That was in reference to regular mail. USPS was one of the first fed agencies and mail was one of the few original functionals of the central government. On top of USPS being one of the largest agencies, there is also legal protection against tempering with mail and usps has its own enforcement branch.

In case of email, it probably should simpler. Gov only serves as an address book, essentially. But does minimum of tech implementing it. Given how domain MX records work, does not seem possible. So i think email relay is the closest thing.