its an old blackhat marketing trick, really, you put a one pixel by one pixel dot img in emails you send out, with the name of the img tagging the email address for each spam, and by checking usage logs on the imgs, lets you know who opened the email and viewed it as well as forwarded it, and lets you tag an ip to each email address for futher customization of advertising. its why most email programs warn you about an email having images on an external site, do you want to load?