Hi Theresa,
I checked out the service and the reason it does not work accurately anymore is because they do the tracking by including hidden images in the e-mail. Most service providers such as Yahoo Mail and desktop e-mail clients such as Microsoft Outlook will either block or warn you before opening links and images in an e-mail. The reason this service used to work is because when someone opened the an e-mail it would open an image that was located on readnotify's web server. If the image was loaded over the web then readnotify and you knew that the e-mail had been opened. Several services used this trick for a while including readnotify and mailing list management tools. Unfortunately spammers started using this trick in an attempt to generate lists of valid e-mail addresses and eventually service providers and desktop clients had to start blocking links and images for this reason and because of viruses in automatically opened content.