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Mail::RBL
Perl extension to access RBL-style host verification services...
RBL Spam Daemon
rblsd is a small, fast, SpamAssassin-compatible spam filter....
qmail-rblchk
qmail-rblchk filter all incoming mail and check if it comes from RBL listed IPs....
Net::Blacklist::Client
Queries multiple RBLs or URIBLs in parallel....