title: .Net TcpClient and SmtpClient won't connect to my Smtp server
body: <p>I have a method that is supposed to connect to an Smtp Server to see if it is online. This method works when testing many mail servers, but not all of them. The code is below, however the failure happens on...</p>
  
  <p>client.Connect(strMailServer, intPort);</p>
  
  <p>... before the logic of talking to the server even begins. It simply won't connect. I've made absolutely sure I'm connecting the the right IP and port (25), and I've used third party web sites like mxtoolbox to test the same server IP with success. This server is receiving regular traffic from the world wide web... only .Net seems unable to connect. I've reviewed firewall rules, and watched with WireShark to see what is happening on the server, but I never see any incoming packets from my test runs. The Firewall is set to allow all connections to port 25 on all interfaces, from anybody. </p>
  
  <p>I've also used SmtpClient to run a similar test shown below, it fails as well.</p>
  
  <pre><code>                var client = new System.Net.Mail.SmtpClient(strMailServer, intPort);
              client.Send("test@mydomain.com", "test@mydomain.com", "test message", "This is meant to test an SMTP server to see if it is online, it expects the message to be rejected.");
  </code></pre>
  
  <p>The error stack here leads to the same underying error as my TcpClient attempt. 
  SocketException: {"No connection could be made because the target machine actively refused it xxx.xxx.xxx.xxx:25"}</p>
  
  <p>How could everybody in the world be able to connect to this server... except for my laptop computer... I don't think this is a firewall issue.</p>
  
  <p>Help!</p>
  
  <pre><code>public static bool TestMailServer(string strMailServer, int intPort, out string strResponse)
      {
          try
          {
              try
              {
                  //First I'll try a basic SMTP HELO
                  using (var client = new TcpClient())
                  {
                      client.Connect(strMailServer, intPort);
                      // As GMail requires SSL we should use SslStream
                      // If your SMTP server doesn't support SSL you can
                      // work directly with the underlying stream
                      using (var stream = client.GetStream())
                      {
                          using (var writer = new StreamWriter(stream))
                          using (var reader = new StreamReader(stream))
                          {
                              writer.WriteLine("EHLO " + strMailServer);
                              writer.Flush();
                              strResponse = reader.ReadLine();
                              if (strResponse == null)
                                  throw new Exception("No Valid Connection");
  
                              stream.Close();
                              client.Close();
                              if (F.StartsWith(strResponse, "220"))
                                  return true;
                              else
                                  return false;
                          }
  
                      }
  
                  }
              }
              catch (Exception ex)
              {
                  //If the above failed, I'll try with SSL
                  using (var client = new TcpClient())
                  {
                      //var server = "smtp.gmail.com";
                      //var port = 465;
                      //client.SendTimeout = 10000;
                      //client.ReceiveTimeout = 10000;
  
                      client.Connect(strMailServer, intPort);
                      // As GMail requires SSL we should use SslStream
                      // If your SMTP server doesn't support SSL you can
                      // work directly with the underlying stream
                      using (var stream = client.GetStream())
                      using (var sslStream = new SslStream(stream))
                      {
                          sslStream.AuthenticateAsClient(strMailServer);
                          using (var writer = new StreamWriter(sslStream))
                          using (var reader = new StreamReader(sslStream))
                          {
                              writer.WriteLine("EHLO " + strMailServer);
                              writer.Flush();
  
                              strResponse = reader.ReadLine();
                              if (strResponse == null)
                                  throw new Exception("No Valid Connection");
  
                              stream.Close();
                              client.Close();
                              if (F.StartsWith(strResponse, "220"))
                                  return true;
                              else
                                  return false;
                              // GMail responds with: 220 mx.google.com ESMTP
                          }
                      }
                  }
              }
          }
          catch (Exception ex)
          {
              strResponse = ex.Message;
              return false;
          }
      }
  </code></pre>