calisun
rpm -qi spamassassin-botnet
or view the rpm using mc
SUMMARY
A botnet plugin for SpamAssassin
DESCRIPTION
Botnet looks for possible botnet sources of email by checking various
DNS values that indicate things such as other ISP's clients or
workstations, or misconfigured DNS settings that are more likely to
happen with client or workstation addresses than servers.
Botnet looks in the Untrusted Relays pseudoheader. It defaults to
looking at the first relay in that list. However, certain options
allow it to skip past relays in that list (or not score a hit if it
finds certain relays).
IIRC also look in bugzilla for some information.
Google for more details