... the same problems apply to blocking incoming SMTP connections, or mails from/to specific addresses, SPAM.
thinking a little bit more about the issue with networked services in general (including SMTP and the spam/abuse problems, as well as filesharing and many more), the conclusive decision would be to define a bullet proof standard on introducer based trust, deriving a certain trust level or metric from a peer-trust based trust chain. ...
A friend of mine recently started Habeas (see http://www.habeas.com/) and since I have no financial or operational connection to them I feel pretty free to state publically that there are non-web-of-trust introducer models available which might pan out pretty well even though they're profitable. -- Paul Vixie