From the BBC:
The use of "invisible" tracking tech in emails is now "endemic", according to a messaging service that analysed its traffic at the BBC's request.
Hey's review indicated that two-thirds of emails sent to its users' personal accounts contained a "spy pixel", even after excluding for spam. Its makers said that many of the largest brands used email pixels, with the exception of the "big tech" firms.
Defenders of the trackers say they are a commonplace marketing tactic. And several of the companies involved noted their use of such tech was mentioned within their wider privacy policies.
Emails pixels can be used to log:- if and when an email is opened- how many times it is opened- what device or devices are involved- the user's rough physical location, deduced from their internet protocol (IP) address - in some cases making it possible to see the street the recipient is onThis information can then be used to determine the impact of a specific email campaign, as well as to feed into more detailed customer profiles.
Tracking pixels are typically a .GIF or .PNG file that is as small as 1x1 pixels, which is inserted into the header, footer or body of an email.Since they often show the colour of the content below, they can be impossible to spot with the naked eye even if you know where to look.Recipients do not need to click on a link or do anything to activate them beyond open an email they are embedded in.
Addendum: an anonymous reader offered this relevant link re blocking these spy pixels.