FidoNet consists of approximately 10,000 systems world-wide which comprise a network which exchanges mail and files via Modems using a proprietary protocol.
They are connected for the purposes of exchanging E-Mail to the Internet thru a series of gateway systems which interact with the Internet via UUCP with cooperating UNIX-based smart-hosts which act as their MX-receivers.
More info on
http://www.fidonet.org/