"I run a Postfix-based mail server that services a few hundred
users with an average load of a couple of thousand legitimate
messages a day--but thanks to spam, the actual load on the server
is much higher. I use Realtime Blackhole Lists (RBL) and
Distributed Checksum Clearinghouse (DCC) clients on Postfix and
SpamAssassin to reduce the impact of spam.
"RBLs are lists of IP addresses of known and potential spam
originators. There are many RBL providers, such as Spamhaus,
Spamcop, and DNSRBL. These lists are also known by various other
names, such as blacklists or blocking lists. Mail servers can
connect to RBL servers to check on IP addresses..."