Thanks for the response, it certainly sounds like there is an extremely interesting story there.

I guess outbound SMTP filtering is an obvious solution when you think but is it expensive or difficult to set up? It is probably less expensive than inbound filtering? I suppose a lot of ISPs just may not be bothered setting it up?

Sorry I thought about the response but forgot to post it :)

Luckily for most of us it is ISP's that are mostly taking the initiative and setting up outbound filtering , it's nothing like censorship or anything as some people assume it's simply looking for known spam patterns as used in 419 Advanced Fee Fraud Spam , phishing spam , enlargement medz etc , outbound filtering can detect unusual sending patterns that should not be coming from their networks , this is helped places like Bigpond /telstra , westnet ,iinet to name a few to avoid regular blacklisting.
If only some of the free mail provider like Gmail ( google ) , yahoo , wannadoo france etc would get their acts together we could rid the world of about 30% of all spam
This is some interesting reading and advice for mail server admins
http://www.wiki.sorbs.net/wiki/bin/view/SORBS/FAQforISPs#Spam_Database


There is also new problem that is causing more blacklistings than spam itself , its called backscatter ( or NDR's ) , It's when mail servers bounce known spam addressed to legitimate users on their networks or to none existent users back to the sender.
Any ISP admin with a brain bigger than a jaffa should know that spammers never use their own address in the " from " field , the from field in spam is always forged meaning that when they bounce spam back to sender they are always sending it back to someone who never had anything to do with spamming to begin with , basically making their problem someone else's .