Managing personal Outlook.com account settings, security, and privacy
I think deleting the 'sent' folder is the key. I totally cleaned out my one drive/outlook storage etc. Even though there were loads of emails that I would like to have kept. I just lost patience with it. But I found I had 12,000 items in my 'sent' folder. Removing then look me down below the stupid threshold. I really resent having to delete all my correspondence though and will probably take my account elsewhere.
It's still saying I'm 82% full and there is literally NOTHING left to delete. I deleted all from the 'deleted' folder too. Sigh. It now says "Enjoy your empty inbox' Total money grab by Outlook.