Walden, So when your email client attempts to "draw" the image it requests that
jpg file which makes a request to the server and that's how they know you opened it.
I think this is the more common technique. Originally done with 1 x 1 pixel "images", I've sometimes wondered how often this is done with "legitimate" sized images which would be harder for a web washer style process to strip out. Doug PS - It is also one of the reasons I intentionally continue to use a text only email client which does not render HTML... (except for these lists I now use gmail)