Based on the discussion http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-Java-Sender-API-fire-amp-forget-adding-callback-methods-tt4735.html, the idea is following:

There should be a way how to let users of Java Sender API know that their request was processed by Unified Push Server. It does not necessarily mean that UnifiedMessage was actually sent, rather it can be used to identify that credentials were correct and such.

It looks like the best option how to let user access this information would be via callbacks like onSuccess (covering HTTP 2xx) and onFailure (HTTP 4xx and 5xx).