The fundamental problem of spam
Recently, yet another article on the death of email ran in the Register, 'The time has come to ditch email' (which I saw due to a Slashdot article). As usual, it advocates replacing SMTP email with something that is more 'secure', whatever exactly this means.
Unfortunately, this misses the fundamental problem of spam:
You want to get email from strangers, but only good strangers.
Telling good strangers from bad strangers is a hard problem, to put it one way. There is no indication that computers are going to be any good at it any time soon, and certainly current technology is not up to the job. Magic new security technology for a new email protocol would have to be very magic to solve the problem, and so far no one has even come close. Worse, a great many people (including the author of the piece in the Register) seem completely oblivious to the issue.
Indeed, today's antispam technology has false positives and false negatives precisely because it has to use heuristics like 'did a copy get emailed to a lot of other people' or 'does it have bad phrases' as a proxy for the real question.
(If you think that assigning people identities on the Internet will solve this problem, please see TwoSidesOfIdentity.)
(This idea isn't original to me; I think I picked it up in Usenet's news.admin.net-abuse.email.)