Install SPF policy server

Check openspf.org for the latest development. For Postfix, http://www.openspf.org/Software currently lists two packages: python-postfix-policyd-spf and  postfix-policyd-spf-perl. It says that python-postfix-policyd-spf is more feature.

Install python-postfix-policyd-spf:

Download the package, extract the tar ball, and follow the README in the source tree.

Install pyspf package:

"emerge -pv pyspf" showed that the package is currently masked. Following emerge and portage man page (man emerge, man portage), let's add the following line into the file /etc/portage/package.keywords:

=dev-python/pyspf-2.0.5

Then run 

# emerge pyspf

Configure postfix:

Edit /etc/postfix/main.cf and /etc/postfix/master.cf as directed by the README file in the python-postfix-policyd-spf package tree. Then reload postfix.

Enable SPF in SpamAssassin

References

http://www.openspf.org/Software

https://help.ubuntu.com/community/Postfix/SPF

http://www.howtoforge.com/postfix_spf

http://www.openspf.org/Implementations