procmail.
Quick Spam Filter (qsf) is a lightweight statistical spam filter written in C.
It is designed to be small, fast, reliable, easy to install, and simple to use
in a procmail recipe
WWW: http://www.ivarch.com/programs/qsf.shtml
PR: 61486
Submitted by: liamfoy@sepulcrum.org
Approved by: erwin (mentor) (implicitly)