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Dear Hans,
May I know whether you have found an answer?
- Export the emails to the .pst files. Then you can upload them to OneDrive for Business and you can download them and import them to Outlook at any time. I think that it is the simplest way to back up the emails.
Export emails, contacts, and calendar items to Outlook using a .pst file - Microsoft Support
- The Power Automate way you mentioned isn't an actual way for backing up the emails. It is just copying emails as the .eml files to OneDrive for Business which will not reduce the Outlook storage. The backup process is to use the Export way in Outlook. Then you can safely delete them. If you delete them and don't back up them, your emails data will be lost.
Hope that the information above helps you!
Thanks for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator