Originally Posted by
gfunkdave
Many such images have custom URL components unique for each user - if that particular URL is fetched then the marketer knows that that user's copy of the image has been requested.
But that request will be made when the mail arrives at Google, not when the user reads it. Thus it doesn't actually tell them anything useful other than it's a valid address (and Google might be fetching the images for invalid addresses just to defeat even this.)