postoffice-1.0.x-dev/extensions/postoffice_image/tests/modules/postoffice_image_compat_test/postoffice_image_compat_test.module

<?php

/**
 * @file
 * Primary module hooks for Postoffice Image Compat Test module.
 */

/**
 * Implements hook_mail().
 */
function postoffice_image_compat_test_mail($key, &$message, $params) {
  /** @var \Drupal\Core\Render\RendererInterface $renderer */
  $renderer = \Drupal::service('renderer');

  $message['subject'] = $params['subject'];
  $message['body'][] = $renderer->renderInIsolation($params['body']);
}