When I wrote email tracking software about 7 years ago, I dropped the 1x1 pixel, because of spam filters tripping over them. I just used any image in the email to track you and assigned it an unique URL. How do you propose catching that one?
At the moment I just scan for the most commonly known sources of tracking and then things that seem to be tracker images. The logic is quite simple, definitely can be improved to pick up a lot more tracking sources. But for now it works quite well :)