The other fascinating thing about this change is that it has, to some extent, fulfilled Bill Gates’s prediction of January 2004 that “Two years from now, spam will be solved“. He thought it would be done by instituting tiny charges per email, or by making an email sender solve a small cryptographic puzzle, or by using a simpler system that would automatically confirm the identity of the sender. Instead, ad-hoc filters, plus education, have done the job.
Actually, if you follow that link, he “pledged” to rid us all from spam in two years. Still waiting, Bill.