title: SmtpClient.Send not working on linux environment
body: <p>The following code, written in the <code>.net core 2</code> environment, works in <code>the windows environment</code> but not in <code>the linux environment</code>.</p>
  
  <pre><code>string host = "10.99.99.10";
  int port = 25;
  string userName = "user@user.com";
  string password = "password";
  string from = userName;
  
  var client = new SmtpClient
  {
      Host = host,
      Port = port,
      EnableSsl = false,
      DeliveryMethod = SmtpDeliveryMethod.Network,
      UseDefaultCredentials = false,
      Credentials = new NetworkCredential(userName, password)
  };
  
  MailMessage mailMessage = new MailMessage();
  mailMessage.From = new MailAddress(from);
  mailMessage.To.Add(userName);
  mailMessage.Body = "This is test mail.";
  mailMessage.Subject = "Testing";                
  client.Send(mailMessage);
  </code></pre>
  
  <p>Exception: <code>Failure sending mail.</code> </p>
  
  <p>InnerExcepiton: </p>
  
  <pre><code>System.ComponentModel.Win32Exception (0x80004005): GSSAPI operation failed with error - An invalid status code was supplied (Unknown error).
     at System.Net.Security.NegotiateStreamPal.AcquireCredentialsHandle(String package, Boolean isServer, NetworkCredential credential)
     at System.Net.NTAuthentication.Initialize(Boolean isServer, String package, NetworkCredential credential, String spn, ContextFlagsPal requestedContextFlags, ChannelBinding channelBinding)
     at System.Net.Mail.SmtpNtlmAuthenticationModule.Authenticate(String challenge, NetworkCredential credential, Object sessionCookie, String spn, ChannelBinding channelBindingToken)
     at System.Net.Mail.SmtpConnection.SetContextAndTryAuthenticate(ISmtpAuthenticationModule module, NetworkCredential credential, ContextAwareResult context)
     at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
     at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
     at System.Net.Mail.SmtpClient.GetConnection()
     at System.Net.Mail.SmtpClient.Send(MailMessage message)
  </code></pre>
  
  <p>StackTrace:</p>
  
  <pre><code>at System.Net.Mail.SmtpClient.Send(MailMessage message)
     at MyProject.Helper.Utils.SendMail() in C:\Test\MyProject\MyProject.Helper\Utils.cs:line 146
  </code></pre>
  
  <p>Linux:    <code>Ubuntu 16.04.3 LTS</code></p>
  
  <p>This is a console app. 
  Why is not working in linux environment?</p>