This inner class extends the javax.mail.Message.RecipientType
class to add additional RecipientTypes. The one additional
RecipientType currently defined here is NEWSGROUPS.
When deserializing a RecipientType, we need to make sure to
return only one of the known static final instances defined
in this class. Subclasses must implement their own
readResolve method that checks for their known
instances before calling this super method.