The rbl.cluecentral.net DNSBLs were created in 2001 or 2002 by Sabri Berisha. The goal: To list "all known assigned IPv4 address space, by originating AS and by country. [This is based on] a full routing view is extracted daily from a router in the default free zone. The AS->country mapping is done via the statistics which are being provided by the four RIR's, ARIN, APNIC, LACNIC and RIPE."
Today, the website warns that the rbl.cluecentral.net service is closed. Sabri notes that "[it has become] more and more difficult and time-consuming to maintain a trustworthy list I started to notice more and more errors. The list is no longer of the quality needed to use in a production environment."
The website warns that if DNS queries continue at a high level, the DNS servers are likely to be configured in a way that will cause 100% of inbound mail attempts to be rejected, for all mail servers still using rbl.cluecentral.net. This makes it imperative that you remove any rbl.cluecentral.net zones from your mail server configuration, as soon as possible.