Brian Merchant:

“I just came across this email,” began the message, a long overdue reply. But I knew the sender was lying. He’d opened my email nearly six months ago. On a Mac. In Palo Alto. At night.
 I knew this because I was running the email tracking service Streak, which notified me as soon as my message had been opened. It told me where, when, and on what kind of device it was read. With Streak enabled, I felt like an inside trader whenever I glanced at my inbox, privy to details that gave me maybe a little too much information. And I certainly wasn’t alone.
 There are some 269 billion emails sent and received daily. That’s roughly 35 emails for every person on the planet, every day. Over 40 percent of those emails are tracked, according to a study published last June by OMC, an “email intelligence” company that also builds anti-tracking tools.
 The 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.