At the BBC’s request, email service “Hey” analyzed its traffic and found two-thirds of emails sent to users contained a spy pixel.

Defenders of the trackers say they are a commonplace marketing tactic. 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.

These pixels are tiny 1×1 images embedded in photos that can track a variety of data points. You could turn off “Load Remote Images” automatically in Settings > Mail, but then of course they would load along with other photos when you want to see them.