When I first heard about these trackers on here, I was curious how they worked. The one I read about used a small image file that would not be visible in the email (think like a 1 pixel image that would look like a tiny black or white dot). The image file is stored on a remote server. When the email is opened, the image file would be accessed and the tracker would know when and where the web server traffic came from. I'm guessing that if the email recipient doesn't have HTML turned on for email, the tracker probably wouldn't work. If the recipient was suspicious and tech savvy, they could examine the source of the email and find the embedded image file. They could also note the packets being sent from their computer and if they found an image file being retrieved from emailtracker.com they could figure out what's going on. I doubt most people would be able to figure this out, but it also wouldn't be hard to get a tool to defend yourself against being tracked like this.