From Here
Up to 50 percent of all spam sent in the last year came with hidden HTML code that spammers embedded in their messages in an effort to fine-tune their distribution lists and send out even more junk e-mail.

When a user opens or even previews an message containing a spam beacon, it signals the spammer, validating the address as live and helping spammers separate the "good" addresses from the "bad."

"Millions of users are unaware that spammers have the ability to track when they view and open their e-mail," said Scott Chasin, the chief technology officer of MX Logic, in a statement. "This reinforces the fact that spammers are using increasingly deceptive tools to invade end users' privacy and harvest valid email addresses."
Even more 'good' news about spam (sic).