The j2ee.jar in IDE-HOME/lib contains the class javax.mail.internet.ParameterList that relies on the following Geronimo-specific classes:
org.apache.geronimo.mail.util.ASCIIUtil
org.apache.geronimo.mail.util.RFC2231Encoder
org.apache.geronimo.mail.util.SessionUtil
This might be OK for IDEA's internal use, however, the j2ee.jar is offered to be added to J2EE modules which can cause serious and hard to track problems when running command line apps that include the jar through some dependency:
The jar that is (and has been) offered to be added as J2EE module-library should be a portable one.
BTW: Someone else had that problem as well: http://www.nabble.com/Question-regarding-MTOM-client-t3142379.html