Postfix/DCC

The Distributed Checksum Clearinghouses (or DCC) is an anti-spam content filter based upon the exchange between mail servers of fuzzy checksums identifying message content and the number of such messages received. The checksums are constant across common variations in bulk messages, including "personalizations." The official project website is at rhyolite.com/dcc/.

Background
The idea of DCC is that if mail recipients could compare the mail they receive, they could recognize unsolicited bulk mail. A DCC server totals reports of checksums of messages from clients and answers queries about the total counts for checksums of mail messages. A DCC client reports the checksums for a mail message to a server and is told the total number of recipients of mail with each checksum. If one of the totals is higher than a threshold set by the client and according to local whitelists the message is unsolicited, the DCC client can log, discard, or reject the message.

Because simplistic checksums of spam would not be effective, the main DCC checksums are fuzzy and ignore aspects of messages. The fuzzy checksums are changed as spam evolves. Since DCC started being used in late 2000, the fuzzy checksums have been modified several times.