Unless they're base64-encoded, images in emails must load off a web server. By customizing the image URLs per recipient, the system serving those images can tell who saw the email.

It's not perfect because:

- Some preview functions load images, even if the email was not actually opened, and

- Some email clients block images from loading by default, so it's possible for the user to open and read the text of the email without triggering an "open" signal.

Clicks are much more reliable signals of intent, but there's far fewer of them.

Mixpanel for example, provides a mechanism [1].

[1] https://help.mixpanel.com/hc/en-us/articles/115004690106-How...

You can quite easily setup tracking, eg using Google Analytics