Malware is used for a harmful purpose. It can be in your software or hardware. Email and free (don't confuse with OSS) or pirated software is the most famous way to spread malware. It is inserted in a system w/o user notification.
Wikipedia defines Malware as:
Malware is software designed to infiltrate or damage a computer system without the owner's informed consent. It is a portmanteau of the words "malicious" and "software". The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.
It will be a nice idea to block malware spreading urls and website. Setting up a mlaware blacklist in Postix MTA is quite easy. The Malware Block List is a free, automated and user contributed system for checking URLs for the presence of Viruses, Trojans, Worms, or any other software considered Malware. The list is available in 25 formats.
Create a blacklist
First you need to create a blacklist, type the following command:
# wget -O - http://www.malware.com.br/cgi/submit?action=list_postfix > /etc/postfix/mbl-body-deny
Open postfix main.cf file:
# vi /etc/postfix/main.cf
Setup postfix body_check directive:
body_checks = regexp:/etc/postfix/mbl-body-deny
Now just restart postfix:
# postmap /etc/postfix/mbl-body-deny
# /etc/init.d/postfix restart
You need to setup a cron job to automate entire procedure. Create a shell script as follows (Download link):
# Script to update malware urls
/usr/bin/wget -O - http://www.malware.com.br/cgi/submit?action=list_postfix > /etc/postfix/mbl-body-deny
Add cronjob as follows:
40 23 * * * /etc/admin/scripts/fetch.postfixmalware.sh >/dev/null 2>&1
You may wan to combine this feature with mime attachments blocking and anti spam blacklist for the best result.