Drupal 8 - Send mail with Swiftmailer

mails.php

<?php

use Drupal\Core\Render\Markup;

/**
* Mail hook
*/
function hook_mail($key, &$message, $params) {
    $options['langcode'] = $message['langcode'];
    
    $headers = array(
        'MIME-Version'              => '1.0',
        'Content-Type'              => 'text/html; charset=UTF-8; format=flowed; delsp=yes',
        'Content-Transfer-Encoding' => '8Bit',
        'X-Mailer'                  => 'Drupal',
    );
    
    switch($key) {
        case 'inline':
            $message['subject'] = t('');
            
            // Add headers
            foreach ($headers as $key => $value) {
                $message['headers'][$key] = $value;
            }
            
            // Usage of inline markup
            $message['body'][] = Markup::create( '<b>'. t("Title") . '</b><br /><br />');
            $message['body'][] = Markup::create( t('Text.').' <br />');
            
            break;

        case 'template':
            $message['subject'] = t('');
            
            // Add headers
            foreach ($headers as $key => $value) {
                $message['headers'][$key] = $value;
            }
            
            // Usage of template
            $renderer = \Drupal::service('renderer');
            $render = [
                '#theme'     => 'module_template_mail',
                '#variables' => array(),
            ];
            $message['body'][] = $renderer->renderRoot($render);
            
            break;
    }
}