A good BBC article on the use of spy pixels in emails.
https://www.bbc.com/news/technology-56071437
This "feature" goes back to the early days of emails with rich text and features like graphics in emails. You can turn off the graphic images and usually can see the text you need to at least understand the email but sometimes functionality doesn't work without the graphics (things like clicking on buttons, etc.). I see they mention this option in the article.
It's so widespread that I'm not sure of any other way to stop it except by government action or users individually demanding or as a group that companies stop using this invasive practice.
Tutanota's secure email service blocks these images by default. You have to click a button to see the images which isn't a bad way to do it. If you do trust an email source, there is no way to enable automatically only that address which could be annoying.
It'd be better if these tracker pixels were just blocked by email clients altogether. I don't know how it is now but originally they were just transparent pixels or just so tiny you couldn't see them and separated from other graphics. It seems easy to block but it might be more complex than I imagine. Of course, you could just block anything in an email that sends back data to a server without the user's explicit consent. However, even getting graphics from a website wouldn't work unless that was allowed and the fetch command to do this would enable some tracking. Images can be embedded in emails though without having to be fetched from a server so there's really no reason to do this other than tracking or getting emails downloaded on very slow internet connections.