/**
 * Implements hook_mail_alter().
 */
function {{ machine_name }}_mail_alter(&$message) {
  if ($message['id'] == 'modulename_messagekey') {
    if (!example_notifications_optin($message['to'], $message['id'])) {
      // If the recipient has opted to not receive such messages, cancel
      // sending.
      $message['send'] = FALSE;
      return;
    }
    $message['body'][] = "--\nMail sent out from " . \Drupal::config('system.site')->get('name');
  }
}