Historically, the term electronic mail was used generically for any electronic document transmission.
Email operates across computer networks, which in the 2010s is primarily the Internet.
Some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging.
Today's email systems are based on a store-and-forward model.
Email servers accept, forward, deliver, and store messages.
Neither the users nor their computers are required to be online simultaneously; they need connect only briefly, typically to a mail server, for as long as it takes to send or receive messages.
Originally an ASCII text-only communications medium, Internet email was extended by Multipurpose Internet Mail Extensions (MIME) to carry text in other character sets and multi-media content attachments.
International email, with internationalized email addresses using UTF-8, has been standardized, but as of 2016 not widely adopted.
The history of modern, global Internet email services reaches back to the early ARPANET, with standards for encoding email messages proposed as early as 1973 (RFC 561).
An email message sent in the early 1970s looks very similar to a basic text email sent today.
Email played an important part in creating the Internet, and the conversion from ARPANET to the Internet in the early 1980s produced the core of the current services.
The ARPANET initially used extensions to the File Transfer Protocol (FTP) to exchange network email, but this is now done with the Simple Mail Transfer Protocol (SMTP), first published as Internet standard 10 (RFC 821) in 1982.