Your email client sent three emails, with different "hidden image file" urls. The cheesiest version just makes the image url be something like: https://exampl

Your email client sent three emails, with different "hidden image file" urls. The cheesiest version just makes the image url be something like: https://example.com/trackingpixel.png?email=dhbanes%40example.net https://example.com/trackingpixel.png?email=dhbanes%40exampl... and rewrites that query parameter for each of the To: Cc: and Bcc: addresses. A real implementation will more likely use a UUID instead of just plonking the email address in there, and store that uuid -> email mapping sevrerside.