I meant MIME types like: [Content-Type: application/hta]. And I meant string filtering like: iptables -A INPUT -p tcp -dport 25 -m string --string "Buy now" -j REJECT --reject-with tcp-reset (needs a kernel patch AFAIK; it is further described in: http://www.linuxsecurity.com/feature_stories/feature_story-148.html ).
