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
Patch README:
postfix-2.7.1-libspf2-1.2.x-4.README.txt
postfix-2.8.0-libspf2-1.2.x-4.README.txt
Patch download:
postfix-2.7.1-libspf2-1.2.x-4.patch
postfix-2.8.0-libspf2-1.2.x-0.patch
