Delete one email that you don’t need. I know, you can probably delete more, but if you put in your mind that you need to delete a dozen or a hundred old emails, the thought can be so overwhelming that you won’t be able to motivate yourself to start.
However, if your goal is one email, then you succeed after deleting that one email. And eliminating one email is better than none. (You can do more, but it's not your goal.)