Solved The external app was creating a new connection for every message (DOH!). I'd copied and pasted the jms message sending code from somewhere that was running inside JBoss. That was expecting JBoss to manage a jms connection pool, whereas the external app was creating a new connection every time it sent a message. So I was running out of threads pretty quickly (even when I had the max number set at 2000). Properly synchonized access to the connection and shared it between the messages, and everything's back under control. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4079788#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...