SimpleX Chat on Nostr: it certainly can be hashed - when user provides the main email and the recovery ...

it certainly can be hashed - when user provides the main email and the recovery email, you can validate it. Obviously, user has to remember what the recovery email was for that to work. But a reasonable compromise would be to store hash, salt and x***@***.com pattern so a user could remember which recovery email was used (with or without correct letter count).