private function failsMultiPartFilter(MessagePart $part)
    {
        if (!($part instanceof MimePart)) {
            return $this->multipart !== static::FILTER_EXCLUDE;
        }
        return ($this->multipart === static::FILTER_EXCLUDE && $part->isMultiPart())
            || ($this->multipart === static::FILTER_INCLUDE && !$part->isMultiPart());
    }