Most email services give you a limited amount of space for storage, most of the time spam gets into your in box. While using outlook you might have received a message that you're running out of space, here's a tip to clean up your inbox.
At the top of outlook, open your tools bar, tap on "accounts", select your email account, tap change which is right above it. When the new window opens, tap the advanced tab to the right. The bottom option will read "remove from server when deleted from deleted items".
Click ok and you're done, your client will sync with the server and delete email that was clogging up the server. Anything that is deleted from outlook will now be reflected on your ISP email website.