public function getRecipientsFromPost(array $referenced_entity) {
    $recipients = [];

    $post = Post::load($referenced_entity['target_id']);
    $recipient_user = $post->get('field_recipient_user')->getValue();
    if (!empty($recipient_user)) {
      $recipients[] = [
        'target_type' => 'user',
        'target_id' => $recipient_user['0']['target_id'],
      ];
    }

    return $recipients;
  }