Drupal 8 - Display communication from swiftmailer with the MTA

example.module

<?php

use Drupal\example\DrupalSwiftMailerMessageLogger;

/**
 * Implements hook_swiftmailer().
 */
function example_swiftmailer_alter($mailer, $m, $message) {
  $logger = new DrupalSwiftMailerMessageLogger(500);
  $mailer->registerPlugin(new Swift_Plugins_LoggerPlugin($logger));
}

src\DrupalSwiftMailerMessageLogger.php

<?php

namespace Drupal\example;

use Drupal\Core\Messenger\MessengerTrait;

/**
 * Class DrupalSwiftMailerMessageLogger.
 *
 * @package Drupal\pfsamplesaustr_profile
 */
class DrupalSwiftMailerMessageLogger implements \Swift_Plugins_Logger {

  use MessengerTrait;

  /**
   * Messages.
   *
   * @var array
   */
  private $log;

  /**
   * Max lines.
   *
   * @var int
   */
  private $size = 0;

  /**
   * DrupalSwiftMailerMessageLogger constructor.
   */
  public function __construct($size = 50) {
    $this->size = $size;
  }

  /**
   * {@inheritdoc}
   */
  public function add($entry) {
    $this->log[] = $entry;
    while (count($this->log) > $this->size) {
      array_shift($this->log);
    }
  }

  /**
   * {@inheritdoc}
   */
  public function dump() {
    $this->messenger()->addMessage(json_encode($this->log));
  }

  /**
   * {@inheritdoc}
   */
  public function clear() {
    $this->log = [];
  }

  /**
   * Display log.
   */
  public function __destruct() {
    $this->dump();
  }

}