I sent email to one of my clients today and promptly got a message back from their email server telling me that my message was rejected because our IP address is being blocked, and to check the relays.osirusoft.com website (purposely not linked) for details.

It turns out that Joe Jared, operator of relays.osirusoft.com and a very aggressive (to put it mildly) anti-spammer, decided yesterday to discontinue his service.  Rather than do something reasonable like just shut down the server or remove all entries from his blocking list, he decided to mark every IP address as a spammer domain so that people would get the message that he’s no longer offering the service.  This affected a huge number of mail servers because over-zealous systems administrators had been relying on that list as their primary or only weapon against spam.  Never mind that there are some well documented cases of the operators of this and similar lists behaving quite irresponsibly.

Yet another argument against using filtering to fight the spam problem.  If you think a client-side filter is subject to false positives, imagine what happens when a legitimate ISP is blocked by one of these filters because one person has a personal score to settle.