Gmail Error :The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required

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

msg = new MailMessage();
           SmtpClient smtp = new SmtpClient();
           emailId = txtEmailId.Text.Trim();
           //sender email address
           msg.From = new MailAddress("yaduvir4@gmail.com");
           //Receiver email address
           msg.To.Add(emailId);
           msg.Subject = "Confirmation email for account activation";
           //For testing replace the local host path with your lost host path and while making online replace with your website domain name
           ActivationUrl = Server.HtmlEncode("http://localhost:8665/MySampleApplication/ActivateAccount.aspx?UserId=" + FetchUserId(emailId) + "&EmailId=" + emailId);

           msg.Body = "Hi " + txtName.Text.Trim() + "!\n" +
                 "Thanks for showing interest and registring in <a href='http://www.webcodeexpert.com'> webcodeexpert.com<a> " +
                 " Please <a href='" + ActivationUrl + "'>click here to activate</a>  your account and enjoy our services. \nThanks!";
           msg.IsBodyHtml = true;
           smtp.Credentials = new NetworkCredential("sendfrommailaddress.com", "password");
           smtp.Port = 587;
           smtp.Host = "smtp.gmail.com";
           smtp.EnableSsl = true;
           smtp.Send(msg);
           clear_controls();
           ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "alert('Confirmation Link to activate your account has been sent to your email address');", true);