There's the email hack where you can insert an image and check the server logs later to see if (and how many times) it was viewed. Its a hack, and not a feature, for a reason. :\
Most sane email clients have automatic loading of remote content disabled by default. The iOS client doesn't. There are loads of similar tricks to the img tag one. Many clients which attempt to disable the automatic loading of remote content fail in different ways. I built an automated web based tool which sends you a specially crafted email which attempts many of these tricks. You can see it here: https://emailprivacytester.com/