The remote certificate is invalid according to the validation procedure.

MailMessage Msg = new MailMessage();
            // Sender e-mail address.
            Msg.From = new MailAddress(txtUsername.Text);
            // Recipient e-mail address.
            Msg.To.Add(txtTo.Text);
            Msg.Subject = txtSubject.Text;
            Msg.Body = txtBody.Text;
            // your remote SMTP server IP.
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.Port = 587;
            smtp.Credentials = new System.Net.NetworkCredential(txtUsername.Text, txtpwd.Text);
            smtp.EnableSsl = true;
            smtp.Send(Msg);
            Msg = null;
            Page.RegisterStartupScript("UserMsg", "<script>alert('Mail sent thank you...');if(alert){ window.location='SendMail.aspx';}</script>");