Electronic mail is a method of exchanging digital messages between computer users; it first entered substantial use in the 1960s and by the mid-1970s had taken the form now recognised as email. it 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. it servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously; they need to 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 it was extended by Multipurpose Internet Mail Extensions (MIME) to carry text in other character sets and multimedia content attachments. International email, with internationalized it addresses using UTF-8, has been standardized, but as of 2016 not widely adopted.