Hi Maciej,
When I change JDK from 1.5 to 1.6, then the testcase passed. And I find there was javax.activation package in the rt.jar of JDK 1.6. I decompile the ObjectDataHandler.class to ObjectDataHandler.java, and saw its content is as same as activation-1.1.1.jar. It means that the JDK 1.6 included a activation-1.1.1.jar. So if someone want to send mail with JDK1.5, he must use activation-1.1.1.jar.
So If Alejandro could modify the pom.xml and change activation.jar from 1.1 to 1.1.1. It will be very appreciate.