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 on.

This information can then be used to determine the impact of a specific email campaign, as well as to feed into more detailed customer profiles.

Hey's co-founder David Heinemeier Hansson says they amount to a "grotesque invasion of privacy". And other experts have also questioned whether companies are being as transparent as required under law about their use.