I want to share how i reduce spam in my box.
Maybe not perfect solution but  works
Always make backup and use it at your own risk !
Tested on CentOS 5
Install Greylist:

# rpm -Uvh http://dl.atrpms.net/el5-i386/atrpms/stable/greylistd-0.8.3.2-8.0.el5.noarch.rpm

Now you need to change permissions:

# chown -R mail.mail /etc/greylistd/
# chown -R mail.mail /var/run/greylistd/
# chown -R mail.mail /var/lib/greylistd/
# chown -R mail.mail /usr/sbin/greylist*

start [...]