You don't start with any specific systems. You start by mapping the data flow of the information requiring protection. True, in normally comes in via email, but it might be coming in via FTP, actual paper being scanned into a file system, or a variety of ways. The email system will most likely be scoped as a classified asset, for processing/transmitting/storing sensitive data; but it's not the starting point usually. And one can't just "secure email", one also first must identify the people who need access and sort out what systems they use as part of their job duties. Identity validation is one one part of NIST RMF. The three criteria are Confidentiality, Availability, and Integrity. Requiring signed email is Integrity. Encryption can provide Confidentiality. Having cloud systems in some HA mode across multiple geographical tenants provides Availability
Side note, S/MIME is used quite often in the DoD world; but it's not a "stand alone" system that just anyone can also use. I have an ECA token that can do it, it costs over $150 a year and filing paperwork that includes my birth certificate and a DCSA clearance. Even then, you also need to use something like DoD InstallRoot to pull in the specific DoD certs into your cert store, configure the email system itself correctly...and there are a PILE of STIG controls to implement to get there; then an independent third-party audit to certify the system via EMASS (this means only very specific 3rd parties can do this certification, called 3PAOs, like the companies that can certify FedRAMP) that it is authorized for whatever clearance level is needed per the interconnection contractual requirements..
It really is FAR more complicated than just technical requirements. The lawmakers in Congress are most likely hopeless outside of their lane on this, and would need hours worth of very specific CDSE training to even begin to understand it, and this would be on top of already having years of IT / sysadmin / cyber security experience. GRC is a WHOLE other world on top of what most think of as cyber security...and I only work in the 800-171 space, not the larger 800-53 control sets.