messages
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From the navigation pane, go to Protect > Office 365.
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The Office 365 apps 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 table, click the app that contains the mailbox.
The app page appears.
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On the Mailboxes tab, in the row for the mailbox that contains the backed-up content that you want to delete, click the Actions button
, and then select Restore > Restore messages. The mailbox contents appear.
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Select the mailbox, folders, or messages that you want to delete:
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To delete a mailbox, in the left pane, select the mailbox, and then at the top of the page, click Delete.
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To delete a folder or a sub-folder, in the left pane, expand the mailbox and/or sub-folders, select the folders you want to delete, and then at the top of the page, click Delete.
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To delete messages, either expand folders in the left pane to locate messages in the right pane, or in the Search box, enter search terms to find the content you want to delete, and then at the top of the page, click Delete.
The Confirm delete dialog box appears.
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In the box, type DELETE, and then click Delete.