Edit the /etc/clamav-milter.conf and change the configuration to "AddHeader Add".
[root@mail ~]# less /etc/clamav-milter.conf
# If this option is set to "Replace" (or "Yes"), an "X-Virus-Scanned" and an
# "X-Virus-Status" headers will be attached to each processed message, possibly
# replacing existing headers.
# If it is set to Add, the X-Virus headers are added possibly on top of the
# existing ones.
# Note that while "Replace" can potentially break DKIM signatures, "Add" may
# confuse procmail and similar filters.
# Default: no
Restart the clamav-milter daemon after changing the configuration file, then try sending emails from/to your user in your mailserver and check the header if the clamav-milter is already working. This is also one way to check if your clamav-milter integration with your email server is working correctly.
X-Virus-Status: Clean X-Virus-Scanned: clamav-milter 0.95.3 at mail.example.com Received: (qmail 49613 invoked by uid 60001); 29 Mar 2010 08:59:04 -0000