отправка электронной почты с помощью gmail с asp.net

Я пытаюсь отправить почту с помощью gmail в asp.net

Мой код:

using (MailMessage mm = new MailMessage(txtEmail.Text, txtTo.Text))
        {
            mm.Subject = txtSubject.Text;
            mm.Body = txtBody.Text;
            if (fuAttachment.HasFile)
            {
                string FileName = Path.GetFileName(fuAttachment.PostedFile.FileName);
                mm.Attachments.Add(new Attachment(fuAttachment.PostedFile.InputStream, FileName));
            }
            mm.IsBodyHtml = false;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
            NetworkCredential NetworkCred = new NetworkCredential(txtEmail.Text, txtPassword.Text);
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            smtp.Port = 587;
            smtp.Send(mm);
            ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
        }

Но я получаю ошибку:

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at

где моя ошибка?