to deliver mail on time
to deliver mail reliably.
delays
errors.
Examples of unreliabilities:
Italy and France : periodic strikes of its mail and national railway services occurs, their national postal services are notoriously unreliable.
South Africa : so unreliable - businesses & individuals no longer trust; vis-a-vis payments. Hence, payments in person or through banks.
Postal pilferage: employees stealing the contents of parcels ... before they reach their destination.