If sending fails during the request and no email fallback succeeds, the API returns an error. When the access point first accepts the document and reports a failure later, the send response can already have succeeded; inspect the failed attempt in deliveries or subscribe to document.delivery_status_changed.
If you requested email fallback with email.when: "on_peppol_failure", it also applies to a later Peppol failure. Check the Peppol and email attempts before deciding whether to resend. See Tracking Delivery.