How to send mail in contact us page through?

See more:

C#

ASP.NET

I use all code but exception occured Following..
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

What I have tried:

i tried follwing code...
but it will give following exception
"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"

try
            {
                System.Net.Mail.MailMessage Msg = new System.Net.Mail.MailMessage();
                // Sender e-mail address.
                Msg.From = new MailAddress(txtEmail.Text);
                // Recipient e-mail address.
               Msg.To.Add("administrator@aspdotnet-suresh.com");
            
                Msg.Subject = txtSubject.Text;
                Msg.Body = txtmsg.Text;
                // your remote SMTP server IP.
                SmtpClient smtp = new SmtpClient();
                smtp.Host = "smtp.gmail.com";
                smtp.Port = 587;
                //smtp.Credentials = new System.Net.NetworkCredential("yourgmailemail@gmail.com", "yourpassword");
                
                smtp.EnableSsl = true;
                smtp.Send(Msg);
                //Msg = null;
                success.Text = "Thanks for Contact us";
                // Clear the textbox valuess
                txtname.Text = "";
                txtSubject.Text = "";
                txtmsg.Text = "";
                txtEmail.Text = "";
            }
            catch (Exception ex)
            {
              // Console.WriteLine("{0} Exception caught.", ex);
            }