无法在通知URL上获得响应

我刚刚整合paypal到我的网站一切正常添加产品，付款，然后返回成功页面但唯一的问题是notify_URL不工作我添加了邮件功能来测试响应，但没有收到任何邮件

echo '<form target="paypal" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
         <input type="hidden" name="business" value="sendbox@creativemediafx.com">
         <input type="hidden" name="cmd" value="_cart">
         <input type="hidden" name="add" value="1">
         <input type="hidden" name="notify_url" value="http://go4qp.com/fbtest/s1.php" />
         <input type="hidden" name="return" value="http://www.urbaniacs.com/hideout/inventory/" />
         <input type="hidden" name="custom" value="'. $custom .'">
         <input type="hidden" name="item_number" value="'. $itemnumber .'">          
         <input type="hidden" name="item_name" value="' . $item->name . '">
         <input type="hidden" name="amount" value="'. number_format($item->price,2) .'">
         <input type="hidden" name="currency_code" value="USD">
         <input type="image" name="submit" border="0" src="https://www.paypalobjects.com/en_US/i/btn/btn_cart_SM.gif" alt="PayPal - The safer, easier way to pay online">
         <img alt="" border="0" width="1" height="1" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" >
         </form>';