well, you can just include one or two parameters in the link just to indicate that any users that visit your website by this link is the one that received your newsletter.
something like this:
[url]http://www.yourwebsite.com/index.php?nwsltr=yes&uid=4[/url]
then in your index.php, you can do something like this:
$newsletter=$_GET['nwsltr'];
$userid=$_GET['uid'];
if ($newsletter=="yes" AND !empty($uid)){
//it means, the user is visiting from the newsletter
//mail a notification here...
}
of course, anyone can fake this by typing it. You can add another routine that check whether the user id exist in database..