We create a transacted session with:
QueueSession session = conn.createQueueSession(true, -1);
It means all the messages created in that session are apart of an atomic unit and there can be none interposing in that order. However the starting of a transacted session doesn't take a destination as a parameter. So it couldn't be blocking at this time, other destinations should be able to accept whatever messages they please.
So is the transaction started when we call
QueueSender senderJMS = session.createSender(Destination);
or in fact is not really a transaction at all so other "transactions" started after this one but who finished before it, actualy get their group of messages before this transaction's group?
