Email senders can use a trick called a 'tracking pixel' to know exactly when you open their emails, just like a read receipt.
These pixels are not only hard to detect, but they also give away lots of personal information!
Using this website, you can receive an email containing my implementation of a tracking pixel to see examples of the information you're giving away simply by opening emails.
This website stores none of your personal information. It's totally stateless with no cookies, databases, logs, or anything else! You can check the source code on my GitHub.