Electronic mail, most commonly abbreviated email or e-mail, is a method of exchanging digital messages. E-mail systems are based on a store-and-forward model in which e-mail server computer systems accept, forward, deliver and store messages on behalf of users, who only need to connect to the e-mail infrastructure, typically an e-mail server, with a network-enabled device for the duration of message submission or retrieval. Originally, e-mail was always transmitted directly from one user's device to another's; nowadays this is rarely the case.
An electronic mail message consists of two components, the message header, and the message body, which is the email's content. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually additional information is added, such as a subject header field.
Originally a text-only communications medium, email was extended to carry multi-media content attachments, which were standardized in with RFC 2045 through RFC 2049, collectively called, Multipurpose Internet Mail Extensions (MIME).
The foundation for today's global Internet e-mail service was created in the early ARPANET and standards for encoding of messages were proposed as early as 1973 (RFC 561). An e-mail sent in the early 1970s looked very similar to one sent on the Internet today. Conversion from the ARPANET to the Internet in the early 1980s produced the core of the current service.
Network-based e-mail was initially exchanged on the ARPANET in extensions to the File Transfer Protocol (FTP), but is today carried by the Simple Mail Transfer Protocol (SMTP), first published as Internet standard 10 (RFC 821) in 1982. In the process of transporting e-mail messages between systems, SMTP communicates delivery parameters using a message envelope separately from the message (header and body) itself.
Spelling
There are several spelling variations that are occasionally the cause of vehement disagreement.
- email is the form officially required by IETF Request for Comments and working groups and is also recognised in most dictionaries.
- e-mail is a form still recommended by some prominent journalistic and technical style guides.
- mail was the form used in the original RFC. The service is referred to as mail and a single piece of electronic mail is called a message.
- eMail, capitalising only the letter M, was common among ARPANET users and early developers from Unix, CMS, AppleLink, eWorld, AOL, GEnie, and Hotmail.
- EMail is a traditional form that has been used in RFCs for the "Author's Address", and is expressly required "...for historical reasons...".
Origin
Electronic mail predates the inception of the Internet, and was in fact a crucial tool in creating the Internet.
MIT first demonstrated the Compatible Time-Sharing System (CTSS) in 1961. It allowed multiple users to log into the IBM 7094 from remote dial-up terminals, and to store files online on disk. This new ability encouraged users to share information in new ways. E-mail started in 1965 as a way for multiple users of a time-sharing mainframe computer to communicate. Although the exact history is murky, among the first systems to have such a facility were SDC's Q32 and MIT's CTSS.
Host-based mail systems
The original email systems allowed communication only between users who logged into the one host or "mainframe", but this could be hundreds or thousands of users within a company or university. By 1966 (or earlier, it is possible that the SAGE system had something similar some time before), such systems allowed email between different companies as long as they ran compatible operating systems, but not to other dissimilar systems.
Examples include BITNET, IBM PROFS, Digital Equipment Corporation ALL-IN-1 and the original Unix mail.
LAN-based mail systems
From the early 1980s networked personal computers on LANs became increasingly important. Server based systems similar to the earlier mainframe systems developed, and again initially allowed communication only between users logged into the same server infrastructure, but these also could generally be linked between different companies as long as they ran the same email system and (proprietary) protocol.
Examples include cc:Mail, WordPerfect Office, Microsoft Mail, Banyan VINES and Lotus Notes - with various vendors supplying gateway software to link these incompatible systems.
Attempts at interoperability
- Novell briefly championed the open MHS protocol but abandoned it after purchasing the non-MHS WordPerfect Office (renamed Groupwise)
- uucp was used as an open "glue" between differing mail systems
- The Coloured Book protocols on UK academic networks until 1992
- X.400 in the early 1990s was mandated for government use under GOSIP but almost immediately abandoned by all but a few — in favour of Internet SMTP
From SNDMSG to MSG
In the early 1970s, Ray Tomlinson updated an existing utility called SNDMSG so that it could copy files over the network. Lawrence Roberts, the project manager for the ARPANET development, updated READMAIL and called the program RD. Barry Wessler then updated RD and called it NRD.
Marty Yonke combined SNDMSG and NRD to include reading, sending, and a help system, and called the utility WRD. John Vittal then updated this version to include message forwarding and an Answer command to create replies with the correct address, and called it MSG. With inclusion of these features, MSG is considered to be the first modern email program, from which many other applications have descended.
The rise of ARPANET mail
The ARPANET computer network made a large contribution to the development of e-mail. There is one report that indicates experimental inter-system e-mail transfers began shortly after its creation in 1969. Ray Tomlinson is credited by some as having sent the first email, initiating the use of the "@" sign to separate the names of the user and the user's machine in 1971, when he sent a message from one Digital Equipment Corporation DEC-10 computer to another DEC-10. The two machines were placed next to each other. The ARPANET significantly increased the popularity of e-mail, and it became the killer app of the ARPANET.
Most other networks had their own email protocols and address formats; as the influence of the ARPANET and later the Internet grew, central sites often hosted email gateways that passed mail between the Internet and these other networks. Internet email addressing is still complicated by the need to handle mail destined for these older networks. Some well-known examples of these were UUCP (mostly Unix computers), BITNET (mostly IBM and VAX mainframes at universities), FidoNet (personal computers), DECNET (various networks) and CSNet a forerunner of NSFNet.
An example of an Internet email address that routed mail to a user at a UUCP host:
hubhost!middlehost!edgehost!user@uucpgateway.somedomain.example.com
This was necessary because in early years UUCP computers did not maintain (or consult servers for) information about the location of all hosts they exchanged mail with, but rather only knew how to communicate with a few network neighbors; email messages (and other data such as Usenet News) were passed along in a chain among hosts who had explicitly agreed to share data with each other.
Operation overview
The diagram to the right shows a typical sequence of events that takes place when Alice composes a message using her mail user agent (MUA). She enters the e-mail address of her correspondent, and hits the "send" button. 
- Her MUA formats the message in e-mail format and uses the Simple Mail Transfer Protocol (SMTP) to send the message to the local mail transfer agent (MTA), in this case smtp.a.org, run by Alice's Internet Service Provider (ISP).
- The MTA looks at the destination address provided in the SMTP protocol (not from the message header), in this case bob@b.org. An Internet e-mail address is a string of the form localpart@exampledomain. The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name or a fully qualified domain name. The MTA resolves a domain name to determine the fully qualified domain name of the mail exchange server in the Domain Name System (DNS).
- The DNS server for the b.org domain, ns.b.org, responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a server run by Bob's ISP.
- smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user bob.
- Bob presses the "get mail" button in his MUA, which picks up the message using the Post Office Protocol (POP3).
That sequence of events applies to the majority of e-mail users. However, there are many alternative possibi