Echomail
FidoNet's public conferencing system — shared topic threads copied from node to node overnight, roughly Usenet's newsgroups delivered over dial-up.
Jeff Rush added the feature in 1986, two years after the network began. A message written in an "echo" (a topic area) was bundled up and forwarded during the nightly mail run until it had propagated across the whole network. Its opposite was netmail, which travelled directly and privately.
Latency could be days, but it was effectively free: a single phone call reached thousands of readers. The system survived well into the 1990s before the internet made it redundant.