Send an Email

Hello everyone, I'm a newbie both in C Sharp and in this group discussion. Sorry if I make mistake with my words and grammar (since I'm an Indonesian and not good enough in English)

I am now try to Send an email but I got an error. Please help me.

This is my code :

private void btnSend_Click(object sender, EventArgs e)

        {

            login = new NetworkCredential(txtUsername.Text, txtPassword.Text);

            client = new SmtpClient(txtSmtp.Text);

            client.Port = Convert.ToInt32(txtPort.Text);

            client.EnableSsl = chkSSL.Checked;

            client.Credentials = login;

            msg = new MailMessage { From = new MailAddress(txtUsername.Text + txtSmtp.Text.Replace("smtp.", "@"), "Suriati Marpaung", Encoding.UTF8) };

            msg.To.Add(new MailAddress(txtTo.Text));

            if (!string.IsNullOrEmpty(txtCC.Text))

                msg.To.Add(new MailAddress(txtCC.Text));

            msg.Subject = txtSubject.Text;

            msg.Body = txtMessage.Text;

            msg.BodyEncoding = Encoding.UTF8;

            msg.IsBodyHtml = true;

            msg.Priority = MailPriority.Normal;

            msg.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

            client.SendCompleted += new SendCompletedEventHandler(SendCompletedCallBack);

            string userstate = "Sending..";

            client.SendAsync(msg, userstate);

        }

And the message error is :

the smtp server requires a secure connection or the client was not authenticated.....

Thank you so much.