Highly doubtful much of anything majorly sensitive got leaked. Firstly even unclassified DoD emails are encrypted by default. Secondly anything classified isn't even on a network that can talk to normal email, it's either 100% point to point encrypted or on an airgapped network. If I hopped on SIPR (DoD Secret-level internet) and emailed a normal email address it simply wouldn't work.
That doesn't stop somebody from being an idiot and mentioning something classified in clearnet communications. Never underestimate the power of stupidity.
Ehhhhh, you're missing the human element. Humans do dumb shit all the time. You can't stop someone from reading something with their eyeballs, remembering it in their meat brain, and using their sausage fingers to type it back into something unsecured. Odds are still low of course, but I wouldn't be so confident.