final protected function doGetContent(string $partNumber): string
    {
        $return = \imap_fetchbody(
            $this->resource->getStream(),
            $this->getNumber(),
            $partNumber,
            \FT_UID | \FT_PEEK
        );

        if (false === $return) {
            throw new ImapFetchbodyException('imap_fetchbody failed');
        }

        return $return;
    }