According to the EJB 2.1 spec, the arguments given at the createSession(boolean, int) method should be ignored by the Application Server. However, I can only send messages when I submit (false, Session.AUTO_ACKNOWLEDGE).

The following section is copied from the EJB 2.1 specification (page 374):

Because the container manages the transactional enlistment of
JMS sessions on behalf of a bean, the parameters of the
createSession(boolean transacted, int acknowledgeMode),
createQueueSession(boolean transacted, int acknowledgeMode)
and createTopicSession(boolean transacted, int acknowledgeMode)
methods are ignored. It is recommended that the Bean Provider
specify that a session is transacted, but provide 0 for the
value of the acknowledgment mode.