public function setSystemMessageType($messageType): Envelope
    {
        if (!in_array($messageType, static::getLetterTypeOptions())) {
            throw new InvalidArgumentException(
                sprintf('Property %s is not supported for %s', $messageType, __FUNCTION__)
            );
        }

        $this->data['letterType']['systemMessageType'] = $messageType;

        return $this;
    }