Donna Brazile's Debate question email was not altered by anyone outside of the Hillary Clinton camp.
DKIM is a technology that email servers use to verify the contents of emails. When an email is sent, the server signs it. The webmaster of the domain publishes a public key which can be used to verify the signature is valid. If one character is changed the signatures don't check out.
While it is conceivable somebody could hack hillaryclinton.com and steal the private key, then forge all of the signatures on the emails, it is rather unlikely. And a lot of work. (I believe these where taken from Podesta's gmail account) But it becomes exponentially more unlikely that the replies from other domains (Yahoo, Google, etc) also where correctly forged..
I haven't seen this discussed anywhere outside of my own brain, so I figured and educational demo was in order..