dear bog

The problem with this is that the denial of service attack just moves.
Hostile parties just start sending bogus signed messages, (i.e.
formatted random numbers), and it is very costly for the mail exploder
to check the signatures, and they shut down the list server, by this.
One day perhaps in 2007, we may be able to exchange cryptographicly signed
messages, and trust the systems we use, but there are still a number
of problems to solve before you should think about doing that.
(Anytime it cost the attacker less than the victim, particuarlly
when its several orders of magnitude difference, these attack will
continue and the protocols to avoid them are complex and not generally
understood, although a lot of the TCP syn attack stuff, will apply,
i.e. compressed state, selective discard, etc.)