install the spamc of the os
as i know right, ispconfig is NOT using or starting the spamd daemon.
for me (at debian) this solution works fine:
1) aptitude install spamassassin (to install the version of my os)
2) change /root/ispconfig/isp/conf/spamassassin.rc: | /usr/bin/spamc
3) update isp_isp_user set status='u' where status ) '';
4) change anything inside one user (to rewrite the files)
after this, everything works fine