Historically, electronic mail (or e-mail) referred to any of a number of technologies that allowed people to send documents to one another through electronic means. It was frequently used to describe both wirephoto [the precursor of the facsimile (fax) machine] and telegraphy. Subsequently, usage of the term focused upon the narrower sense, of asynchronous transmission of messages by using computers and data-communication networks.
According to the standard account, computer-based messaging systems emerged alongside computer networks of the early 1960s, such as the pioneering 'time-sharing' computer system installed on the campus of Massachusetts Institute of Technology (MIT). The MIT system and those that followed were intended to allow multiple users, sometimes spread out in various computer labs around campus, to access a central computer using keyboard and monitor terminals. The geographic dispersion of the terminals led to a desire for a convenient text message service. The resulting service at MIT was called 'Mailbox', and may have been the first, but there were many similar programs written at about the same time.
In 1971, the first e-mail was typed into the Teletype terminal connected to the Digital Equipment PDP-10. The message was transmitted via ARPAnet, the progenitor of the Internet, to the PDP-10 in front, in the Bolt, Beranek and Newman datacenter. Electronic mail, along with the electronic bulletin board, became by far the most popular applications by the mid-1970s. As the volume of mail grew, programmers at various institutions around the United States and in Europe collaboratively improved the mail system and imposed technical standards to allow universal service. It was estimated that less than ten thousand electronic mail messages were being transmitted per day in 1976, compared to about 140 million postal messages. By the end of the decade there were an estimated 400,000 unique electronic mailboxes across the country.
The use of electronic mail grew continuously until the late 1980s but never achieved widespread use outside of work groups or corporations. The limiting factor was the complicated addressing that had to be worked out before a message could be successfully transmitted.
There were two proposed methods to solve the problem of mail-system identification and routing. The Organization for International Standardization (ISO) formulated the X.400 standard, and the Internet community developed an extended use of the domain name system (DNS). Many impediments to the spread of X.400, such as high software costs and delays in standardization, caused the freely available DNS solution to become the de facto standard.
The DNS describes a worldwide distributed database in which each site maintains its own information about how to route messages to a computer within its administrative domain. A computer wishing to send a message to another asks the DNS for the routing information and uses the information returned to make the connection. This allows a person on virtually any online networking service to send mail to another person by giving only the personal identification and the e-mail system name of the recipient.
From the time the usage of the term narrowed to exclude facsimile until the early 1990s, generally only coded textual information could be transferred via e-mail. The transmission of nontextual data required special preprocessing, postprocessing, and prior arrangements between the sending and receiving parties. It was very difficult to make these kinds of transfers if the sending and receiving computers were different types.
This restriction was lifted with the adoption of the MIME (Multimedia Internet Mail Enhancements) standard. It described a way of encoding an arbitrary list of media types within a normal textual message in an operating-system-independent manner. Finally, different types of systems could send executable, sound, picture, movie, and other kinds of files to each other via e-mail.
The spread of electronic mail was also hampered by its lack of security. As mail was passed from one site to another closer to its destination, system administrators at each intermediate site could read messages. Also, the source of an e-mail message may be fairly easily forged to make it either untraceable or appear to come from another person. This limited the use of e-mail to so-called friendly applications. Public-key cryptography has been applied to e-mail messaging, notably in PEM (Privacy Enhanced Mail), in response to these security concerns.
Since the communications speeds required for e-mail are quite modest, messages are sometimes transmitted by wireless means. Cell phones and personal digital assistants can send and receive e-mail through Earth-satellite relay.