All written communication for government business is supposed to be saved.
The CISA document you referenced doesn't seem to be clear on this point, although it is marked as not applying to sensitive communications anyway: "Sources may use TLP:CLEAR when information carries minimal or no foreseeable risk of misuse". https://www.cisa.gov/sites/default/files/2024-12/guidance-mo...
It does seem like members of both political parties have had issues following the law on this point though - after all, Hillary's famous personal email server was an issue for the same reason.