public function isValid($value)
 {
     $translate = Zend_Registry::get('translate');
     $this->_messageTemplates[self::EXIST] = $translate->_('This email really exist, please choose another');
     $value = (string) $value;
     $user_tbl = new VC_DbTable_User();
     $row = $user_tbl->fetchRow($user_tbl->select("id")->where("email = ?", $value));
     if ($row && sizeof($row)) {
         //really exist
         $this->_error(self::EXIST);
         return false;
     }
     return true;
 }