Akka actor receiving messages from mailbox while Actor system termination is in progress

Question

I have my java application uses Akka framework for message routing. During application startup I create Actor System and then creates number of top level actors (via actorSystem.actorOf()). Now while shutting down my application, I call actorSystem.terminate() in the expectation that as soon as this termination begins, no new message should be routed by any actor in the system but actually my logs shows that new message is received by one of the akka actor from mailbox while termination is in progress. Is this expected ? if yes how can I be sure that it does not happen ? Thanks