Unfortunately,
spammers lately seem to have confused
opt-out with
opt in. They claim in their
junk mail that they are an
opt in mailing list, even though you've never heard of them
before but you're on their list.
Sometimes they even prefix their letter with claims
that you subscribed to their list and actually
asked for it.
I doubt a technical solution will ever fix the spam problem.
We'll need legislation like the Telephone Consumer Protection Act which has specific wording to the effect of "prior business relationship", although this is probably too simple. For example, I don't mind if a linux vendor finds
my linux web page and sends me a job offer or even a product announcement. I do mind when a diet hoax spammer finds my thin client page.... but that rant belongs in a different node. Perhaps it is enough that the FTC is beginning to prosecute fraudulent email scams.