Install SpamAssassin
SpamAssassin is a spam scanning engine. It's written in Perl, needs lots of resources, but is still very helpful. It is called via the spamassassin plugin.
apt-get install -y spamassassin
sed -i.bak -e 's/ENABLED=0/ENABLED=1/' /etc/default/spamassassin
sed -i.bak -e 's/CRON=0/CRON=1/' /etc/default/spamassassin
update-rc.d spamassassin enable
service spamassassin start
sed -i.bak -e 's/#spamassassin/spamassassin/' $HARAKA_CONF/plugins