“I JUST Stumbled on this email,” began the message, a lengthy 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 informed 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 provided me with maybe a little too many details. And I certainly wasn’t alone.

There are several 269 billion emails sent and received daily. That’s roughly 35 emails for everyone on the planet, every day. Over 40 percent of the emails are tracked, based on 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 your body of your email-usually in a 1×1 pixel image, so tiny it’s invisible, but also in elements like hyperlinks and custom fonts. Whenever a recipient opens the email, the tracking client understands that pixel has become downloaded, as well as where as well as on what device. Newsletter services, marketers, and advertisers used the technique for many years, to collect data regarding their open rates; major tech companies like Facebook and Twitter followed suit within their ongoing pursuit to profile and predict our behavior online.

But lately, an unexpected-and growing-number of tracked emails are being sent not from corporations, but acquaintances. “We happen to be in contact with users that were tracked by their spouses, business partners, competitors,” says Florian Seroussi, the founder of OMC. “It’s the wild, wild west on the market.”

According to OMC’s data, an entire 19 percent of all the “conversational” email is currently tracked. That’s 1 in 5 in the emails you receive from your friends. And also you probably never noticed.

“Surprisingly, while there is a huge literature on web tracking, email tracker gmail free has seen little research,” noted an October 2017 paper published by three Princeton computer scientists. All of this signifies that vast amounts of emails are sent every single day to thousands of people who may have never consented in any respect to be tracked, but are being tracked nonetheless. And Seroussi believes that some, at the very least, have been in serious danger as a result.

As recently since the mid-2000s, email tracking was almost entirely unknown for the mainstream public. Then in 2006, a young tracking service called ReadNotify made waves when a lawsuit revealed that HP had used the product to trace the origins of any scandalous email which had leaked for the press. The intrusiveness (and simplicity) in the tactic came as something of any shock, despite the fact that newsletter services, salespeople, and marketers had long used email tracking to assemble data.

Seroussi says that Gmail was the ice breaker here-he points to the period when sponsored links first started turning up within our inboxes, based upon tracked data. During the time it seemed invasive, even unsettling. “Now,” he says, “it’s common knowledge and everyone’s fine with it.” Gmail’s foray was the signal flare; when advertisers and salespeople realized they too could send targeted ads according to tracked data, with little lasting pushback, the practice grew more pervasive.

“I do not know of a single established sales team in [the online sales industry] that will not use some form of email open tracking,” says John-Henry Scherck, a content marketing pro and the principal consultant at Growth Plays. “I think it will be a point of time before either everyone uses them,” Scherck says, “or major email providers block them entirely.”

That’s partly concerning spam. “Competent spammers will track any activity on your email because they have a tendency to buy entire lists of addresses and will actively try to rule out spam traps or unused emails,” says Andrei Afloarei, a pnifcc researcher with Bitdefender. “If you simply click any link in a single of their messages they are going to know your address has been used and can actually cause them to send more spam the right path.”

But marketing and web-based sales-even spammers-are no longer accountable for the majority of the tracking. “Now, it’s the key tech companies,” Seroussi says. “Amazon has been using them a lot, Facebook has become using them. Facebook is the main tracker besides MailChimp.” When Facebook sends you an email notifying you about new activity on your own account, “it opens an app in background, and today Facebook knows where you are, the product you’re using, the last picture you’ve taken-they get everything.”