Vipul's Razor is a checksum-based, distributed, collaborative, spam-detection-and-filtering network. Through user contribution, Razor establishes a distributed and constantly updating catalogue of spam in propagation that is consulted by email clients to filter out known spam. Detection is done with statistical and randomized signatures that efficiently spot mutating spam content. User input is validated through reputation assignments based on consensus on report and revoke assertions which in turn is used for computing confidence values associated with individual signatures.
Some e-mail clients directly use Razor. It is also used by some server-side spam filters (for example, SpamAssassin).