I find this *really* annoying

[aluminumsulfate at earthlink.net]

   From: Derek Martin <invalid at pizzashack.org>
   Date: Thu, 7 Apr 2005 22:53:46 -0400

   > 
   > I'm afraid signing SMTP won't help the spam problem.  

   Sure it will, if implemented well.

If you want a crypto solution to the spam problem, how about this:

 (1) Whenever you give out your email address, you generate a new
public/private key pair and transmit the public key with the address.

 (2) Everyone sending you mail signs the message with their public
key for you.

 (3) Accept any message signed with a valid key.

 (4) When spam arrives, signed with a valid key, then retract the key.

That way, you'll get at most as many spam messages as occasions on
which you've given out your address.

This too, though, would require 100% participation.... :(

Dave