/**
  * add/clear flags on imap server
  *
  * @param array $_imapMessageUids
  * @param array $_flags
  * @param Expressomail_Backend_ImapProxy $_imapBackend
  * @throws Expressomail_Exception_IMAP
  */
 protected function _updateFlagsOnImap($_imapMessageUids, $_flags, $_imapBackend, $_mode)
 {
     $flagsToChange = array_intersect($_flags, array_keys(self::$_allowedFlags));
     if (empty($flagsToChange)) {
         return;
     }
     if (Tinebase_Core::isLogLevel(Zend_Log::TRACE)) {
         Tinebase_Core::getLogger()->trace(__METHOD__ . '::' . __LINE__ . ' ' . $_mode . 'ing flags on IMAP server for ' . print_r($_imapMessageUids, TRUE) . ' messages:' . print_r($flagsToChange, TRUE));
     }
     try {
         if ($_mode === 'add') {
             $_imapBackend->addFlags($_imapMessageUids, $flagsToChange);
         } else {
             if ($_mode === 'clear') {
                 $_imapBackend->clearFlags($_imapMessageUids, $flagsToChange);
             }
         }
     } catch (Zend_Mail_Storage_Exception $zmse) {
         throw new Expressomail_Exception_IMAP($zmse->getMessage());
     }
 }