Interesting, but you have to remember to try and not stop legitimate email.
Personally, I have use something like this but it's my own creation. I have (over time) built up a list of dodgy IP addresses and address blocks that are suspect. What I do is use the access feature of postfix, list them all in there (they are stored in a mysql db with a script that updates the access.cidr), that way I can give these IP's a reject, well error 554, message and direct them to a webpage which has a form they can complete (if they are a genuine sender) to get their IP removed from the list.
This avoids blacklisting legitimate senders (as long as they read the mailserver reply), but of course it does not block like using iptables, postfix just rejects their mail.
Postfix access manual - http://www.postfix.org/access.5.html