A proposal for trustworthy e-mail. I mentioned this idea when I (re?)thought of it, sending an e-mail to M-manda a short while ago (I guess I shouldn't really use M- here, since most people don't speak native (X)Emacs).
Sendmail creator EricA  suggests that "a small number of people are polluting a great medium", e-mail. He argues that spam makes economic sense, at least for the senders, and points out that current approaches put the burden on the shoulders of the recipients, rather than the senders.
Instead of arguing for direct permission-based mail, EricA believes that the only viable long-term solution it to "make spammers pay more than we do".
I suggest an alternative, which may or may not be feasible, or new, for that matter. A trustworthy e-mail can be sent between Bob and Carol iff there exists a connected path, on some social network, of length at most k for some small constant k.
That is, instead of having to give permission to every possible sender, you have to only give permission to a smaller 1-ring of people you trust. Trust is then transitive, to some degree, even though it may decay exponentially along with path length.
All other e-mails may be received, but perhaps get filed under "shady".
I imagine that you can also specify different levels of trust. You could say, I trust Paul to send me e-mails, but not as a gateway from other people. We can also define greater aggregate entities worthy of trust. Communities, organizations, and countries, for example.
Implementation-wise, this requires an integration of social networking software and e-mail systems (or at least interop), which is no small feat for Internet-scale topologies, but maybe, just maybe it could even work.
Back to finishing up conference paper to be submitted to SIGxxx on social networks.
1: Wikipedia quote: "There is some sort of perverse pleasure in knowing that it's basically impossible to send a piece of hate mail through the Internet without its being touched by a gay program. That's kind of funny."