Net::DNSBL::MultiDaemon is the Perl module that implements the multi_dnsbl 
daemon.

multi_dnsbl is a DNS emulator daemon that increases the efficacy of DNSBL 
look-ups in a mail system. multi_dnsbl may be used as a stand-alone DNSBL or as 
a plug-in for a standard BIND 9 installation. multi_dnsbl shares a common 
configuration file format with the Mail::SpamCannibal sc_BLcheck.pl script so 
that DNSBL's can be maintained in a common configuration file for an entire 
mail installation.

Because DNSBL usefulness is dependent on the nature and source of spam sent to 
a specific site and because sometimes DNSBL's may provide intermittant service, 
multi_dnsbl interrogates them sorted in the order of greatest successful hits. 
DNSBL's that do not respond within the configured timeout period are not 
interrogated at all after 6 consecutive failures, and thereafter will be 
retried not more often than once every hour until they come back online. This 
eliminates the need to place DNSBL's in a particular order in your MTA's config 
file or periodically monitor the DNSBL statistics and/or update the MTA config 
file.

WWW: http://search.cpan.org/dist/Net-DNSBL-MultiDaemon/