With Five Links for example, I can see when you opened the email (or that you haven’t opened it yet). I can see that perhaps you opened it six times before clicking. I can see which links you clicked, and so I can track which links were most popular. And this is all the default offered by my newsletter provider - if I really wanted to, I could add more and more tracking features to do more and more spying.

Read Five Links Issue #017 here