Senders can then gauge the effectiveness of sales pitches and other online solicitations.
That's great for the senders, but Web bugs can reveal much more than e-mail usage patterns. If an IP address other than that of the original recipient calls for the image, senders can determine when and how often their e-mail has been forwarded inside the receiving company. That's where privacy problems loom. HTML makes browsers launch, letting senders place cookies on every PC that accepts an e-mail message containing such a bug. Once a sender places those cookies, it can gather such information as recipients' IP addresses, the types of browsers they use and the Web sites they have visited.