This mail host isn't allowed
OAuth new-account path hit isBannedMailHostEmail (oauth_email_provider_not_allowed). Product shows This mail host isn't allowed. (no host name in the string).
Why this happens
Same policy as email registration’s host block, but OAuth uses this shorter message. Disposable/blocked domains are rejected at signup.
Diagnose and fix
1. Use a different email domain, or sign in if an account already exists.
2. Admin: edit Disallowed mail hosts.
3. Email/password register shows the variant with host in the message.
4. Distinct from captcha / registration-disabled errors.