static function user2email($user, $first=true, $extended=false)
    {
        $rcube = rcube::get_instance();
        $plugin = $rcube->plugins->exec_hook('user2email',
            array('email' => NULL, 'user' => $user,
                'first' => $first, 'extended' => $extended));

        return empty($plugin['email']) ? NULL : $plugin['email'];
    }