I’m not sure if this is legal in all countries but you can send the emailadress back to you by adapting it in the link the user clicks to your website.
e.g. The email they receive has a button that links to your website.
the url of the button looks like this:
<a href="http://www.yourwebsite.com?email="[email protected]""><img src="button.jpg"></a>
Then all you have to do is use a $GET_['email'] request and get the emailaddress and save it to a database.