Log swiftmailer mails in eml files

MailerLoggerUtil.php

<?php

namespace AppBundle\Util;

use Gedmo\Sluggable\Util as Sluggable;
use Swift_Events_SendEvent;
use Swift_Events_SendListener;
use Symfony\Component\Filesystem\Exception\IOExceptionInterface;
use Symfony\Component\Filesystem\Filesystem;

/**
 * https://stackoverflow.com/q/18033210/5947371
 */
class MailerLoggerUtil implements Swift_Events_SendListener
{
    protected $logDir;

    /**
     * MailerLoggerUtil constructor.
     *
     * @param string          $logDir
     */
    public function __construct($logDir)
    {
        $this->logDir = $logDir;
    }

    /**
     * @param Swift_Events_SendEvent $evt
     */
    public function beforeSendPerformed(Swift_Events_SendEvent $evt)
    {
        // ...
    }

    /**
     * @param Swift_Events_SendEvent $evt
     */
    public function sendPerformed(Swift_Events_SendEvent $evt)
    {
        $message = $evt->getMessage();

        $emails = implode(', ', array_keys($message->getTo()));

        /**
         * Replace this with your own slug function if you don't have Sluggable Util
         */
        $emailSlug = $slug = Sluggable\Urlizer::urlize($emails, '-');
        $subjectSlug = $slug = Sluggable\Urlizer::urlize($message->getSubject(), '-');

        $dir = $this->logDir.'/mail/'.date('Y/m/d');
        $file = date('Ymd_His').'_'.$emailSlug.'_'.$subjectSlug.'.eml';

        $fileSystem = new Filesystem();

        try {
            $fileSystem->mkdir($dir);
        } catch (IOExceptionInterface $exception) {
            echo "An error occurred while creating your directory at ".$exception->getPath();
        }

        file_put_contents($dir.'/'.$file, $message->toString());
    }
}

service.yml

services:
    _defaults:
         bind:
            $logDir: '%kernel.logs_dir%'

    AppBundle\Util\MailerLoggerUtil:
        tags:
            - { name: monolog.logger, channel: mailer }
            - { name: "swiftmailer.default.plugin" }