问C# SMTP电子邮件程序未正确使用发件人地址

EN

    public void SendEmail()
    {
        //smtp host and port for gmail
        string host = txtHost.Text;
        int port;
        if (!Int32.TryParse(txtPort.Text, out port))
        {
            MessageBox.Show("Please enter a valid port number.");
            return;
        }          

        //compose email
        MailMessage msg = new MailMessage();
        msg.Sender = new MailAddress(txtFrom.Text, txtFrom.Text);
        msg.From = new MailAddress(txtFrom.Text, txtFrom.Text);

        msg.To.Add(txtTo.Text);

        msg.Subject = txtSubject.Text;
        msg.Body = rTxtMessage.Text;

        msg.IsBodyHtml = chkHtml.Checked;


        //msg.From = new MailAddress(txtFrom.Text, txtFrom.Text);

        //create smtp client
        SmtpClient smtp = new SmtpClient(host, port);

        //TODO: Move constants to the NetworkCredentials call
        string username = SMTP_USERNAME;
        string password = SMTP_PASSWORD;

        smtp.UseDefaultCredentials = false;
        smtp.Credentials = new NetworkCredential(username, password);
        smtp.EnableSsl = true;

        try
        {
            //Send email
            smtp.Send(msg);
        }
        catch (Exception exp)
        {
            //Log if any errors occur
            MessageBox.Show(exp.Message);
        }
    }