sslstream不适用于SMTP协议

Question

我正在使用的代码仅用于测试，这里我用gmail测试：

C# code: 
private void button1_Click(object sender, EventArgs e)
        {
        string server = "smtp.gmail.com";
        int port = 25;
        client = new TcpClient();
        client.Connect(server, port);
        var stream = client.GetStream();
        sslStream = new SslStream(stream);
        sslStream.AuthenticateAsClient(server);
        clearTextReader = new StreamReader(sslStream);
        clearTextWriter = new StreamWriter(sslStream);
        clearTextWriter.WriteLine("EHLO " + server);
        clearTextWriter.Flush();
        var response = clearTextReader.ReadLine();
        Error(response);
    }

但是这个错误出现了这个错误：

System.IO.IOException occurred
  HResult=0x80131620
  Message=The handshake failed due to an unexpected packet format.
  Source=System
  StackTrace:
   at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost, X509CertificateCollection clientCertificates, SslProtocols enabledSslProtocols, Boolean checkCertificateRevocation)
   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost)

当我尝试在端口443上执行此操作时，它工作正常，但端口25，只是一直出错。我已经尝试了一切，但我找不到解决方案。证书是有效的，当我尝试不同的服务器时它也不起作用。