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.

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 on

British Airways, TalkTalk, Vodafone, Sainsbury's, Tesco, HSBC, Marks & Spencer, Asos and Unilever are among UK brands Hey detected to be using them.