This has been discovered in JES 5. The classes in activation.jar for Solaris have been compiled with J2SE 5. As a result, J2SE 4 clients will experience a class loader error:
Exception in thread "main" java.lang.UnsupportedClassVersionError: javax/activation/DataSource (Unsupported major.minor version 49.0)

Observation: In HPUX and In Windows the error is not reproducible as the activation.jar file has been built using Java SE 1.4.2.