Hi Thomas Nemeth,
Good day.
Thank you for posting to the Microsoft Community. We are glad to assist.
May I know what version of Outlook you have? What version of Outlook do I have? - Microsoft Support. Do you get an error message when you try to delete emails if you do please share a screenshot with us.
Firstly I advise you to reach out to your admin to help you delete using PowerShell since you have already tried several measures. How do I find my Microsoft 365 admin? - Microsoft Support
In the meantime here are the steps to clean up your mailbox in Outlook that you can try on your own:
- Open Outlook and click on the File tab.
- Click on Tools and then click on Mailbox Cleanup.
- You will see a list of options to help you clean up your mailbox. You can view the total size of your mailbox and individual folders within it, find items older than a certain date or larger than a certain size, archive items by using AutoArchive, view the size of your Deleted Items folder, and empty your Deleted Items folder, and view the size of your Conflicts folder and delete this folder.
- Choose the option that best suits your needs and follow the prompts to complete the cleanup process.
Your admin can also Get support - Microsoft 365 admin | Microsoft Learn the support team there will have access to assist remotely and collect logs to investigate the issue from the back end.
Should you have any questions or concerns please do not hesitate to contact me. Look forward to hearing from you.
Please understand that our initial response may not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.
Sincerely,
Microsoft Community Moderator