Hi!

This is a bit offtopic, but i thought that maybe is useful for you:

There is a project called SPF (sender policy framework), which is a draft in the IETF as long is i know, that aims to build a network of trusted senders of emails.

Basically, a host exports a list of valid smtp hosts, so origin of emails is clearly defined.

This will not terminate spam, but at least can spot where it was originated.

More info in Newsforge.

Mail::SPF::Query module may help you to identify trusted hosts.