How does my email tracking service know WHO opened my email?

Question

I've been using an email client that has an email open/click tracking feature for a while. I like to track opens for certain types of emails like sales or invoices.Somehow, when I send an email to multiple recipients using either multiple TOs, CCs, or BCCs (or a combination thereof), my tracker seems to know which individual opened the email each time is it opened.For example, I sent an email this afternoon to one TO recipient and two BCCs. One of the BCCed parties opened the email and the tracking software was able to determine that it was that specific BCC recipient rather than one of the other recipients.Since the tracking is achieved with a hidden image file and the email is one single entity, how is this achieved?

bigiain · Accepted Answer

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%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.

neximo64 · Answer

Each one gets a different image in the email, when they open the email it fetches the image, so the tracking service knows they opened the email.The image is usually a 1px by 1px transparent png at the end of the email