The IESG has approved “SIEVE Spamtest and Virustest Extensions” as a Proposed Standard.
This document has been reviewed in the IETF but is not the product of an IETF Working Group.
The SIEVE mail filtering language “spamtest” and “virustest” extensions permit users to use simple, portable commands for spam and virus tests on email messages. Each extension provides a new test using matches against numeric ’scores’. It is the responsibility of the underlying SIEVE implementation to do the actual checks that result in values returned by the tests.
http://www.ietf.org/internet-drafts/draft-daboo-sieve-spamtest-04.txt