title: FCM service unavailable nack
body: <p>I have a python3 program that connects to the FCM XMPP test server successfully. However, after sending a JSON, it returns me a <code>SERVICE_UNAVAILABLE</code> nack message every time. The very same piece of code worked perfectly last week.</p>
  
  <p>The <a href="https://firebase.google.com/docs/cloud-messaging/xmpp-server-ref#error-codes" rel="nofollow">documentation</a> advises to retry to send the message, several times, what I did. It also says that I might be blacklisted.</p>
  
  <ul>
  <li>How can I know if I am blacklisted?</li>
  <li>What behavior can get you blacklisted? Is it possible to get blacklisted from the FCM <strong>test</strong> server?</li>
  <li>Is it a technical problem from FCM? It seems to <a href="https://status.firebase.google.com/" rel="nofollow">be working</a>.</li>
  </ul>
  
  <p>The request that provokes the nack is:</p>
  
  <pre><code>&lt;message id=""&gt;&lt;gcm xmlns="google:mobile:data"&gt;{"message_id": "37ea7059-a832-413b-b008-378f8d8e1a46", "delivery_receipt_requested": true, "time_to_live": 600, "notification": {"body": "BAR", "color": "#666", "icon": "ic_launcher", "title": "FOO", "sound": "content://settings/system/notification_home"}, "data": {"url": "http://google.com"}, "to": "FOOBAR"}&lt;/gcm&gt;&lt;/message&gt;
  </code></pre>