Resources:
SpamAssassin can look up a sender's IP address in a set of DNSBLs or RBLs to determine whether they have been listed as known spam source, open proxy or relay, dialup host, etc. Many ISPs perform these checks in the MTA itself in order to reject connections from such hosts at the earliest possible point. If you do that, you can prevent SpamAssassin from doing its own lookups by setting the directive skip_rbl_checks to 1; the default is 0. It is also possible to perform lookups against one set of DNSBLs at the MTA and a different set in SpamAssassin.
The DSBL (Distributed Sender Blackhole List) is now dead. Please refer to the website dsbl.org for their notice.
DSBL used to provide three services:
All three have to be removed from your MTA/spamfilter configuration unless they are able to restore their service. There is no indication of such up to this date yet.
Even the website previously at http://www.ordb.org/ is gone. Thus; you should remove relays.ordb.org from your MTA/spamfilter configuration.
A spam clearinghouse is a server (or a distributed network of servers) that gathers spam messages reported by thousands of users around the world and provides a mechanism for a client to check a new message to see if it matches a message in the clearinghouse.
As of version 3.0, SpamAssassin can consult three clearinghouses:
SpamAssassin can also be used to report spam to the clearinghouses. Each clearinghouse uses its own client software, and you should install these clients before you install SpamAssassin.
SpamAssassin is able to check the URLs contained in an email message body against the DNSRB lists. For SpamAssassin to use Trend Micro's web reputation network service, read here.