Lets see an example :(To Check whether the mail recipients has read the mail or not)
Now-a-days an email is written in HTML Format.So you can insert the below code easily.
<img src="webbug.php" width="1" height="1">
What will happen here is,the Email client will try to fetch the image from the src path, But instead of actual image the PHP script will get executed.So this way you can check if the user has read the mail or not.
Note : In most of the email clients images are blocked by default,in that case this will not help.