title: System.Net.Mail.SmtpException: Mailbox unavailable
body: <p>I've been getting the following exception when trying to send an e-mail using System.Net.Mail:</p>
  
  <pre><code>Unhandled exception in Service Thread:
  System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: Requested action not taken: mailbox unavailable
     at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
     at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
     at System.Net.ClosableStream.Close()
     at System.Net.Mail.MailWriter.Close()
     at System.Net.Mail.SmtpClient.Send(MailMessage message)
  </code></pre>
  
  <p>Does anyone know what this could be caused by?</p>