This may sound paradoxical, but Persona has nothing to do with actual email. Well, except for Mozilla-provided fallback/compatibility authenticator that, indeed, uses actual email.
It just a protocol that - oversimplifying things - allows a certain server (identified by domain name) to issue you a certificate that says that you have a name associated with that server.
It's usually an email, but can be anything that could be represented as (name, domain) pair by concatenating those with "@" character. For example, XMPP ID, forum nickname or system account.
That is true, however "login with your email" will be understood by The Average Internet User. Any discourse on login identifiers, domains, xmpp, certificates, et al will just scare them. That's a non-starter.
It just a protocol that - oversimplifying things - allows a certain server (identified by domain name) to issue you a certificate that says that you have a name associated with that server.
It's usually an email, but can be anything that could be represented as (name, domain) pair by concatenating those with "@" character. For example, XMPP ID, forum nickname or system account.