A tracking tool puts a small invisible image or redirected links into the email. When your mail app loads the image or you click a link, the tool records it.
If the email was sent with HawkOpen tracking, it ends with the notice “This message contains HawkOpen tracking elements.” and the link “Do not track my activity”, with which you refuse tracking by that sender’s company. HawkOpen tracks you only if your consent for that company is recorded, and classifies an image load at most as likely human activity, never as proof of reading.
To stop tracking images from reporting opens, turn off automatic image loading in your mail app.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.