Unable to back up Microsoft 365 user emails.
Your administrative user needs to implement the User Impersonation in Microsoft's Exchange Online.
How to configure Exchange Online Impersonation:
1. Log in as administrator at https://portal.microsoftonline.com/
2. Go to the ‘Exchange” Admin section.
3. In ‘Exchange admin center’ go to Permissions> Admin roles and choose Discovery Management
4. On the Discovery Management screen, the ‘ApplicationImpersonation’ role should be added (+ button), and the admin should be a member.