If you are using John Simpsons qmail-smtpd-run script SPF Checking is now done within his script at the SMTP level. See his web page for additional configuration information.

To install SPF, do the following:

# cd ~root/qmail/
# wget http://freebsdrocks.net/files/tarballs/Mail-SPF-Query-1.997.tar.gz
# tar xvzf Mail-SPF-Query-1.997.tar.gz
# cd Mail-SPF-Query-1.997
# perl Makefile.PL && make && make install

You can test this installation (and that PER5LIB is set correctly) with perl -e 'require Mail::SPF::Query'.

Now we can let spamassassin check for SPF headers