The Spamhaus Data Query Service (DQS) plugin for SpamAssassin enhances existing functions by checking HELO/EHLO, From, Reply-To, Envelope-From and Return-Path against Spamhaus DBL/ZRD blacklists. It also scans the e-mail body for e-mail addresses and performs blacklist lookups against the domains or its authoritative nameservers. Further checks cover the reverse DNS matches in DBL/ZRD blacklists or the SBL/CSS lookups for IP addresses or IP addresses of authoritative nameservers of domains being part of the e-mail body.
While the DQS usage is free under the same terms like when using public mirrors (which are shipped in SpamAssassin as default configuration), a registration procedure for a free DQS key is mandatory nevertheless.