Arpanet: 50 years of internetworking

As noted on the Centre for Computer History's website, Ray Tomlinson is recognised as the inventor of email. Tomlinson wrote a program called SNDMSG that the ARPANET programmers and researchers could use to leave messages for each other. As the Centre for Computer History notes, SNDMSG was a "local" electronic message program. This meant that users on the PDP-10 minicomputers at the centre were only able leave messages on the computer that they were sat at. Other people could then see these messages when they gained access to the same machine. According to the Centre for Computer History, Tomlinson developed a file transfer protocol called CYPNE, which he then adapted for the SNDMSG program, which enabled electronic messages (email)  to be sent to any computer on the Arpanet network.