ClamAV

http://www.clamav.netdownload http://prdownloads.sourceforge.net/clamav

command line scan tool
/usr/local/bin/clamdscan
command line scan tool, use with clamd, scan speed fast
/usr/local/bin/clamscan
virus pattern update tool
/usr/local/bin/freshclam
clamav daemon
/usr/local/sbin/clamd
lib
/usr/local/lib/libclamav.so.1 -> libclamav.so.1.0.4
/usr/local/lib/libclamav.so.1.0.4
config
/usr/local/etc/freshclam.conf
/usr/local/etc/clamav.conf
virus pattern
/usr/local/share/clamav/daily.cvd
/usr/local/share/clamav/main.cvd

use clamassassin for mail scan
http://drivel.com/clamassassin/

To pass your mail through clamassassin, add the following command to your
procmail rc file:

:0fw
| /usr/local/bin/clamassassin

If you installed the program elsewhere, change the path. The above will
scan the mesage for viruses and add the headers X-Virus-Status and
X-Virus-Report as appropriate. Procmail then continues processing the
message as normal. Nothing else will happen unless you add additional
rules to sort or discard detected viruses. To do this, you might do the
following:

:0:
* ^X-Virus-Status: Yes
IN.virus

ps. http://monkey.org/openbsd/archive/misc/0310/msg00002.html
The following are the open source antivirus solution:

(a) Open Antivirus
http://www.openantivirus.org

(b) Clam AV
http://clamav.elektrapro.com

(c) Trophie
http://www.vanja.com/tools/trophie/
(based on Trend Micro Virus scanning engine library)

(d) Sophie
http://www.vanja.com/tools/sophie/
(based on Sophos SAVI AV interface)