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There is a weakness here though. The Korean government can only require it of emailings done from Korea. In the US, the bulk of the spam emails are coming from outside the US. So setting up such a system would not cover the largest amount of spam.
In the US States don't regulate interstate commerce so any law is only binding to mail sent within the State. The federal government regulates interstate commerce.
With the bulk of the spam traffic coming from outside the State's jurisdiction, I can't see how these laws can be effective.