Most email sent now includes some form of tracking, intended to tell the sender if the recipient has opened the email, who they forwarded it to, how long it was read, and so on. The techniques used to "spy" on people have moved on some from the era of invisible pixels (though that's still the key method) and even savvy users may not realize their email is revealing things about them.
British Airways, TalkTalk, Vodafone, Sainsbury's, Tesco, HSBC, Marks & Spencer, Asos and Unilever are among UK brands Hey detected to be using them. But their use was much more widespread despite many members of the public being unaware of it, said Mr Hansson.
"It's not like there's a flag saying 'this email includes a spy pixel' in most email software," he added.
Privacy-averse free mail services can never really be trusted, but even traditional offline apps might be inadvertantly betraying you to nosey senders. Thunderbird claims to have everything relevant off by default.
Conversely, this is the same tech that powers data-sharing that your newsletter recipients think they've opted into. If your audience is blocking the tracking in significant numbers—say, your newsletter is for Email Privacy Experts—your stats may be inaccurate.