MailMessage mail = new MailMessage();
mail.To.Add(hdn_patemail.Value);
mail.From = new MailAddress("abc@gmail.com");
mail.Subject = "Your account details";
mail.Body = "<html><head></head><body><table><tr><td>Dear friend,</td></tr></table></body></html>" + Session["PatName"].ToString() + "<br/>" + hdn_pwdd.Value + "<br/><html><body><table><tr><td>Regards,</td></tr><tr><td>Team-InfyMED</td></tr></table></body><html>";
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Credentials = new System.Net.NetworkCredential("abc@gmail.com", "abc@123");
smtp.EnableSsl = true;
smtp.Send(mail);
use smtp.gmail.com instead 19.168.1.100 and uncomment ssl statement.