Spam control isn't precisely an email problem, but a "system where arbitrary people not in my contact list can reach me" problem, same as telemarketers and SMS. Federated, open, lots of ways to describe it.
I agree that positively approving contacts is a great way to avoid the threat of spam, but that concept does have occasional downsides.
I agree that positively approving contacts is a great way to avoid the threat of spam, but that concept does have occasional downsides.