How Can a Newsletter Track You
There are fundamentally two ways a newsletter can track you:
They embed an image in between the text, but that image is unique to each recipient. When the image loads, the sender of the newsletter knows exactly who opened it.
The links included in the newsletter don't go to the final destination directly. Instead, they go to an intermediate address that is unique to each recipient. Therefore, instead of going to google.com you go to mynewsletter.com/your-unique-id/google.com With this it is straightforward to know who clicked on what.
These are the other notes that link to this one.