This email provider is not allowed
Legacy/mapped OAuth wording for a banned mail host during social registration. Live redirects more often show This mail host isn't allowed. Same root cause: isBannedMailHostEmail.
Why this happens
Admin Disallowed mail hosts (or equivalent) blocks the email domain. OAuth signup refuses before creating the user.
Diagnose and fix
1. Member: register with an allowed email domain (or password register if allowed).
2. Admin: review Disallowed mail hosts; remove the domain if it should be permitted.
3. See sibling articles without/with {host} for register vs OAuth wording.