Most people have return receipts turned off so that's going to under represent the number of people who have read the email.
A better way (but still far from perfect), is to construct an HTML email and include a graphic in the form:
<img src="<? print "http://www.mydomain.com/logo.php?x=$user_email"; ?>">
This way, when you send out each email, the image tag will contain the user's email address. When the recipient opens the email, their browser will often try to retrieve the image from your web server.
Then create a PHP script called logo.php that writes $x to a database (this will be your list of who has read the emails), and then output a graphic like this:
header("Content-Type: image/x-icon");
$file="/the/path/to/your/images/filename.gif";
@readfile($file);
It's not perfect (there is no perfect solution) but it will return more results than the "Return Receipt" method.
