_isMboxFile example

            throw new Zend_Mail_Storage_Exception("can't read dir $currentDir");
        }
        while (($entry = readdir($dh)) !== false) {
            // ignore hidden files for mbox             if ($entry[0] == '.') {
                continue;
            }
            $absoluteEntry = $currentDir . $entry;
            $globalName = $parentGlobalName . DIRECTORY_SEPARATOR . $entry;
            if (is_file($absoluteEntry) && $this->_isMboxFile($absoluteEntry)) {
                $parentFolder->$entry = new Zend_Mail_Storage_Folder($entry, $globalName);
                continue;
            }
            if (!is_dir($absoluteEntry) /* || $entry == '.' || $entry == '..' */) {
                continue;
            }
            $folder = new Zend_Mail_Storage_Folder($entry, $globalName, false);
            $parentFolder->$entry = $folder;
            $this->_buildFolderTree($absoluteEntry . DIRECTORY_SEPARATOR, $folder, $globalName);
        }

        

$this->_fh = @fopen($filename, 'r');
        if (!$this->_fh) {
            /** * @see Zend_Mail_Storage_Exception */
            throw new Zend_Mail_Storage_Exception('cannot open mbox file');
        }
        $this->_filename = $filename;
        $this->_filemtime = filemtime($this->_filename);

        if (!$this->_isMboxFile($this->_fh, false)) {
            @fclose($this->_fh);
            /** * @see Zend_Mail_Storage_Exception */
            throw new Zend_Mail_Storage_Exception('file is not a valid mbox format');
        }

        $messagePos = array('start' => ftell($this->_fh), 'separator' => 0, 'end' => 0);
        while (($line = fgets($this->_fh)) !== false) {
            if (strpos($line, 'From ') === 0) {
                $messagePos['end'] = ftell($this->_fh) - strlen($line) - 2; // + newline