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 ARPANET, 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 article, 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 article on the standard input.)
It is not a requirement that USENET sites have mail systems capable of understanding the ARPA 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 site 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 site generating or forwarding news messages must have an internet address that allows them to receive mail from sites with internet mailers, and they must include their internet address on their From line.