UUCP (UNIX-to-UNIX CoPy) was a queued dialup technology to reliably transfer files between UNIX systems, and optionally execute (uux) a command with the resulting file, first in V7 UNIX in 1978.
UUCP was mainly used to send e-mail.
An address ucbvax!mark (or ucbvax^mark) sent a message to UUCP host ucbvax (U.C. Berkeley’s VAX,) user mark.
If ucbvax wasn’t directly connected to the sending host, the sender could manually route the message, e.g. duke!research!ucbvax!mark sent the message to Duke University, then to Bell Labs Research in New Jersey, then to ucbvax.
Since all mail had to be routed, your e-mail address depended on where the sender was. End users had to know how to route e-mail.
E-mail addresses like research!greg@Berkeley were ambiguous, which presented a special challenge to users and mail software. Should the message be sent first to Berkeley and then to research, or first to research and then to Berkeley? Depending upon the context, either interpretation might be the one that would work.
With no registrar, duplicate names were common.
Several Netizens proposed to create a map of UUCP, asking everyone to send their UUCP connectivity. Each was buried under a huge pile of data and gave up.
The UUCP email message below was sent by Tim Thompson (stargate!starcube!tgt) to Mark Horton, Kurt Shoens (author of the “mailx” command, originally “Mail”) and Tom Truscott (a Usenet founder.) The mailx “reply” command had to untangle the email addresses in the To and Cc lines, and the multiple “From” lines at the top, to get the reply to all the right people.