Mail Message

           MailMessage mm = new MailMessage("teztag.web1@gmail.com", "teztag.web2@gmail.com");
            mm.Subject = "Password";
            mm.Body = "body";
            mm.IsBodyHtml = true;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
            NetworkCred.UserName = "teztag.web1@gmail.com";
            NetworkCred.Password = "password";
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            smtp.Port = 587;
            smtp.Send(mm);

Check this link
https://myaccount.google.com/lesssecureapps?pli=1
& change the setting as bellow:-
Allow less secure apps: ON