Mirrors the existing client-side check in the v3 signup form (company-email-validator via Vuelidate) so non-business emails can no longer slip in via Google OAuth, direct API calls, or any other path that bypassed the form. The deny list ships as config/deny_listed_email_domains.yml — picked up automatically by ValidEmail2 — and is trimmed to the 6.5k consumer-only domains the gem's disposable list doesn't already cover.