It's also strange that the Korean side didn't destroy that evidence and handed it over in discovery. Most companies who destroy evidence at least try to do a decent job and go all the way. If the servers and individuals responsible were in Korea, an American civil suit would have a hard time reaching them.

When the command comes to destroy evidence, the company already unravels. The CEOS and upper echelons do no longer hold the power of career life and death, instead they are just "visibly" pawns in the larger game of society, playing "prison" or "no prison". In that moment, even the most loyal individual realizes that the game is lost- and that deleting evidence that proofs your innocence, would increment you for nothing.

Thus that command - is never followed. There are always copies of the bad shit everywhere. The underlings are not that stupid.

Maybe they destroyed every relevant mail but missed that one?

I assume most company executives are not experts at evidence destruction. And in this case, it seems that they destroyed all of the documents and then just sent out a "We've now destroyed everything you asked us" email reply back without thinking to include that in the list of things to destroy.