client.search(
                'ou=domain,dc=sub,dc=net',
                opts, (err, res) => {
                    if (err) {
                        cb(err, null);
                        return;
                    }
                    res.on(
                        'searchEntry', entry => {
                            if (entry.object.hasOwnProperty('mail')) {
                                retrievedUsers[entry.object.uid] = entry.object;
                            }
                        }
                    );
                    res.on(
                        'error', err => {
                            cb(err, null);
                            client.unbind(
                                err => {
                                    if (err) {
                                        logger.error(err);
                                    }
                                }
                            );
                        }
                    );
                    res.on(
                        'end', () => {
                            client.unbind(
                                err => { // << --- never called
                                    cb(err, retrievedUsers);
                                }
                            );
                        }
                    );
                }
            );