title: Outlook Rest API - 503 Service Unavailable for only one user in the domain, other users work
body: <p>I am trying to help a user connect to the Outlook API.  Three of this person's colleagues, who all have the same email domain, are able to successfully connect and executed API calls, but the one user is receiving 503 Service Unavailable responses.</p>
  
  <p>The user is able to successfully get an access token, and that access token can also successfully be refreshed. </p>
  
  <p>Here is an example request and response from the "Outlook Dev Center - Oauth Sandbox Tool" <a href="https://oauthplay.azurewebsites.net/" rel="nofollow noreferrer">https://oauthplay.azurewebsites.net/</a>.</p>
  
  <pre><code>GET https://outlook.office.com/api/v2.0/me/mailfolders/sentitems/messages?$filter=sentdatetime%20ge%202017-02-13T00:00:00Z HTTP/1.1
  Accept: text/*, application/xml, application/json; odata.metadata=none
  User-Agent: PlayGroundAgent/1.0
  Authorization: Bearer {hidden}
  client-request-id: ce0330c6-56cd-4ab9-b3ab-69670b565cac
  X-AnchorMailbox: {hidden}
  
  
  HTTP/1.1 503 Service Unavailable
  Transfer-Encoding: chunked
  request-id: c81e91cc-705d-4abe-a6e5-5bafbdd62b87
  X-CalculatedFETarget: MWHPR01CU001.internal.outlook.com
  X-BackEndHttpStatus: 503, 503
  X-FEProxyInfo: MWHPR01CA0012.PROD.EXCHANGELABS.COM
  X-CalculatedBETarget: MWHPR16MB1583.namprd16.prod.outlook.com
  OData-Version: 4.0
  X-DiagInfo: MWHPR16MB1583
  X-BEServer: MWHPR16MB1583
  X-FEServer: MWHPR01CA0012, MWHPR15CA0056
  Cache-Control: private
  Date: Mon, 13 Feb 2017 20:11:39 GMT
  Set-Cookie: exchangecookie={hidden}; expires=Tue, 13-Feb-2018 20:11:40 GMT; path=/; HttpOnly
  Server: Microsoft-IIS/8.5
  X-AspNet-Version: 4.0.30319
  X-Powered-By: ASP.NET
  
  {
      "error": {
          "code": "ErrorServiceUnavailable",
          "message": "Active Directory operation did not succeed. Try again later."
      }
  }
  </code></pre>
  
  <p>My understanding is that this user was once on Exchange and has been migrated to Office 365 in recent years.  The three users who are able to connect were not originally on Exchange, they all had their accounts created, net new, on Office 365.  My assumption is that something in the migration process from Exchange to Office 365 is causing the 503 Service Unavailable responses.</p>