From their docs: "The protocol does not require that [identity provider]-backed identities are SMTP-routable, but it does require that identities follow the use

From their docs: "The protocol does not require that [identity provider]-backed identities are SMTP-routable, but it does require that identities follow the user@domain format." The protocol still requires an identity provider to attest to your identity, but the actual identitifier doesn't appear to have to be a real email address. It seems like you could use anything in email address format.