Description:

Distributed Checksum Clearinghouse or DCC is a cooperative,
distributed system intended to detect "bulk" mail or mail sent to
many people. It allows individuals receiving a single mail message
to determine that many other people have been sent essentially
identical copies of the message and so reject the message. It can
identify some unsolicited bulk mail using "spam traps" and other
detectors, but that is not its focus.

The DCC can be viewed as a tool for end users to enforce their
right to "opt-in" to streams of bulk mail by refusing all bulk mail
except from sources in a "white list." White lists are generally
the responsibility of DCC clients, since only they know which bulk
mail they solicited.

NB to use DCC to reject SPAM you need to configure
/usr/share/dcc/dcc_conf and either use procmail or sendmail to
feed the messages to DCC