Here. Excerpt:
* * * Trackers, which come in many forms including a single invisible pixel inserted into an email or the hyperlinks embedded inside a message, are frequently being used to detect when someone opens a message and even where that person is when the email is opened. By some estimates, trackers are now used in as much as 60 percent of all sent emails.
* * *
Yet, the prevalence of these trackers raises consumer questions. Because trackers are invisible, many people are unaware of them and have no inkling of how to dodge them. “It’s definitely a privacy concern,” said Cooper Quintin, a technologist and privacy advocate for the Electronic Frontier Foundation. “There’s no mechanism for people to opt out.”
