Render Twig in hook_mail()
$body_data = [
  '#theme' =&gt; 'something',
  '#somearg1' =&gt; 'somevalue1',
  '#somearg2' =&gt; 'somevalue2',
];
$body_content = \Drupal::service('renderer')-&gt;render($body_data);
$message['body'] = explode("\n", $body_content);