Spy pixels or tracking pixels are small images (often in GIF or PNG format with dimensions of 1×1 px) that are inserted into the body of HTML e-mails we receive in such a way that they are impossible to recognise with the naked eye.
Companies that use them claim that this is a common marketing tactic, which is often stated in their privacy policies.
It is these accesses that allow the owner of the server to know:
Whether an email has been opened or not, at what times it is opened and how many times it has been opened in total, from which device(s), are invisible and tell-tale.
Link to the news: https://cutt.ly/vlUPLh5
