From Wizard Industries:
A number of new services have emerged that offer tracking of e-mails sent by their users. These services include http://www.didtheyreadit.com, http://www.msgtag.com, and http://www.readnotify.com among others. These services invade your privacy to report if, when, how, and for how long you have read the e-mail their user sent you. There is no reason for anyone to secretly track e-mails behind your back. Our E-mail-Tracking Blocker will block all of these services - to those services it will always appear, as the e-mail has never been opened.