Check out Haraka, built by an old SpamAssassin developer (ie me). It's all that and has custom anti-spam features like the Karma plugin too. It's in place at some large installs (eg craigslist, who went from 20 postfix servers to just 7 Haraka servers).
I'll look into it, thanks!
crm114 isn't actively maintained, but it still works great too.