SpyPig is a simple email tracking system that sends you a notification email as soon as the recipient opens and reads your message.

SpyPig allows you to embed a small or invisible picture inside an e-mail.  When that e-mail is opened in an e-mail client that supports loading of web pictures (most modern e-mail clients) SpyPig logs that access and allows you to see when that e-mail was opened.  Actually works the same way a lot of advertising and spam e-mails work, this is how they know your e-mail address exists.  Now you can  use the technology for yourself.

SpyPig