According to an Intercept story on them, Powell's defense is that he viewed email more like a phone call than a cable machine, so the written record was not important. He used an AOL account for non-classified communication and the State Department computers for classified, so he didn't violate security rules. He also claims he discusses the situation in greater detail in a chapter of his book, It Worked for Me: In Life and Leadership.
In one of his email, he also added, “Everything HRC touches she kind of screws up with hubris.” And then, “I told you about the gig I lost at a University because she so overcharged them they came under heat and couldn’t any fees for awhile. I should send her a bill.”
Update: Richard Wolff's Guardian article on the topic doesn't add much that's new, but he shows a remarkable self-awareness as he writes:
To be clear: hacking personal emails is reprehensible, and those behind the hack are scumbags who are trying to manipulate American voters.
To be doubly clear: the news media (including this column) have no principles and no shame in exploiting the ill-gotten product of those scumbags. Guilty, as charged.
And yet the Powell emails are so insightful and direct, it seems a shame – even if we have no sense of shame to let them pass by without comment.