It has been a long time requirement to find out when your mail was read by the receiver but none of the email service actually provides it. In case out Spy Pig is an email tracking service which lets you find when your email was read by the person without letting them even know about it. It uses a unique image which you can embed in your email. This email is tracked and you get notified in some time.
This is pretty neat service but I wonder if poeple would take so much of effort. If they come out with some plugin or some script it would be more useful. But hey there are some people who would always like to use it.
So go ahead and give Spy Pig a try Oink!!