Why can I send an email to my usrName@localhost but not to a real email.

<?php 

    require 'vendor/autoload.php';

    date_default_timezone_set('America/Toroto');

    $app = new \Slim\Slim( array(

    'view' => new \Slim\Views\Twig()
  )); 

  $view = $app->view();
  $view->parserOptions = array('debug' => true);
  $view->parserExtensions = array(new \Slim\Views\TwigExtension()); //gives us helpers like baseURL, currentURL, siteURL, urlFor

//this displays the root of the site (index page)
$app->get('/', function() use($app) {

  $app->render('about.twig');
})->name('home');

//this displays the root of the site (index page)
  $app->get('/contact', function() use($app) { 

    $app->render('contact.twig');
  })->name('contact');


$app->post('/contact', function() use($app){
  // var_dump($app->request->post());

  $name = $app->request->post('name');
  $email = $app->request->post('email');
  $msg = $app->request->post('msg');

  if(!empty($name) && !empty($email) && !empty($msg)) {
    $sanitizedName = filter_var($name, FILTER_SANITIZE_STRING);
    $sanitizedEmail = filter_var($email, FILTER_SANITIZE_EMAIL);
    $sanitizedMsg = filter_var($msg, FILTER_SANITIZE_STRING);
  } else {
    //message the user that there was a problem
    $app->redirect('/contact');
  }



  // :: means static operator
  $transport = Swift_MailTransport::newInstance();
  $mailer = \Swift_Mailer::newInstance($transport);
  $message = \Swift_Message::newInstance();

  $message->setSubject('Email From Test Site');
  $message->setFrom(array(

     $sanitizedEmail => $sanitizedName
    ));
  $message->setTo(array('kipp0@localhost'));
  $message->setBody($sanitizedMsg); 


  $result = $mailer->send($message); //sends mail and stores a boolean in result for if it failed

  if($result < 1) { //message failed
    //send user a message saying that the email failed
    //log the failed attempt
    //redirect to contact

    $app->redirect('/contact');

  } else {
    //tell the user it worked and thank them
    //redirect to contact
    $app->redirect('/');    
    // var_dump($message);
  }

});

    $app->run(); //runs app (Obviously!)
 ?>