I've done some experimenting with the application that you sent me, and, at long last, I think I may have cracked it.
I managed to reproduce the problem on my Mac development box using Java 6 (1.6.0_07). I then tried it on my Mac using Java 5, and could not reproduce the problem. Crucially, in the Java 5 case, the JVM would be using the javax.activation API and implementation from the 1.1.1 javax activation bundle, whereas on Java 6, it would be using them from the JRE (anything that's part of the JRE takes precedence over what's in dm Server's lib directory). I then tried the app on an Ubuntu box, this time using Java 6 1.6.0_10. I could not reproduce the problem.
It would appear that a fix has been made to the Activation implementation somewhere between the 0_07 and 0_10 releases of Java 6. This tallys with Kryptolus's experience where updating the version of JDK 6 fixed the problem.
Is upgrading the version of SE 6 that you're using an option for you?