There is no reliable way to do that. You can make it an HTML email and put a link to a graphic in the email. Host the graphic on your web site and see how many times the graphic gets requested.
You could make the graphic served up by a PHP page which also records a special code number so you can know which people opened the email. For example, maybe the script image.php serves up a GIF. The you construct your html emails like this:
<img src="http://www.mydomain.com/image.php?x=bob@msn.com">
Now when image.php serves up the GIF, it also writes the value of X to a database and you know that bob at least opened the email.
Many people have HTML email turned off and many others don't permit their email to obtain external graphics for exactly the reason you are pursuing.