These two commands will enable SpamAssassin for all users on a Plesk 8 server:
# mysql -u admin -p`cat /etc/psa/.psa.shadow` psa
mysql> update mail set spamfilter = 'true' where postbox = 'true';
# /usr/local/psa/admin/bin/mchk --with-spam
Thanks to Sean R. for this one!
