Inspired by spypig.com, a site that helps you track when your emails are opened, I decided to see if I could write a script with the same functionality. It tuned out to be a rather interesting project.
Here it is: http://www.classicalcode.com/t.php
It works by generating an image which you embed in your email. Then when your recipeint reads their email, their email client downloads that image. Downloading that image triggers a script on my server which then sends you an email.
In order to make the image look less suspicious I implemented a feature where the image is a string of text of the senders choosing. Example:
Give it a try!