protected function getAddressesForPrincipal($principal)
    {
        $CUAS = '{'.self::NS_CALDAV.'}calendar-user-address-set';

        $properties = $this->server->getProperties(
            $principal,
            [$CUAS]
        );

        // If we can't find this information, we'll stop processing
        if (!isset($properties[$CUAS])) {
            return [];
        }

        $addresses = $properties[$CUAS]->getHrefs();

        return $addresses;
    }