rfc1036 December 1987

2.1.3.  Newsgroups

    The "Newsgroups" line specifies the newsgroup or newsgroups in which
    the message belongs.  Multiple newsgroups may be specified,
    separated by a comma.  Newsgroups specified must all be the names of
    existing newsgroups, as no new newsgroups will be created by simply
    posting to them.
    Wildcards (e.g., the word "all") are never allowed in a "News-
    groups" line.  For example, a newsgroup comp.all is illegal,
    although a newsgroup rec.sport.football is permitted.

    If a message is received with a "Newsgroups" line listing some valid
    newsgroups and some invalid newsgroups, a host should not remove
    invalid newsgroups from the list.  Instead, the invalid newsgroups
    should be ignored.  For example, suppose host A subscribes to the
    classes btl.all and comp.all, and exchanges news messages with host
    B, which subscribes to comp.all but not btl.all.  Suppose A receives
    a message with Newsgroups: comp.unix,btl.general.

    This message is passed on to B because B receives comp.unix, but B
    does not receive btl.general.  A must leave the "Newsgroups" line
    unchanged.  If it were to remove btl.general, the edited header
    could eventually re-enter the btl.all class, resulting in a message
    that is not shown to users subscribing to btl.general.  Also,
    follow-ups from outside btl.all would not be shown to such users.