Dear Konstantinos Varvaropoulos
Welcome to Microsoft Community.
It sounds like you're having a frustrating issue with Outlook and Microsoft storage. Here are some explanations and some steps you can take to resolve this issue:
Understand your storage limits:
Outlook.com email storage: You have 15 GB of free storage dedicated to Outlook.com email.
Microsoft cloud storage: You have 5 GB of free cloud storage, which is shared between OneDrive, attachments in Outlook.com, and other Microsoft 365 apps(Word, Excel, PowerPoint or OneNote.etc).
Why is there a difference?:
Attachments in Outlook emails count toward your OneDrive storage limit. This means that even though your email storage shows as 4.6 GB, the attachments in these emails count toward your 5 GB OneDrive limit.
If you click Save all to OneDrive below the attachments in Outlook.com, then these attachments may be stored both ways in your Microsoft cloud storage.
Steps to resolve the issue:
Since you mentioned that the space is all from Outlook attachments, then we mainly need to clean them up.
Delete large attachments: Go through your emails and delete any large attachments that you no longer need. You can also save these attachments to local storage before deleting them from the email.
Empty the Deleted Items and Trash folders: Make sure to empty your Deleted Items and Trash folders as these also take up space.
The process of cleaning up your emails can be found at: Storage limits in Outlook.com - Microsoft Support
If you need further assistance, feel free to ask. I am here for you!
Thank you for your time and understanding.
Kind Regards,
Hardyjoe.J|Microsoft Community Support Specialist