Hanane,
Dublin core is a content standard, as a result it would be up to you to ensure your metadata meets Dublin Core's standards. If you would like some information about how to ensure this happens I would be happy to provide some resources.
I can't really comment on METS or 239.50 as I'm not familiar with either. However, you can export metadata from the administration console as XML, which does meet a number of transmission guidelines, including Dublin Core's. Metadata is one of my major interests so if you'd like to chat privately I'd be happy to.
Good luck,
Allan