he returned futures will return <code>null</code> on
 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Future.html?is-external=true#get--" title="class or interface in java.util.concurrent"><code>Future.get()</code></a> if the message was successfully delivered and otherwise throw <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutionException.html?is-external=true" title="class or interface in java.util.concurrent"><code>ExecutionException</code></a>.</dd>
