A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft presents a mechanism to simplify one of the cumbersome aspects of mailing. The basic intention is to minimize the complication when a mail needs to be sent to n - m mail IDs i.e. send it to a group Id of n and exclude m from the alias list.
http://www.ietf.org/internet-drafts/draft-arun-ncc-smtp-00.txt