http://ezine.daemonnews.org/200309/postfix-spamassassin.html
48 5 * * * /usr/local/bin/sa-update --channel updates.spamassassin.org && /usr/local/etc/rc.d/sa-spamd restart
To filter before acception mails, e.g. with amavisd it is quiet simple. In my case amavisd is accepting mails on port 10024 and transfer it back on port 10025. We add the following into the master.cf:
smtp inet n - n - 20 smtpd -o smtpd_proxy_filter=127.0.0.1:10024 -o smtpd_client_connection_count_limit=10 -o smtpd_proxy_options=speed_adjust localhost:10025 inet n - n - - smtpd -o smtpd_authorized_xforward_hosts=127.0.0.0/8 -o smtpd_client_restrictions -o smtpd_helo_restrictions -o smtpd_sender_restrictions -o smtpd_recipient_restrictions=permit_mynetworks,reject -o smtpd_data_restrictions -o mynetworks=127.0.0.0/8 -o receive_override_options=no_unknown_recipient_checks
Make sure amavisd is running and restart postfix.