What Is Email?
An email message is an electronic letter composed on one computer and transmitted across a network to another computer (which might be nearby, or on the other side of the world). Email developed early in the history of networking. Almost as soon as computers were linked into networks, computer engineers began to wonder if humans as well as machines could communicate across those same network links.
The current Internet email system dates back to ARPAnet days. Most of the Internet's email infrastructure is derived from a pair of documents published in 1982: RFC 821 (Simple Mail Transfer Protocol) and RFC 822 (Standard for the Format of ARPA Internet Text Messages). Other proposed email formats have developed since then (such as the X.400 system, as well as several proprietary formats), but the simplicity and versatility of SMTP-based electronic mail have made it the dominant form and the de facto standard for the Internet.
Electronic mail was invented in the days of the text-based user interface, and the original purpose of email was to transmit text. The email message format is designed to transmit text efficiently. The original email specifications did not include provisions for sending binary files. One of the primary reasons for the efficiency of email is that ASCII text is light and simple to transmit. But emphasis on ASCII text ultimately proved limiting. In the 1990s, the email format was extended to include binary attachments. An attachment can be any type of file, as long as it doesn't exceed the maximum size allowed for the email application. As you'll learn in this hour, these attachments are typically encoded in Multipurpose Internet Mail Extensions (MIME) format. Users today commonly attach graphics files, spreadsheets, or word processing files to their email messages.