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You want to know if Dimitri is the person regularly posting these messages. So, you use your powers as ISP to block his access to all remailers. If the public messages suddenly stop then you can be reasonably certain that Dimitri was sending them. I'm not following something...just how to your "powers as ISP" affect a remailer in, say, Holland, or one for that matter on another ISP? (As a

Packet filtering at the ISP's router. If Dimitri can't connect to a remailer, he can't send anonymous messages. Sure there are a lot of hacks that could be done (like have sendmail on another system send it to a remailer) but such things could be detected and blocked by clever filters. In fact, the ISP could just claim to be "going down for maintenance" and completely block Dimitri from the internet for a while.

Just as the Nazis could isolate spy transmitters by selectively turning off electricity to different neigborhoods, so, too, can various

Isolating spy transmitters by selectively cutting power is exactly alalogous to what I have suggested.