Install DCC, Razor and Pyrzor as this helped spam assassin flag a lot more spam.
Take a look at these
https://marcocarcano.wordpress.com/postfix-mail-server-how-to-set-up-a-centos6-based-one-with-antivirus-and-antispam-part-3/
http://www.iredmail.org/forum/topic481-iredmail-support-install-pyzor-razor2-and-dcc-on-your-centosrhel-iredmail-server.html
Below is how I did it on CentOS 6 but do not blindly follow verbatim.
Also take a look at what I posted on https://www.virtualmin.com/node/43369#comment-765808 re configuring Postfix as this has been the most successful at reducing spam by over 90% so far as it doesn’t even reach spam assassin it gets rejected by the mail server before it hits it.
In Spam assassin my threshold is set to 2.
INSTALL AND SETUP PYRZOR
Open the following ports on your firewall
DCC port 6277 UDP
Razor2 port 2703 TCP
Pyzor port 24441 UDP
enable epl.repo
yum install pyzor perl-Razor-Agent
create user and group amavis including home directory
mkdir /etc/mail/spamassassin/.razor
mkdir /etc/mail/spamassassin/.pyzor
chown amavis:amavis /etc/mail/spamassassin/.razor
chown amavis:amavis /etc/mail/spamassassin/.pyzor
su -m amavis -c ‘pyzor --homedir /etc/mail/spamassassin/.pyzor discover’
su -m amavis -c ‘razor-admin -home=/etc/mail/spamassassin/.razor -create’
su -m amavis -c ‘razor-admin -home=/etc/mail/spamassassin/.razor -register’
INSTALL DCC
cd /tmp
wget http://www.dcc-servers.net/dcc/source/dcc-dccproc.tar.Z
tar xzvf dcc-dccproc.tar.Z
cd dcc-dccproc-1.3.158
./configure --with-uid=amavis
make
make install
chown -R amavis:amavis /var/dcc
ln -s /var/dcc/libexec/dccifd /usr/local/bin/dccifd
Add the following to /etc/mail/spamassassin/local.cf
dcc
use_dcc 1
dcc_home /var/dcc
dcc_path /usr/local/bin/dccproc
dcc_timeout 10
add_header all DCC DCCB: DCCR
pyzor
use_pyzor 1
pyzor_path /usr/bin/pyzor
add_header all Pyzor PYZOR
razor
use_razor2 1
razor_config /etc/mail/spamassassin/.razor/razor-agent.conf
score RAZOR2_CHECK 2.500
score PYZOR_CHECK 2.500
score DCC_CHECK 4.000
Enable the DCC plugin in /etc/mail/spamassassin/v310.pre
loadplugin Mail::SpamAssassin::Plugin::DCC
testing commands
spamassassin -t -D razor2 < /usr/share/doc/spamassassin-3.3.1/sample-spam.txt
spamassassin -t -D pyzor < /usr/share/doc/spamassassin-3.3.1/sample-spam.txt
cdcc info
echo “test” | spamassassin -D pyzor 2>&1 | less
spamassassin -t -D < /tmp/spam