Well, I finally solved the "554 denied by gorbs" mystery: Verio Inc. If you send an email to someone and have it bounce back as blocked with an error message of "554 denied by gorbs", then you've tried to send to someone that hosts with Verio Hosting (directly) or another company that outsources their hosting to Verio (like SBC Webhosting and Pacific Bell Internet Services), and they have blocked your IP address (or more likely your ISP's entire netblock!) to "help" reduce spam.

What's goofy about it is that they're apparently maintaining their own list, not referencing any of the other wider-known DNSBL/RBLs. They're also apparently only basing their decisions upon whether or not your mail server is an open relay (and not on mismatched reverse DNS entries) - how antiquated! There are already other databases tracking open mail relays, so I'm not sure why they went to all the trouble of reinventing the wheel...