Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email entered limited use in the 1960s, but users could only send to users of the same computer, and some early email systems required the author and the recipient to both be online simultaneously, similar to instant messaging. Ray Tomlinson is credited as the inventor of email; in 1971, he developed the first system able to send mail between users on different hosts across the ARPANET, using the @ sign to link the user name with a destination server. By the mid-1970s, this was the form recognized as email.
This screenshot shows the "Inbox" page of an email client; users can see new emails and take actions, such as reading, deleting, saving, or responding to these messages.
The at sign, a part of every SMTP email address[1]
Email operates across computer networks, primarily the Internet. 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 to connect, typically to a mail server or a webmail interface to send or receive messages or download it.
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 multimedia content attachments. International email, with internationalized email addresses using UTF-8, is standardized but not widely adopted.[2]
The history of modern Internet email services reaches back to the early ARPANET, with standards for encoding email messages published as early as 1973 (RFC 561). An email message sent in the early 1970s is similar to a basic email sent today.
Terminology
Historically, the term electronic mail is any electronic document transmission. For example, several writers in the early 1970s used the term to refer to fax document transmission.[3][4] As a result, finding its first use is difficult with the specific meaning it has today.
The term electronic mail has been in use with its current meaning since at least 1975, and variations of the shorter E-mail have been in use since at least 1979:[5][6]
email is now the common form, and recommended by style guides.[7][8] It is the form required by IETF Requests for Comments (RFC) and working groups.[9] This spelling also appears in most dictionaries.[10][11][12][13][14][15][16]
e-mail is the form favored in edited published American English and British English writing as reflected in the Corpus of Contemporary American English data,[17] but is falling out of favor in some style guides.[8][18]
EMail is a traditional form used in RFCs for the "Author's Address" and is required "for historical reasons".[19]
E-mail is sometimes used, capitalizing the initial E as in similar abbreviations like E-piano, E-guitar, A-bomb, and H-bomb.[20]
In the original protocol, RFC 524, none of these forms was used. The service is simply referred to as mail, and a single piece of electronic mail is called a message.
An Internet e-mail consists of an envelope and content;[21] the content consists of a header and a body.[22]
Origin
Main article: History of email
Computer-based mail and messaging became possible with the advent of time-sharing computers in the early 1960s, and informal methods of using shared files to pass messages were soon expanded into the first mail systems. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. Over time, a complex web of gateways and routing systems linked many of them. Many US universities were part of the ARPANET (created in the late 1960s), which aimed at software portability between its systems. In 1971 the first ARPANET network email was sent, introducing the now-familiar address syntax with the '@' symbol designating the user's system address.[23] The Simple Mail Transfer Protocol (SMTP) protocol was introduced in 1981.
For a time in the late 1980s and early 1990s, it seemed likely that either a proprietary commercial system or the X.400 email system, part of the Government Open Systems Interconnection Profile (GOSIP), would predominate.[nb 1] However, once the final restrictions on carrying commercial traffic over the Internet ended in 1995,[24][25] a combination of factors made the current Internet suite of SMTP, POP3 and IMAP email protocols the standard.
Operation
The following is a typical sequence of events that takes place when sender Alice transmits a message using a mail user agent (MUA) addressed to the email address of the recipient.[26]
Email operation
The MUA formats the message in email format and uses the submission protocol, a profile of the Simple Mail Transfer Protocol (SMTP), to send the message content to the local mail submission agent (MSA), in this case smtp.a.org.
The MSA determines the destination address provided in the SMTP protocol (not from the message header) — in this case, bob@b.org — which is a fully qualified domain address (FQDA). 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. The MSA resolves a domain name to determine the fully qualified domain name of the mail server in the Domain Name System (DNS).
The DNS server for the domain b.org (ns.b.org) responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a message transfer agent (MTA) server run by the recipient's ISP.[27]
smtp.a.org sends the message to mx.b.org using SMTP. This server may need to forward the message to other MTAs before the message reaches the final message delivery agent (MDA).
The MDA delivers it to the mailbox of user bob.
Bob's MUA picks up the message using either the Post Office Protocol (POP3) or the Internet Message Access Protocol (IMAP).
In addition to this example, alternatives and complications exist in the email system:
Alice or Bob may use a client connected to a corporate email system, such as IBM Lotus Notes or Microsoft Exchange. These systems often have their own internal email format and their clients typically communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate email system.
Alice may not have an MUA on her computer but instead may connect to a webmail service.
Alice's computer may run its own MTA, so avoiding the transfer at step 1.
Bob may pick up his email in many ways, for example logging into mx.b.org and reading it directly, or by using a webmail service.
Domains usually have several mail exchange servers so that they can continue to accept mail even if the primary is not available.
Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays. This was very important in the early days of the Internet when network connections were unreliable.[28][29] However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as a consequence open mail relays have become rare,[30] and many MTAs do not accept messages from open mail relays.
Web-based email
Main article: Webmail
Many email providers have a web-based email client (e.g. AOL Mail, Gmail, Outlook.com and Yahoo! Mail). This allows users to log into the email account by using any compatible web browser to send and receive their email. Mail is typically not downloaded to the web client, so can't be read without a current Internet connection.
POP3 email servers
The Post Office Protocol 3 (POP3) is a mail access protocol used by a client application to read messages from the mail server. Received messages are often deleted from the server. POP supports simple download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC's).[61]POP3 allows you to download email messages on your local computer and read them even when you are offline.[62][63]
IMAP email servers
The Internet Message Access Protocol (IMAP) provides features to manage a mailbox from multiple devices. Small portable devices like smartphones are increasingly used to check email while traveling and to make brief replies, larger devices with better keyboard access being used to reply at greater length. IMAP shows the headers of messages, the sender and the subject and the device needs to request to download specific messages. Usually, the mail is left in folders in the mail server.
MAPI email servers
Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook to communicate to Microsoft Exchange Server - and to a range of other email server products such as Axigen Mail Server, Kerio Connect, Scalix, Zimbra, HP OpenMail, IBM Lotus Notes, Zarafa, and Bynari where vendors have added MAPI support to allow their products to be accessed directly via Outlook.