Welcome to the Java Programming Forums.
I suggest you Google the exception.
NoClassDefFoundError (Java Platform SE 6)
Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.
So this suggests that no definition of the class could be found. It is caused by javax.mail.Message
Have you imported the JavaMail API into your project correctly?
JavaMail API Download