SpyPig is a free email tracking service that tracks your email and notifies your when your email is read. All you have to do is send email using SpyPig form and select an image to track. If your recipient reads your email you will get notification.
How it tracks: It tracks emails by including a small image at the end of your email, when your recipient opens email the spypig image will be loaded and it leaves a log entry in spypig server and you will get the notification.
The service is useful particularly in some situations, but it has some limitations. Your recipients should accept HTML emails to track the email, if they use plain text you don’t know whether your email is read or not. Also it is not useful if you want to send an email several people, current notification limit is 5. If the same recipient opens email several times, you will get notifications every time to annoy you.
SpyPig could be great for responding to classifieds or some others where you want to know your email is read. But it may pose some privacy concerns, your recipient may feel bad that you are spying on him, use it responsibly.