[megous]

Technically, if you hack a mail server, you'll get access to DKIM private keys. I would only trust these mails DKIM signature if I personally received them prior to the breach.

Otherwise it's a very weak proof.

[saltyfever]

Clinton's people would've come out and said if any of the emails were forged. They didn't.

[megous]

That'd be just a damage control decision. It's not a verifiable claim for outside observers just from the DKIM keys.