Checking the DKIM key doesn't prove anything with certainty - the keys could have been leaked or stolen, or the mail server hacked. Or, the operator of the mail

Checking the DKIM key doesn't prove anything with certainty - the keys could have been leaked or stolen, or the mail server hacked. Or, the operator of the mail server could even forge the message. Many possibilities. The purpose of DKIM is to help prevent spam, not to verify the authenticity of the sender.