public function getEmbeddedMessage(): EmbeddedMessageInterface
    {
        if (!$this->isEmbeddedMessage()) {
            throw new NotEmbeddedMessageException(\sprintf(
                'Attachment "%s" in message "%s" is not embedded message',
                $this->getPartNumber(),
                $this->getNumber()
            ));
        }

        return new EmbeddedMessage($this->resource, $this->getNumber(), $this->getPartNumber(), $this->getStructure()->parts[0]);
    }