Using classic Outlook for Windows in business environments
You have to clean up Onedrive. Go there and remove, either by deleting or moving the files to your folders on your computer, to free up space. In my email account, I deleted the majority of emails, emptied most folders, and always keep deleted and junk folders empty. I also turned off One Drive, so it doesn't save every single thing on my computer. This seems to have done the trick.