They eventually changed to greylisting, which filters out non-rfc-compliant mailservers. (and pays no attention to origin or content) Greylisting is much more effective, which surprises me. You'd think the spammers would simply upgrade their engines, but they don't?
Not surprising. Spammers usually send spam through people's PCs that are infected with viruses. The email engine has to be small and lightweight enough to fit into a computer virus, has to be able to send very large amounts of email in a very short time, and will likely be sending large volumes of email to email addresses that are unreachable for whatever reason (invalid, incorrect, closed, whatever--spam lists tend to be pretty dirty). It's not worth the effort to make the email engine retry deferred connections.
