The domain is now parked by a parking company. This DNS change has rendered the RBL issuing 100% false positives, marking every IP address as being blacklisted. Any mail server that is still using this RBL to filter out emails will see all sending IPs as being blacklisted (false positives) and in consequence will be rejecting all the incoming emails.
Posting this update in order to clear up the confusion, as some users think that this RBL is now back online and functioning because they are seeing their emails being rejected based on responses from this RBL, which (as explained above) are 100% false positives.
We have not been monitoring this RBL on our platform since April 2017, when it went dark.