rfc1036 December 1987

4.  Transmission Methods

    USENET is not a physical network, but rather a logical network
    resting on top of several existing physical networks.  These
    networks include, but are not limited to, UUCP, the Internet, an
    Ethernet, the BLICN network, an NSC Hyperchannel, and a BERKNET.
    What is important is that two neighboring systems on USENET have
    some method to get a new message, in the format listed here, from
    one system to the other, and once on the receiving system, processed
    by the netnews software on that system.  (On UNIX systems, this
    usually means the rnews program being run with the message on the
    standard input. <1>)

    It is not a requirement that USENET hosts have mail systems capable
    of understanding the Internet mail syntax, but it is strongly
    recommended.  Since "From", "Reply-To", and "Sender" lines use the
    Internet syntax, replies will be difficult or impossible without an
    Internet mailer.  A host without an Internet mailer can attempt to
    use the "Path" header line for replies, but this field is not
    guaranteed to be a working path for replies.  In any event, any host
    generating or forwarding news messages must have an Internet address
    that allows them to receive mail from hosts with Internet mailers,
    and they must include their Internet address on their From line.