Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email entered limited use in the 1960s, but users could only send to users of the same computer. Some systems also supported a form of instant messaging, where sender and receiver needed to be online simultaneously. Ray Tomlinson is credited as the inventor of networked email; in 1971, he developed the first system able to send mail between users on different hosts across the ARPANET, using the @ sign to link the user name with a destination server. By the mid-1970s, this was the form recognized as email. Email describes the overall method and "an email" is a message in the system.
Source : DBpedia.org