blacklist - What are the best methods for catching snowshoe spam?

I'm using Smartermail for my small mailserver. We've been having a problem lately of getting waves of snowshoe spam that follow the same pattern. They come in batches of 3 or 4 at a time. The bodies are almost identical save for the domain name they link to. The source IPs tend to be from the same /24 block for awhile, then they switch to another /24. The domains tend to be brand new. They have valid PTR and SPF records and have random gibberish at the bottom of the body to spoof bayesian filters.

I'm using a dozen or so different RBLs including Barracuda, Spamhaus, SURBL and URIBL. They do a decent job catching most of them, but we still get a lot the slip through because the IPs and domains haven't been blacklisted.

Are there any strategies I can employ, including RBLs that block newly created domains or deal specifically with snoeshow spam? I'm hoping to avoid having to use a 3rd party filtering service.