Smtp server requires a secure connection or client was not authenticated. The server response was 5.7.0 authenticated required?

<pre>  [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Register(RegisterViewModel model)
        {
            if (ModelState.IsValid)
            {
                var user = new ApplicationUser() { UserName = model.UserName };
                user.Email = model.Email;
                user.ConfirmedEmail = false;
                var result = await UserManager.CreateAsync(user, model.Password);
                if (result.Succeeded)
                {
                    System.Net.Mail.MailMessage m = new System.Net.Mail.MailMessage(
                        new System.Net.Mail.MailAddress("mandelacampus@gmail.com", "Web Registration"),
                        new System.Net.Mail.MailAddress(user.Email));
                    m.Subject = "Email confirmation";
                    m.Body = string.Format("Dear {0}<BR/>Thank you for your registration, please click on the below link to complete your registration: <a href=\"{1}\" title=\"User Email Confirm\">{1}</a>", user.UserName, Url.Action("ConfirmEmail", "Account", new { Token = user.Id, Email = user.Email }, Request.Url.Scheme));
                    m.IsBodyHtml = true;
                    
                    System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("smtp.gmail.com");
                    smtp.Credentials = new System.Net.NetworkCredential("ggcobani@gmail.com", "password");
                    smtp.EnableSsl = true;
                    smtp.UseDefaultCredentials = false;
                    smtp.Port = 587;
                    smtp.Host = "smtp.gmail.com";
                    smtp.Send(m);
                    return RedirectToAction("Confirm", "Account", new { Email = user.Email });
                }
                else
                {
                    AddErrors(result);
                }
            }

            // If we got this far, something failed, redisplay form  478216
            return View(model);
        }