Guaranteed push notifications - is there a service for this?

Question

Push technologies

It does not matter whether such a service is paid or free, it is important that there is a guaranteed delivery, albeit not instantaneous. The services I have seen usually work on top of the same Google Cloud Messaging or a similar Apple service, which does not guarantee either the timing or the fact of delivery.

Answer 1

OneSignal

Answer 2

Apple itself does not guarantee the delivery of push notifications.
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