What was the name of the first email system on ARPANET?

The first email system on ARPANET was SNDMSG.

SNDMSG began as a local electronic-mail program on TENEX time-sharing computers, allowing users of the same machine to leave messages for one another. In 1971, programmer Ray Tomlinson modified it so messages could travel between separate computers connected to ARPANET.

Tomlinson combined SNDMSG with code from CPYNET, an experimental file-transfer program. He also introduced the now-famous @ symbol to separate a user’s name from the destination computer, creating the basic pattern behind modern email addresses. The first networked email was sent between two nearby PDP-10 computers, rather than across a far-flung global network.

SNDMSG is sometimes confused with MAIL or with Ray Tomlinson himself. MAIL and MLFL were later FTP commands used to move messages, while Tomlinson was the programmer. SNDMSG was the existing mail program he extended to make networked email possible.