title: When sending messages using GCM, I keep getting the response 'MismatchSenderId'
body: <p>I'm implementing a push notification server with GCM. I send a post request with a sender ID (application key) that I got from Google API Console, and a registration ID that I got from the GCM service.</p>
  
  <p>The posted JSON includes one registration ID. I always get the same response:</p>
  
  <pre><code>{
    "multicast_id":8546528660791862014,
    "success":0,
    "failure":1,
    "canonical_ids":0,
    "results":[
      {
        "error":"MismatchSenderId"
      }
    ]
  }
  </code></pre>
  
  <p>What might be the problem? What am I doing wrong?</p>