title: .NET SMTP Client - Client does not have permissions to send as this sender
body: <p>I'm getting strange occurance on our servers when I am trying to send an email using SmtpClient class via an ASP MVC3 project. This is the code I am using.</p>
  
  <pre><code>try
  {
      var client = new SmtpClient("MailServer");
      client.UseDefaultCredentials = true;
  
      MailMessage message = new MailMessage("me@mydomain.com", "friend@mydomain.com", "Test Message", "Test Body");
      client.Send(message);
  }
  catch (Exception ex)
  {
      // Do Nothing
  }
  </code></pre>
  
  <p>I have deployed on three environments; on Windows 7 (using VS 2010 IIS) it sends the email fine, on the Windows 2003 IIS6 machine it sends the email fine, finally on the Windows 2008 R2 II7 server I get the following error:</p>
  
  <pre><code>Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender using username 
  </code></pre>
  
  <p>Can anybody advise on what may be causing this. I have noticed that when I view <code>User.Identity.Name</code>, this is returning an empty string.</p>