By this thread: https://forum.phalcon.io/discussion/9243/which-mailer-and-how-to-install, I set up $di like this:
$di->set('swiftmailer',function() use ($config) {
   include 'path to swift_required.php';
    $transport = Swift_SmtpTransport::newInstance($config->smtp->address,$config->smtp->port)
        ->setUsername($config->smtp->username)
        ->setPassword($config->smtp->password);
    $mailer = Swift_Mailer::newInstance($transport);
    return $mailer;
});And then, How to send a mail in the Controllers?