you can make a php script that will use gd to output a blank 1x1 image and have it track the number of times its viewed and log ip's to prevent it counting the same person opening the email twice. then in your email put <img src="http://www.yoursite.com/track.php">
inside the image, just put whatever logging code you want and make sure to output the header for content type and then push the image with imagepng()