/**
  * Returns the cache identifier for the mail.
  *
  * @param \Drupal\simplenews\Mail\MailInterface $mail
  *   The mail object.
  *
  * @return string
  */
 protected function getCid(MailInterface $mail)
 {
     $entity_id = $mail->getEntity()->id();
     return $mail->getEntity()->getEntityTypeId() . ':' . $entity_id . ':' . $mail->getLanguage();
 }