Networked email introduces the user@host address

Before this, messaging systems let users leave notes only for other accounts on the same shared computer, with no way to address someone on a different machine. Ray Tomlinson adapted an existing local mail program to also work across ARPANET's separate host computers, and picked the @ symbol, chosen mainly because it rarely appeared in names, to separate a user from the specific host holding their mailbox. The user@host format he introduced set the addressing pattern every Internet mail system has used since, turning the network from a tool for remote computing sessions into a channel for everyday communication between institutions.