Class 'Swift_SendmailTransport' not found. What should i do?

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('/lr/contact');
}

//Setup TRANSPORT for email
$transport = Swift_SendmailTransport::newInstance('C:/sendmail/ -bs');
//MAILER
$mailer = \Swift_Mailer::newIsntance($transport);
//MASSaGE OBJECT
$message = \Swift_Message::newInstance();
$message->setSubject('Email from our website');
$message->setFrom(array(
    $cleanEmail => $cleanName

));
//where to send message
$message -> setTo(array('treehouse@localhost'));
//body of message
$message -> setBody($cleanMsg);

//Sending Message
$result = $mailer->send($message);

if($result > 0){
    // send a message that says thank you!
    $app->redirect('/');

} else {
    //send message to the user that the message failed to send
    //log that there was an error
    $app->redirect('/lr/contact');
}