How did Ray Tomlinson create the first e-mail program on ARPANET?

In 1971, Ray Tomlinson, an engineer at BBN, developed the first e-mail program while working on ARPANET, the experimental network that preceded the Internet.12

While creating the code to enable users to send messages across the network, Tomlinson required a specific marker to distinguish between an individual user's name and the name of their host terminal. He selected the at sign (@) for this purpose because it was one of the least utilized characters in the American Standard Code for Information Interchange (ASCII). Tomlinson chose this symbol because it was unlikely to appear in user or computer names, thereby minimizing the potential for confusion within the operating system command lines. The ARPANET community adopted this innovation almost immediately.21