clapf is an open source email content filter application. It includes a
statistical (inverse chi-square) antispam module, interface to several
anti-virus products, a minefield (blackhole), spam quarantine, support for
various RBL lists, training utilities and many more.

clapf can be used as an after queue content filter with Postfix (including as
an appliance) or via a local delivery agent (LDA), preferably maildrop.