Why didn't I think of that 🤦♂️
I think you underestimate how many individual mailservers there are in the US government. I know of an example where legitimate mail from one branch of the military was sent to the spam folder of another branch of the military.
On the other hand, filtering out specific servers like gmail or proton mail sounds incredibly easy.
The report was never given to lawmakers.

*Profits of insurance companies are limited by regulators to about 4.5% — hardly enticing to investors, considering the risk of hurricanes.
However, insurance executives in Florida have used financial workarounds to reward investors and themselves.
While the profits and executive compensation of the insurance company are capped, the profits of affiliate and parent companies are not.
So executives create sister companies that charge the insurance company for basic services, such as claims handling, underwriting, accounting and issuing policies. (Large national insurers typically handle all of those services internally.)*