There's little need to nationalize, since emails don't have a last-mile or natural monopoly issue.

I think, however, that modern countries should provide all citizens with an official, public, mail address. Restrict them to only send/receive emails to/from a whitelist of publicly registered service providers with very strict personal data governance requirements (government, banks, utilities, medical services, higher education).

You can let private companies continue to offer email addresses for everything else, anonymously and under whatever terms. (Much like how you can still buy a mailbox if you want, but you're always guaranteed to receive the mail at your address.) Just ensure that people aren't cut off from essential services.