If you've had an e-mail address for any length of time, you no doubt noticed a gradual increase in the amount of spam you receive (if not, you're one of the lucky ones!). And if you were unlucky enough to accidentally open spam e-mail, you may have noticed it was quickly followed by more spam. It's no coincidence -- spammers use clever tracking mechanisms to monitor whether their e-mail is deleted, opened, or even forwarded to another person.

The most obvious and basic tracking mechanism is a read receipt. A read receipt is a flag in the e-mail header that tells your e-mail client to return the status of the e-mail to its sender. Through this system, the spammer can obtain basic information about whether you read or deleted the e-mail. Every popular e-mail client (such as Outlook or The Bat) has an option to deny read receipts for public e-mail received from the Internet. If you use a corporate e-mail system, such as Microsoft Exchange, the system administrator usually has the ability to force your e-mail client to return read receipts so beware!

Web Bugs

A more sophisticated tracking system is achieved through web bugs. In Lessons 1 and 2, you learned how adware systems are used to track your Web browsing and application usage habits. The same principle applies to e-mail. Using HTML e-mail, a spammer can include a reference to a script on its server that's executed every time the e-mail is opened and the content is loaded. To achieve this, a single pixel, transparent GIF image is included in the e-mail. This GIF is invisible to the reader, but essential to the tracking system. When your e-mail client loads the image, the tracking script on the spammer's server is executed, completing the vicious cycle. This is the reason you should never open an e-mail you have good reason to believe is spam.

If you forward an e-mail with a Web bug in it, the spammer will know exactly who you forwarded it to and retrieve personal information on them. E-mail client security vulnerabilities are an absolute goldmine for spammers, the chief culprit being IFRAMEs, which are discussed in the following section.