public function onReceivedAtChanged($event)
    {
        $sender = $event->sender;
        $raAttribute = $sender->receivedAtAttribute;
        if (!is_string($raAttribute) || empty($raAttribute)) {
            return;
        }
        // If it is permitted to change receiving time, then it will return directly.
        if ($sender->permitChangeReceivedAt) {
            return;
        }
        $oldRa = $sender->getOldAttribute($raAttribute);
        if ($oldRa != null && !$sender->isInitDatetime($oldRa) && $sender->$raAttribute != $oldRa) {
            $sender->$raAttribute = $oldRa;
        }
    }