Every NDR is a delivery attempt that failed. The courier assigns a reason code (consignee unavailable, refused, wrong address, office closed, COD not ready) and holds the shipment at the delivery hub for a re-attempt. If nobody acts, most couriers make 2–3 attempts and then return the parcel to origin as an RTO.
Fast NDR action — reaching the buyer within a few hours to fix the address or confirm availability — is the single highest-leverage way to cut RTO on both prepaid and COD orders.