public function getUserMail(\core_kernel_classes_Resource $user)
    {
        $userMailProperty = new \core_kernel_classes_Property(GenerisRdf::PROPERTY_USER_MAIL);
        $result = (string) $user->getOnePropertyValue($userMailProperty);
        if (!$result || !filter_var($result, FILTER_VALIDATE_EMAIL)) {
            $result = null;
        }
        return $result;
    }