According to OMC’s data, a full 19 percent of all “conversational” email is now tracked. That’s one in five of the emails you get from your friends. And you probably never noticed.“Surprisingly, while there is a vast literature on web tracking, email tracking has seen little research,” noted an October 2017 paper published by three Princeton computer scientists. All of this means that billions of emails are sent every day to millions of people who have never consented in any way to be tracked, but are being tracked nonetheless. And Seroussi believes that some, at least, are in serious danger as a result.