This mail host isn't allowed: {host}
POST /api/auth/register rejected the address because the domain after @ is on the banned mail-host list (AUTH_MAIL_HOST_NOT_ALLOWED). The UI substitutes the real host for {host}.
Why this happens
Admin policy blocks that domain (spam/disposable lists, etc.). Activity may log register_rejected / blocked_mail_host. Same policy as OAuth’s shorter This mail host isn't allowed.
Diagnose and fix
1. Register with an allowed email provider.
2. Admin: Disallowed mail hosts — remove or adjust the domain if it should be allowed.
3. Confirm the member typed the address correctly (typos can hit unexpected hosts).
4. After allowing the host, retry registration.