code: @Override
    public void onException(final Throwable cause, final SocketAddress remoteAddress, final T message) {
      if (LOG.isLoggable(Level.FINEST)) {
        LOG.log(Level.FINEST, "Error sending message " + message + " to " + remoteAddress, cause);
      }
    }
docs: Called when the sent message to remoteAddress is failed to be transferred.