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.