The user command was uucp, it used explicit path routing, a!b!c!user, send to send it by connecting to "a" and then asking it to send to "b!c!user" and so on. Once the UUCP Mapping Project got underway, and the pathalias program was widely used (https://www.cs.columbia.edu/~smb/papers/pathalias.paper.pdf) the routing decreased on user@xxxxxx (everyone knew it was a fake domain) become common. The most common application, in terms of users and bytes, was Usenet, or netnews. The history section at https://en.wikipedia.org/wiki/Usenet is pretty accurate (more than several postings here :). Former Security AD Steven Bellovin was a co-creator of Netnews. Netnews, also known as Usenet, could also run over TCP/IP, via the IETF's NNTP protocol. I wrote an implementation that was widely used. UUNET was originally a non-profit created by Rick Adams and funded by the Usenix association. It morphed into a commercial entity, got a big deal with AOL, hosted the first fully commercial ISP (world.std.com), and so on. The "History" section of https://en.wikipedia.org/wiki/UUNET is also pretty accurate. Hope this helps.
