Page "Web bug" Paragraph 23

Web bugs embedded in emails have greater privacy implications than bugs embedded in web pages.

Through the use of unique identifiers contained in the URL of the web bugs, the sender of an email containing a web bug is able to record the exact time that a message was read, as well as the IP address of the computer used to read the mail or the proxy server that the user went through.

In this way, the sender can gather detailed information about when and where each particular recipient reads email.

Every subsequent time the email message is displayed can also send information back to the sender.