Problem with protocol

Hi,
With these codes

                    MailMessage message = new MailMessage();
                    message.From = new MailAddress("Administrator@my-friend.co");
                    message.To.Add("peterchen@gmail.com");
                    message.Subject = "test message";
                    message.Body = "This is a test e-mail message sent by an application. ";

                    SmtpClient client = new SmtpClient("WIN-AIGPMD763AL.my-friend.co", 587);
                    client.DeliveryMethod = SmtpDeliveryMethod.Network;
                    client.Credentials = new System.Net.NetworkCredential("Administrator@my-friend.co", "?????");
                    client.EnableSsl = true;
                    client.Send(message);
                    ...

I get these exception and Stacktrace. Any ideas?
The server committed a protocol violation The server response was: UGFzc3dvcmQ6
   at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)

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Many Thanks & Best Regards, Hua Min