public function listIdentities($identity_type = '')
    {
        $this->_action = 'ListIdentities';
        $this->_method = 'GET';
        $this->_refreshDate();

        if (!preg_match('/^(EmailAddress|Domain|)$/', $identity_type)) {
            throw new Exception('IdentityType must be EmailAddress or Domain');
            return;
        }

        $parameters = array();
        if ($identity_type) {
            $parameters['IdentityType'] = $identity_type;
        }

        $this->_generateSignature($parameters);
        $res = $this->_request();
        if ($res['code'] == 200) {
            $members = $res['body']->ListIdentitiesResult->Identities->member;
            $ret = [];
            foreach ($members as $member) {
                $ret[] = (string) $member;
            }
            return $ret;
        } else {
            return new SimpleEmailServiceError((string) $res['body']->Error->Code);
        }
    }