Blackmail existed long before DKIM was a thing. Being able to say "no I didn't say that" is far more powerful than the reverse because the reverse has existed

Blackmail existed long before DKIM was a thing. Being able to say "no I didn't say that" is far more powerful than the reverse because the reverse has existed for thousands of years. But being able to definitively prove that you did not say something is brand new and very powerful.