Error to send email from localhost

<?php 

require 'vendor/autoload.php';
date_default_timezone_set('Africa/Addis_Ababa'); 

//$log = new Monolog\Logger('name');
//$log->pushHandler(new Monolog\Handler\StreamHandler('app.txt', Monolog\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 a problem
        $app->redirect('contact');
    }

    $transport = Swift_SendmailTransport::newInstance('/usr/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('yone920@gmail.com'));
    $message->setBody($cleanMsg);

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

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


});


$app->run();

?>