Based on the previous post, in  your controller you would have to do something like this:

[HttpPost]
         public ViewResult Index(Contact_Us model)
         {
             if (ModelState.IsValid)
             {
                 //I need the Email sender to go here.
                 MailMessage message = new MailMessage();
                 message.From = new MailAddress("sender@foo.bar.com");
                 message.To.Add(new MailAddress(model.email));
 
                 message.Subject = "New Message from Website";
                 message.Body = model.message;
 
                 SmtpClient client = new SmtpClient();
                 client.Send(message);

                 return View("Thanks", Index);
             }
             else
             {
                 return View();
             }
         }