Swift_TransportException while submitting an email during PHP course

<?php

require 'vendor/autoload.php';
date_default_timezone_set('America/Chicago');

//$log = new Logger('name');
//$log->pushHandler(new StreamHandler('app.txt', Logger::WARNING));
//$log->addWarning('Foo');

$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(),
);

$app->get('/', function () use ($app){
  $app->render('about.twig');
})->name('home');

$app->get('/contact', function () use ($app){
  $app->render('contact.twig');
})->name('contact');

$app->post('/contact', function () use ($app){
  $name = $app->request->post('name');
  $email = $app->request->post('email');
  $msg = $app->request->post('msg');

  if(!empty($name) && !empty($email) && !empty($msg)) {
    $cleanName = filter_var($name, FILTER_SANITIZE_STRING);
    $cleanEmail = filter_var($email, FILTER_SANITIZE_EMAIL);
    $cleanMsg = filter_var($msg, FILTER_SANITIZE_STRING);
  } else {
   //message the user that there was an error
   $app->redirect('/contact');
  }

  $transport = Swift_SendmailTransport::newInstance('/user/sbin/sendmail -bs');
  $mailer = \Swift_Mailer::newInstance($transport);

  $message = \Swift_Message::newInstance();
  $message->setSubject('Email from our website');
  $message->setFrom(array(
    $cleanEmail => $cleanName
  ));

  $message->setTo(array('treehouse@localhost'));
  $message->setBody($cleanMsg);

  $result = $mailer->send($message);

  if($result>0) {
     //send a message saying thank you
    $app->redirect('/');
  } else {
    //send message to the user that the message failed to send
    //log that was an error
    $app->redirect('/contact');
  }

});


$app->run();
?>