private function _validateLocalPart()
    {
        // First try to match the local part on the common dot-atom format
        $result = false;

        // Dot-atom characters are: 1*atext *("." 1*atext)
        // atext: ALPHA / DIGIT / and "!", "#", "$", "%", "&", "'", "*",
        //        "+", "-", "/", "=", "?", "^", "_", "`", "{", "|", "}", "~"
        $atext = 'a-zA-Z0-9\x21\x23\x24\x25\x26\x27\x2a\x2b\x2d\x2f\x3d\x3f\x5e\x5f\x60\x7b\x7c\x7d\x7e';
        if (preg_match('/^[' . $atext . ']+(\x2e+[' . $atext . ']+)*$/', $this->_localPart)) {
            $result = true;
        } else {
            // Try quoted string format

            // Quoted-string characters are: DQUOTE *([FWS] qtext/quoted-pair) [FWS] DQUOTE
            // qtext: Non white space controls, and the rest of the US-ASCII characters not
            //   including "\" or the quote character
            $noWsCtl = '\x01-\x08\x0b\x0c\x0e-\x1f\x7f';
            $qtext   = $noWsCtl . '\x21\x23-\x5b\x5d-\x7e';
            $ws      = '\x20\x09';
            if (preg_match('/^\x22([' . $ws . $qtext . '])*[$ws]?\x22$/', $this->_localPart)) {
                $result = true;
            } else {
                $this->_error(self::DOT_ATOM);
                $this->_error(self::QUOTED_STRING);
                $this->_error(self::INVALID_LOCAL_PART);
            }
        }

        return $result;
    }