Believe it or not email has been around for 40 years as the first electronic message was sent in 1971.
ARPANET (the world's first operational packet switching network and the core network of a set that came to compose the global Internet) made a large contribution to the development of email however Ray Tomlinson is generally credited as having sent the first email across a network, initiating the use of the "@" sign to separate the names of the user and the user's machine in 1971, when he sent a message from one Digital Equipment Corporation DEC-10 computer to another DEC-10. The two machines were placed next to each other.
Since then, there have been countless innovations, upgrades, and technological advancements in email. Here is a detailed infographic from ReachMail with all major milestones.