wheel - 4:36 pm on May 23, 2007 (gmt 0)

The difficulty will likely be in getting everyone on board. Though some phase in would be great. To start, add the key to all outgoing mail for a year. Incoming email that has the key gets through, without it it's forced through a spam filter. After a year or so move to allow emails with keys only.

I may be missing something though - how does this stop someone from sending spam from their local internet connection (via their ISP). Or from someone registering a domain in a third world country and sending the spam from behind that kind of wall - just like they do now.