I was thinking, why are all e-mail addresses not encrypted as soon as they leave the authors mail client, surely this would stop anyone seeing the address, apart from the mail client at the other end the message was intended for. And when a user mails a mailing list the e-mail address could be read by the mailing list software, but stays encrypted for the broadcast out to the subscribers of the list.
All you need to do to stop spam is have e-mail addresses encrpyted and only readable by the person they were sent to. perhaps to make it nicer, leave the user@ side of the e-mail address showing, but encrypt the @domain side of the e-mail address.
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