the Distributed Checksum Clearinghouse

The 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 received essentially identical copies of the message and so reject or discard the message.

Source for the server, client, and utilities is available at Rhyolite Software, LLC, http://www.rhyolite.com/dcc/  It is free for organizations that do not sell spam or virus filtering services.