private function get_realm()
    {
        // When user enters a username without domain part, realm
        // allows to add it to the username (and display correct username in the table)

        if (isset($_SESSION['acl_username_realm'])) {
            return $_SESSION['acl_username_realm'];
        }

        // find realm in username of logged user (?)
        list($name, $domain) = explode('@', $_SESSION['username']);

        // Use (always existent) ACL entry on the INBOX for the user to determine
        // whether or not the user ID in ACL entries need to be qualified and how
        // they would need to be qualified.
        if (empty($domain)) {
            $acl = $this->rc->storage->get_acl('INBOX');
            if (is_array($acl)) {
                $regexp = '/^' . preg_quote($_SESSION['username'], '/') . '@(.*)$/';
                foreach (array_keys($acl) as $name) {
                    if (preg_match($regexp, $name, $matches)) {
                        $domain = $matches[1];
                        break;
                    }
                }
            }
        }

        return $_SESSION['acl_username_realm'] = $domain;
    }