SPF (sender policy framework) is a working but disputable method of SPAM protection.
It prevents fake sender addresses.
I have updated the SPF patch from Nigel Kukard for Postfix 2.7.1, fixed IPv6 support and added the permissive option “permit_spf_valid_sender”. With this patch the libspf2 library is called directly from Postfix. This patch is already available in the FreeBSD mail/postfix port as a selectable option.
Update 2011-02-07: I have updated the patch for 2.8.0
License: Secure Mailer License (Postfix License)
I personally use this patch for greylisting positives (= not greylisting mail from valid SPF senders).
Original patch homepage