Thanks for the response!...
Yeah, I kind of suspected as much. I seem to be encountering this more and more frequently. The downside to doing an administrative whitelist on the IP address, though, is that it will then allow the messages from that mail server to anyone on the inside of the organization and, potentially, even mailings not associated with that organization's company. I've run into this kind of "no win" situation before where you whitelist an IP address so a company's mailing can get through but the mailing is being sent by a third party mass mailing outfit that also sends mailings your users don't want to receive. As an alternative, you can whitelist the domain, but domains are easily spoofed.
Hum... Assuming there's a pattern to the local name (even when the address contains random characters), what if I created a custom dictionary that contributed to the ESP and heavily negative weighted particularly odd senders using matching regular expressions against the header only? If I negative weight it enough, could I tip the ESP scale in favor of delivering the message?