Vipul's Razor is a 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.

Razor Agents are written in Perl and will work on most Unix operating
systems and others OSes for which perl is available.

Vipul's Razor v2 is almost a complete rewrite of Razor v1.

Homepage:
https://razor.sourceforge.net/
