It works like this, when you open an image in a spam mail, the image is accessed from the server where it is stored. A message can then be generated by that server letting the spammer know that this is a valid email address. The owner of said address can then be sent numerous messages about the latest male enhancement products. Even if said owner is happy with the current state of his enhancement.
The application, SpyPig, works in the same way. It uses a little picture of a pig that you embed in your email. When your boyfriend opens the note where you remind him of dinner with your parents, the pig image is also opened. The server where the image is stored notes this occurrence and notifies you that said image has been opened. When your boyfriend then says,
"Honey, I was so busy at work, I didn't have a chance to open your email. If I had, I never would have gone out with the guys. You know I love your mom and dad.......even if they do hate me and all my tattoos!"
You can then scream with a frying pan in your hand,
"You bum! You two faced nerd noodle! We are through!"
It isn't foolproof, as many folks have mail setups that show images as attachments. They have to deliberately open the picture to make it work. So, it is possible for someone to open your email and for you not to get the notice. There are also people who resent these kinds of intrusions into their privacy. But, now you know.
["SpyPig: Track Your Email...Like Spammers Do." AppScout: Stalking the Killer App. 14 Sept 2007. PC Magazine. 15 Sep 2007