/**
  * @param OutboundMessage $message
  * @param ReplyableMessage $parentMessage
  */
 public function modifyToReplyMessage(OutboundMessage $message, ReplyableMessage $parentMessage)
 {
     $references = $parentMessage->getReferences() . ' ' . $parentMessage->getMessageId();
     $message->setReferences($references);
     $message->setInReplyTo($parentMessage->getMessageId());
     $message->setSubject(static::SUBJECT_PREFIX . $parentMessage->getSubject());
 }