A service that automatically notifies the sender that an e-mail message was previewed or opened by the user. When users send mail from their e-mail clients, they can request read receipts (return receipts) from the recipient, but the recipient has to oblige that request. On the other hand, e-mail trackers do not rely on the recipient's cooperation and automatically send a response. Some services silently return the receipt, while others place a note at the bottom of the message indicating a receipt was sent. The read receipt indicates the time the message was previewed or opened, as well as possibly the location of the recipient's computer and the duration the message was kept open. See read receipt. A Web Bug Does the Trick The tracker software in the user's machine inserts a Web bug into the message, which is an invisible GIF with a unique name obtained from the tracker's server. When the recipient previews or opens the message, the GIF is downloaded from the tracker's server, which reads the name and notifies the sender. If mail was relayed to a text-only, handheld device, the Web bug will not work because the e-mail must be displayed via HTML and be online to the Internet. See Web bug. Two Tracking Methods The first method requires a software installation in the user's machine; either a plug-in for Outlook or a program that resides in the background for other e-mail clients. When messages are sent, only the unique GIF names are downloaded from the tracker's servers. However, this method generally does not work when a Web browser is the e-mail client. The second way is to route the message through the tracker's server by adding a suffix to the address. This method works with any computer and any e-mail client or Web browser. For example, the user would modify the address from "email@example.com" to "firstname.lastname@example.org" and the message would go to the DidTheyReadIt server, which inserts the bug. Examples of e-mail tracker services are DidTheyReadIt (www.didtheyreadit.com), MSGTAG (www.msgtag.com) and ReadNotify (www.readnotify.com).