Install:
apt-get install spamassassin postfix
in /etc/postfix/master.cf modify;
smtp inet n - - - - smtpd
-o content_filter=spamassassin
and add:
spamassassin unix - n n - - pipe
flags=Rq user=nobody argv=/usr/bin/spamc -u ${user} -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}
Spamc will now be run as the user reciving the email and then delivered with sendmail back
service spamassassin restart
postfix reload