Ray Tomlinson chose the @ symbol to separate a username from the host computer in an email address. His 1971 change made it possible to identify both a person and the machine where that person’s mailbox existed.
Before Tomlinson’s work, electronic mail generally operated only between users of the same computer. While working with ARPANET, the network that preceded today’s Internet, he adapted existing mail programs so messages could travel between different hosts. The format user@host clearly separated the local account name from the destination machine.
Tomlinson selected @ because it was already available on keyboards but was uncommon in usernames and programming. It also had a useful meaning in ordinary language: “at,” as in a user at a particular host. That made the notation compact and intuitive.
The first test message is often misreported as “QWERTYUIOP,” but Tomlinson later said he could not remember its contents. The lasting achievement was the addressing convention, which survived as email moved from ARPANET to the global Internet.