this code can't work, cause you are working here with namespace OOP but in the base.php only Classes with "Sabre_" gets autoloaded, so there occurs an error!
correct definition:
```
if($email !== null) {
	$caldavPlugin = new Sabre_CalDAV_Plugin();
	$caldavPlugin->setIMipHandler(
		new Sabre_CalDAV_Schedule_IMip($email)
	);
	$server->addPlugin($caldavPlugin);
}
```