Get customer e-mail inside iframe src?

I am trying to connect our order process to a review system. For it to work, I need to insert the customer e-mail inside an url, which will be called through an iframe. What I have so far:

<?php
echo '<iframe width="1" height="1" frameborder="0"
src="http://this-is-a-long-url.nl/targetMail=' . $order->billing_email; . '&delay=0" ></iframe>';
?>

For some reason, the e-mail address is not being inserted into the url.

I am trying to put this code inside woocommerce/checkout/thankyou.php but apparently this code is incorrect.

Hope someone can help me out with this!

https://wordpress.org/plugins/woocommerce/