Vipul's Razor is a distributed, collaborative, spam detection and
filtering network. Razor establishes a distributed and constantly
updating catalogue of spam in propagation. This catalogue is used by
clients to filter out known spam. On receiving a spam, a Razor
Reporting Agent (run by an end-user or a troll box) calculates and
submits a 20-character unique identification of the spam (a SHA Digest)
to its closest Razor Catalogue Server. The Catalogue Server echos this
signature to other trusted servers after storing it in its database.
Prior to manual processing or transport-level reception, Razor
Filtering Agents (end-users and MTAs) check their incoming mail against
a Catalogue Server and filter out or deny transport in case of a
signature match. Catalogued spam, once identified and reported by a
Reporting Agent, can be blocked out by the rest of the Filtering Agents
on the network.

Can be used as one of the spamassassin rules.