title: Webhook not performing subscription validation
body: <p>We are creating a Webhook/Subscription using Microsoft Graph. However, the following JSON is returned and we can see no corresponding HTTP request to our validation public web server:</p>
  
  <pre class="lang-json prettyprint-override"><code>"error": {
      "code": "InvalidRequest",
      "message": "Subscription validation request failed. Must respond with 200 OK to this request.",
      "innerError": {
          "request-id": "d2c4eeca-1bf9-4657-a904-ba6925428bbd",
          "date": "2018-02-24T17:19:56"
      }
  }
  </code></pre>
  
  <p>Our HTTP POST request to <code>https://graph.microsoft.com/v1.0/subscriptions</code> contains this JSON:</p>
  
  <pre class="lang-json prettyprint-override"><code>{
      "changeType": "created,updated",
      "clientState": "subscription-identifier",
      "expirationDateTime": "2018-02-27T15:46:18.835Z",
      "notificationUrl": "https://[public_web_server]/api/watl/o365",
      "resource": "Users/[email address]/mailfolders('inbox')/messages"
  }
  </code></pre>
  
  <p>When testing the request via <code>Hurl.it</code> and other online HTTP requests we can see the request hit our web server. However, we can not see anything coming from Microsoft Graph.</p>
  
  <p>Our web server is responding with a <code>200 OK</code> and including the <code>ValidationToken</code> as the body.</p>
  
  <p>Any suggestions?</p>