PubNub is not a guaranteed message delivery service, though we want to provide as much information as possible. For example, while a Sent status code returned to the message publisher does not guarantee that the message is delivered, it does mean that the PubNub Network has received the message and has already sent it along to all currently connected subscribers.
However, there is no way to guarantee that the network connection between the subscribers and PubNub has not been interrupted. We are incrementally adding automatic features which will improve deliverability but the QoS of the last mile of connectivity is not within PubNub's control.
Subscribers with troubled connections or that were offline by intention can still retrieve missed messages.