Originally Posted by
gfunkdave
having its servers get these images for you, so that Google can scan them for known malware and hide your IP from the email marketers.
Well that's a start, but isn't it missing the mark? Don't the embedded images contained references to the destination email address? That is, a unique serial number in the URL is linked to the email address the marketer/spammer is trying to reach. And displaying the image validates that the destination email address is still working, right? If I was a spammer, I'd ensure that an image could not be retrieved merely by shortening the URL.
In any case, great info and thanks for sharing!