title: IPN is not fired when customer closes paypal site
body: <p>My website has a option for customer to pay by paypal.
  After fill the order, customer will be redirected to paypal site. If customer paid, an IPN event will be fired to my server and I will know this customer is paid. But if this customer close the paypal site without payment, I have no IPN event so I don't know this customer paid or not.</p>
  
  <p>How can I handle this problem?</p>