def get(list_spec):
    """Return the mailing list with the given specification, if it exists.

        :param list_spec: Either the fully qualified name of the mailing list,
            or its List-ID.  If list_spec has an `@` in it, it's considered an
            FQDN listname, otherwise it's considered a List-ID.
        :type list_spec: str
        :return: the matching mailing list or None if no matching list is
            found.
        :rtype: IMailingList
        """
