No problem. It looks like the issue is that you are referencing a variable called Model. However, in your case the model being passed in as input parameter is called Index. The parameter that gets passed into the action method is the model that gets binded, so you need to change your action method to this:
 

[HttpPost]
         public ViewResult Index(Contact_Us Index) // Your model is passed in here
         {
             if (ModelState.IsValid)
             {
                 MailMessage message = new MailMessage();
                 message.From = new MailAddress("server@steamkingal.com");
                 message.To.Add(new MailAddress("services@steamkingal.com"));
                 message.Subject = "This is my subject";
                 
                 // You need to use Index because that is the name declared above
                 message.Body = Index.message;

                 SmtpClient client = new SmtpClient();
                 client.Send(message);
                 return View("Thanks", Index);
             }
             else
             {
                 return View();
             }
         }