APNs does not guarantee the delivery of your push notifications. For example during
multiple message sending if the smartphone is switched off only the last message will be
delivered while all the others will be discarded.
If your application is closed by the user in settings, the push notification will be
received by the smartphone but it will be discarded and it will not be accessible by the
Competitors' "best effort" push services
Parse, Urban Airship, Pushwoosh and other main push providers use Apple APNs and Android
FCM / HMS best effort push services as content delivery channel thus they cannot neither
guarantee the delivery of your push messages nor provide a real time delivery status
Go beyond APNs and FCM “best effort” push delivery if notifications are an important part
of your real time app!