title: PayPal IPN not calling notify_url
body: <p>So I have this form in my <em>Wordpress</em> site and I've configured it to use the <em>PayPal</em> sandbox:</p>
  
  <pre><code>&lt;form action="https://sandbox.paypal.com/cgi-bin/webscr" method="post"&gt;
    &lt;input type="hidden" name="business" value="daniel-facilitator@domain.com"&gt;
    &lt;input type="hidden" name="item_name" value="Test - 11/11/2017"&gt;
    &lt;input type="hidden" name="item_number" value="13877"&gt;
    &lt;input type="hidden" name="notify_url"  value="http://b1248fa5.ngrok.io/wp-admin/admin-ajax.php?action=eab_paypal_ipn&amp;amp;blog_id=1&amp;amp;booking_id=1"&gt;
    &lt;input type="hidden" name="amount" value="5"&gt;
    &lt;input type="hidden" name="return" value="http://b1248fa5.ngrok.io/events/2017/11/test/"&gt;
    &lt;input type="hidden" name="currency_code" value="EUR"&gt;
    &lt;input type="hidden" name="no_shipping" value="1"&gt;
    &lt;input type="hidden" name="cmd" value="_xclick"&gt;
    &lt;input type="hidden" name="quantity" value="1" max="1"&gt;
    &lt;p&gt;Haz click aquí para pagar tu entrada&lt;/p&gt;&lt;input type="image" name="submit" border="0" src="https://www.paypal.com/en_US/i/btn/btn_paynow_SM.gif" alt="PayPal - The safer, easier way to pay online"&gt;
    &lt;img alt="" border="0" width="1" height="1" src="https://www.paypal.com/en_US/i/scr/pixel.gif"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>It is generated by the <code>Event +</code> plugin. I has been working fine all year round until now.</p>
  
  <p>This is what is happening:</p>
  
  <ol>
  <li>I submit the form.</li>
  <li>I get to my sandbox account's login page. I login.</li>
  <li>My sandbox account's dashboard opens up.</li>
  <li>Nothing happens... I'm waiting for <em>PayPal</em> to call my <code>notify_url</code> handler, but it doesn't!</li>
  </ol>
  
  <p>If I use the <em>PayPal</em> IPN simulator with the same <code>notify_url</code>, then it is a whole other story. I get a message saying: </p>
  
  <blockquote>
    <p>IPN was not sent, and the handshake was not verified. Please review your information.</p>
  </blockquote>
  
  <p>However, in this case, my <code>notify_url</code> does get called.</p>
  
  <p>What could be going on here? Why <em>PayPal</em> is not calling my <code>notify_url</code> when I submit my form?</p>