Microsoft 365 backups – what happens to the backup data of old users once they’ve left the company and their accounts are deleted?

A good backup strategy must include the ability to retain the backup data of employees who have left an organization. This article examines how BackupAssist 365 handles this once a user has been deleted from Microsoft 365 or Office 365.

By default, after you delete a user from Microsoft 365, that user’s data is retained for a period of 30 days. Unless you take specific actions, after the retention period the data is deleted forever. 

Microsoft outlines how to delete a user, stop paying for their license, and choose what to do with their email and OneDrive content

However, BackupAssist 365 has you covered by default, keeping the backup data of deleted users. 

TL;DR summary

  1. The backup data of deleted users is retained and kept intact by default in the
    BackupAssist 365 backups. This applies to Mailboxes, OneDrive for Business storage accounts, and SharePoint documents. 
  2. There are no extra licensing costs for retaining data of deleted users. 
  3. You can archive or manually delete old users’ data from the backups, if required by your organization’s data handling policies. 


BackupAssist 365 creates backups of Microsoft 365 mailboxes by storing the data as PST files in the backup directory. 

There are two typical scenarios that system administrators adopt after an employee leaves a company: 

  1. The user’s mailbox is converted to a shared mailbox, and the user license is unassigned from the user. 
  2. The user license is unassigned from the user, and the user is deleted. 

In scenario #1, if the mailbox is converted to a shared mailbox, it will continue to be backed up provided your backup is configured to include “All user mailboxes”. Any future email arriving to that mailbox will be included in the backup. Under the licensing for BackupAssist 365, backing up shared mailboxes is free, so that means the former employee’s mailbox will continue to be backed up, even after the employee has left the company. 

In scenario #2, the mailbox is deleted from Microsoft 365, but the last backup of that mailbox will remain in the backup directory. That PST file will not be modified in further backups. 

In both scenarios, you will retain all the backup data for the deleted user’s mailbox. 

You may choose to archive the PST files associated with deleted users or delete them altogether, if required by legislation such as the right to be forgotten. However, we find that the most common scenario is that user data must be retained. 


Deleting a user from a Microsoft 365 tenant has no effect on the documents that user created in SharePoint. 

Your SharePoint backups will continue to work as if nothing happened. 

OneDrive for Business 

Once a user’s Microsoft 365 license has been unassigned, BackupAssist 365 will no longer attempt to back up that user’s OneDrive for Business account. 

That means the directory containing the backups for that user’s OneDrive for Business account will no longer be written to – in effect it’s treated as “frozen” and kept indefinitely. In other words, it lives forever in the backup. 


  • Backup data of the account will not be subject to any data retention rules that you may have set (such as keeping the past 50 versions, or keeping 365 days of history). 
  • Files from deleted users are available for immediate restore. When you restore data, you can choose “Last available backup”, instead of having to choose a point in time where that user last existed.  

If for some reason (such as the right to be forgotten) you need to delete the user’s OneDrive for Business account data, you can do so easily by manually deleting the appropriate folder from the backup directory. 


Without BackupAssist 365, a mistake by the system administrator could result in permanent data loss, which could be very costly for a business. 

With BackupAssist 365, the data of former employees is retained by default, and it’s done at no extra cost. That’s why backing up Microsoft 365 with BackupAssist 365 is a great way to protect your business from data loss. 

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