Craigslist requires that you have a ReverseDNS on the connecting IP address that matches the domain in the FROM HEADER, not the HELO/EHLO. That's insane. Only large corporations can do that in the first place.
Wowee zowee... I never realized that my half-dozen computers and my big, lazy butt qualified as a large corporation. But since I've had proper reverse DNS on my mail server for that last 5 years, that's obviously the only explanation.