Although this is sad to see, one of the most trusted DNSBL providers I know are going away.
Since yesterday, they have stopped serving DNSBL requests from all of their public hostnames (dnsbl.ahbl.org, ircbl.ahbl.org, rhsbl.ahbl.org). Instead, you'll get no data. The name server records for the DNSBL zones have already been removed.
My recommendation to you is to remove these DNSBL zones ASAP. The first (and most obvious) reason is that the DNSBL no longer works. The second (and more dangerous) reason is that on January 1st, 2015, AHBL will wildcard all DNSBL zones to return positive! In real world terms, this means that any lookup will return a positive hit, no questions asked.
In my opinion, it really is sad to see them leave, but they say that they will continue to have private DNSBLs for folks they invite. Maybe I'll make the cut :)
Source: AHBL Blog