I think he was so sure that he wouldn't be caught or rather that he wouldn't be let on trial because was so well connected with big figures that authorities just didn't bother. Of course, he did get murdered kill himself so first hand evidence witness testimony would be gone. So it could also be that it was set up as secondary evidence trail against his colleagues for when he obviously did get murdered kill himself for holding too much information. But that does make one wonder why in the ever-loving fuck would billionaires and presidents not have any OPSEC at all, not even to think "this conversation might be compromised at some point". Maybe they thought at the time that email was extreme security 101, who knows, really.