This line is added to the header of a posted article by the
    server. The contents of the header is either the IP address
    or the fully qualified domain name of the client host posting
    the article. The fully qualified domain name should be
    determined by doing a reverse lookup in the DNS on the IP
    address of the client. If the client article contains this line,
    it is removed by the server before acceptance of the article
    by the Usenet transport system.

    This header provides some idea of the actual host posting
    the article as opposed to information in the Sender or From
    lines that may be present in the article. This is not a
    fool-proof methodology since reverse lookups in the DNS
    are vulnerable to certain types of spoofing, but such
    discussions are outside the scope of this document.