Where O Where Did My ContentDisposition Go?

I've got application code that is using the javax.mail.internet.ContentDisposition class and it runs under some J2EE containers and not others.
This class appears in the JavaMail 1.2 spec on which the J2EE spec is supposed to be based. However, in the J2EE API v1.3 and v.1.2.1 this class is not public.
Does anyone know why it is not public? What is the best alternative?
Any and all suggestions are greatly appreciated!
- Kelly