Below is the full text for part 4 of the guide. If you have any questions concerning this guide, please ask at the Postfix Mail Server of HughesJR.com.
Here comes the last part of the PostFix E-mail server install.

That is to install Mailscanner utilizing Spamassassin and ClamAV.

We will install ClamAV from Dag Wieers' RHEL 4 yum repository. I will show you how to only use Dag's repo for getting ClamAV and unrar (a program similar to unzip for checking compressed files). I want to be careful with Dag's repo, because it can over-write system files if we are not careful. You can add other programs from Dag's repo by adding the name of the program in the includepkgs= line in the [dag] repo section below.

To use Dag's repo, you will need to create a new .repo file in your /etc/yum.repos.d directory to point to the mirror that is closest to you. A list of mirrors available can be found here. There is a mirror in the United States here (that is not listed at the above site):

http://dag.linux.iastate.edu/dag/redhat/el$releasever/en/$basearch/dag

That is the main mirror location I will use (we will add a couple backup mirrors as well). Create a file named /etc/yum.repos.d/dag.repo and have it contain the following lines:

[dag]
name=Dag-RHEL-Yum
baseurl=http://dag.linux.iastate.edu/dag/redhat/el$releasever/en/$basearch/dag
http://www.mirrorservice.org/sites/apt.sw.be/redhat/el$releasever/en/$basearch/dag
http://mirrors.ircam.fr/pub/dag/redhat/el$releasever/en/$basearch/dag
http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
enabled=1
gpgcheck=1
includepkgs=clamav clamav-devel clamav-db unrar

With this repo file, the only packages that can install from Dag's repo are the ones we need for this install (clamav).
Install Dag's rpm signing key. (Dag's instructions). This should do it:

rpm --import http://dag.wieers.com/packages/RPM-GPG-KEY.dag.txt

Now to install the spamassassin and clamav like this (also includes other packages that will be required later for other parts of this install):

yum install spamassassin clamav clamav-devel sendmail-devel bzip2-devel gmp-devel zlib-devel autoconf automake rpm-build rpm-devel gcc perl-CPAN curl-devel unrar
(this is all one line)

2. Install the ClamAV Perl module via these instructions. I was able to accept all the defaults in the install all the way down to picking the closest CPAN servers (for step 1). I chose a couple from the first USA page. I did not upgrade CPAN ... I just installed the module via Steps 2 and 3. SKIP steps 1, 4, and 5 until we install MailScanner below....you are finished with this page and should now have the ClamAV perl module installed.

3.Download the latest MailScanner from here:

http://www.sng.ecs.soton.ac.uk/mailscanner/files/4/rpm/

At the time I wrote this guide, the lastest version is MailScanner-4.42.5-1.rpm.tar.gz ... you can download this version with:

wget http://www.sng.ecs.soton.ac.uk/mailscanner/files/4/rpm/MailScanner-4.42.5-1.rpm.tar.gz

4. Now we will unpack it with the command:

tar -xvzf MailScanner-4.42.5-1.rpm.tar.gz

5.Now we will install MailScanner by going into the extracted directory and issuing these commands:

cd MailScanner-4.42.5-1
export LANG=C; ./install.sh

6. Stop postfix with the command:

/etc/init.d/postfix stop

Ensure postfix doesn't restart, as Mailscanner will start it from now on, with the command:

chkconfig postfix off

7. Make the following changes to the /etc/postfix/main.cf file... add the line:

header_checks = regexp:/etc/postfix/header_checks

8. Edit the file /etc/postfix/header_checks, add this line to the file:

/^Received:/ HOLD

9.Now we are ready to setup the file /etc/MailScanner/MailScanner.conf, which is the configuration file for MailScanner. In this file, edit it to show the following values:

%org-name% = yoursite
Run As User = postfix
Run As Group = postfix
Incoming Queue Dir = /var/spool/postfix/hold
Outgoing Queue Dir = /var/spool/postfix/incoming
MTA = postfix
File Timeout = 120
Maximum Archive Depth = 20
Virus Scanners = clamavmodule
Monitors for ClamAV Updates = /var/clamav/*.cvd
Use SpamAssassin = yes
SpamAssassin User State Dir = /var/spool/MailScanner/spamassassin

The MailScanner file is very well documented, and here is a guide to help you configure other parts of this file so you can get the most out of your Spamassassin and have more control over how mail is delivered to your users.

Here are a couple other optional setting in MailScanner.conf:

Spam List = ORDB-RBL SBL+XBL SORBS-DNSBL CBL RSL DSBL spamcop
Allow IFrame Tags = yes
Log IFrame Tags = yes
Allow Script Tags = yes
Allow Object Codebase Tags = yes
Convert Dangerous HTML To Text = yes
Minimum Stars If On Spam List = 3
Spam Lists To Reach High Score = 3

10. You will need to ensure that the user "postfix" can write to /var/spool/MailScanner/incoming and /var/spool/MailScanner/quarantine with the commands:

chown postfix.postfix /var/spool/MailScanner/incoming
chown postfix.postfix /var/spool/MailScanner/quarantine
mkdir -m 700 /var/spool/MailScanner/spamassassin
chown postfix.postfix /var/spool/MailScanner/spamassassin

If you upgrade your copy of MailScanner, these directories may be changed back to being owned by root, so you may have to do the chown commands again.

11. ClamAV should try to update hourly, controlled by MailScanner (via the cron job /etc/cron.hourly/update_virus_scanners).

To setup ClamAV for auto updates, edit the file /etc/MailScanner/virus.scanners.conf. Find the line: