title: 401 Unauthorized when trying to send emails from EWS Managed API
body: <p>Ok so I am developing an addin which must automatically send emails to others. I know that office-js does not have permissions to do so and I have started using EWS Managed API. I have implemented SSO token and I get it with:</p>
  
  <pre><code>Office.context.auth.getAccessTokenAsync()
  </code></pre>
  
  <p>After I get the token I make a request call to my server where I have my EWS Managed API and try to send an email with the folowing code:</p>
  
  <pre><code>ExchangeService exService = new ExchangeService();
  exService.Url = new Uri(ewsUrl);
  ExchangeCredentials credentials = new OAuthCredentials(ssoToken);
  exService.Credentials = credentials;
  EmailMessage emailMessage = new EmailMessage(exService);
  </code></pre>
  
  <p>When I call <code>emailMessage.SendAndSaveCopy();</code> it throws 401 Error. </p>
  
  <p>If I do use <code>WebCredentials(user, pass)</code> instead of <code>OAuthCredentials(ssoToken)</code> it does work. I do not understand why (I also have set permissions on Azure Portal)</p>