“[Email tracking] tech is pretty simple. Tracking clients embed a line of code in the body of an email—usually in a 1×1 pixel image, so tiny it’s invisible, but also in elements like hyperlinks and custom fonts. When a recipient opens the email, the tracking client recognizes that pixel has been downloaded, as well as where and on what device. Newsletter services, marketers, and advertisers have used the technique for years, to collect data about their open rates; major tech companies like Facebook and Twitter followed suit in their ongoing quest to profile and predict our behavior online…
…To prevent third-parties from leaking your email, meanwhile, Princeton’s Englehart says “the only surefire solution right now is to block images by default.” That is, turn on image-blocking in your email client, so you can’t receive any images at all.”
—Brian Merchant. “How Email Open Tracking Quietly Took Over The Web.” Wired. December 11, 2017.
As discussed in my post A Text Only World there is no surefire way to stop this kind of tracking. Even if you use text only email, which isn’t a bad idea, you will still be tracked if you follow links and so forth. But, sticking with text over HTML is often a more secure and less convenient option.