HOME

TheInfoList



upThe at_sign,_a_part_of_every_SMTP_
at_sign,_a_part_of_every_SMTP_email_address">email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_devices._Email_was_thus_conceived_as_the_
at_sign,_a_part_of_every_SMTP_email_address">email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_devices._Email_was_thus_conceived_as_the_electronics">electronic Electronic_may_refer_to: *Electronics _Electronics_comprises_the_physics,_engineering,_technology_and_applications_that_deal_with_the_emission,_flow_and_control_of_electrons_in_vacuum_and_matter._It_uses_active_devices_to_control_electron_flow_b_...
_(digital_media.html" "title="electronics.html" "title="email_address.html" ;"title="email_address.html" ;"title="at sign, a part of every SMTP email address">at sign, a part of every SMTP email address">email_address.html" ;"title="at sign, a part of every SMTP email address">at sign, a part of every SMTP email address Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronics">electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow b ...
(digital media">digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics comprises the physics, engineering, technology a ...
) version of, or counterpart to, mail, at a time when "mail" meant only physical mail (hence ''wikt:e-#Etymology_2, e- + mail''). Email later became a ubiquitous (very widely used) communication medium, to the point that in current use, an e-mail address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries. Email is the medium, and each message sent therewith is called an email ( mass/count distinction). Email's earliest development began in the 1960s, but at first users could send e-mail only to other users of the same computer. Some systems also supported a form of
instant messaging Instant messaging (IM) technology is a type of online chat allowing real-time text transmission over the Internet or another computer network. Messages are typically transmitted between two or more parties, when each user inputs text and trigger ...

instant messaging
, where sender and receiver needed to be
online In computer technology and telecommunications, online indicates a state of connectivity, and offline indicates a disconnected state. In modern terminology this usually refers to an Internet connection, but (especially when expressed "on line" o ...
simultaneously. The history of modern Internet email services reaches back to the early
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switching In telecommunications, packet switching is a method of grouping data that is transmitted over a digital network into '' packets''. Packets are ...
, 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.
Ray Tomlinson Raymond Samuel Tomlinson (April 23, 1941 – March 5, 2016) was a pioneering American computer programmer who implemented the first email program on the ARPANET system, the precursor to the Internet, in 1971; he is internationally known and cred ...

Ray Tomlinson
is credited as the inventor of networked 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. At the time, though, email, like most computing, was mostly just for "computer geeks" in certain environments, such as engineering and the sciences. During the 1980s and 1990s, use of email became common in the worlds of business management, government, universities, and defense/military industries, but much of the
public In public relations Public relations (PR) is the practice of deliberately managing the release and spread of information between an individual or an organization (such as a business, government agency, or a nonprofit organization) and the ...

public
did not use it yet. Starting with the advent of
web browser A web browser (commonly referred to as a browser) is application software for accessing the World Wide Web upright=1.35, A global map of the web index for countries in 2014 The World Wide Web (WWW), commonly known as the Web, is an in ...

web browser
s in the mid-1990s, use of email began to extend to the rest of the public, no longer something only for geeks in certain professions or industries. By the 2010s,
webmail Webmail (or web-based email) is an Email, email service that can be accessed using a standard web browser. It contrasts with email service accessible through a specialised email client, email client software. Examples of Comparison of webmail provi ...
(the web-era form of email) had gained its ubiquitous status. Email operates across
computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by the Node (networking), network nodes. The ...
s, primarily the
Internet The Internet (Capitalization of Internet, or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''network of networks'' t ...
. Today's email systems are based on a
store-and-forward Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or Node (ne ...
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 Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using Simple Mail Transfer Protocol, SMTP. The terms mail server ...
or a
webmail Webmail (or web-based email) is an Email, email service that can be accessed using a standard web browser. It contrasts with email service accessible through a specialised email client, email client software. Examples of Comparison of webmail provi ...
interface to send or receive messages or download it. Originally an
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Most mod ...
text-only communications medium, Internet email was extended by
Multipurpose Internet Mail Extensions#REDIRECT Mime artist A mime artist or just mime (from Greek , , "imitator, actor") is a person who uses mime as a theatrical medium or as a performance art Performance art is an artwork or art exhibition created through actions executed b ...
(MIME) to carry text in other character sets and multimedia content attachments.
International emailInternational email arises from the combined provision of ''internationalized domain names'' (IDN) and ''email address internationalization'' (EAI).Started with: The result is email that contains international characters (characters which do not ex ...
, with internationalized email addresses using
UTF-8 UTF-8 is a variable-width character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development ...
, is standardized but not widely adopted.


Terminology

Prior to the spread of electronic mail services, the word ''email'', here derived from the French word ''émail'', primarily referred to
vitreous enamel Vitreous enamel, also called porcelain enamel, is a material made by fusing powdered glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decor ...
or sometimes
ceramic glaze Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonm ...
. A rare term, it was mainly used by art historians and medievalists. 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 , which was modern for fax machines at that time. which required special, relatively expensive thermal paper Thermal paper (sometimes referred to as an audit roll) is a special fine paper that is coated with a material formulated to change col ...

fax
document transmission. 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: * ''email'' is now the common form, and recommended by
style guide A style guide or manual of style is a set of standards for the writing, formatting, and design of documents A document is a writing, written, drawing, drawn, presented, or memorialized representation of thought, often the manifestation of n ...
s. It is the form required by
IETF The Internet Engineering Task Force (IETF) is an open standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function ...
Requests for Comments A Request for Comments (RFC) is a publication from the Internet Society The Internet Society (ISOC) is an American nonprofit organization A nonprofit organization (NPO), also known as a non-business entity, not-for-profit organization, or ...
(RFC) and working groups. This spelling also appears in most dictionaries.Random House Unabridged Dictionary, 2006The American Heritage Dictionary of the English Language, Fourth EditionPrinceton University WordNet 3.0The American Heritage Science Dictionary, 2002 * ''e-mail'' is the form favored in edited published American English and British English writing as reflected in the
Corpus of Contemporary American English The Corpus of Contemporary American English (COCA) is a more than 560-million-word corpus of American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties ...
data, but is falling out of favor in some style guides. * ''EMail'' is a traditional form used in RFCs for the "Author's Address" and is required "for historical reasons". * ''E-mail'' is sometimes used, capitalizing the initial ''E'' as in similar abbreviations like ''
E-piano An electric piano is a musical instrument A musical instrument is a device created or adapted to make Music, musical sounds. In principle, any object that produces sound can be considered a musical instrument—it is through purpose that the obje ...
'', ''
E-guitar An electric guitar is a guitar The guitar is a fretted musical instrument that typically has six string instrument, strings. It is held flat against the player's body and played by strumming or Plucked string instrument, plucking the stri ...
'', ''
A-bomb A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fission (fission bomb) or from a ...

A-bomb
'', and ''
H-bomb lenses2) Uranium-238 ("tamper") lined with beryllium reflector3) Vacuum A vacuum is space devoid of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All every ...
''. 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 email consists of an envelope and content; the content consists of a header and a body.


Origin

Computer-based mail and messaging became possible with the advent of
time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
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 Portability in high-level computer programming is the usability of the same software Software is a collection of instructions that tell a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or ...
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. The
Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
(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 X.400 is a suite of ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H. ...
email system, part of the
Government Open Systems Interconnection ProfileThe Government Open Systems Interconnection Profile (GOSIP) was a specification that profiled open networking products for procurement by governments in the late 1980s and early 1990s. Timeline * 1988 - GOSIP: Government Open Systems Interconnection ...
(GOSIP), would predominate. However, once the final restrictions on carrying commercial traffic over the Internet ended in 1995, a combination of factors made the current Internet suite of SMTP,
POP3 In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail client Image:Mozilla Thunderbird 78.6.0 screenshot.png, 250px, Mozilla Thunderbird email client user interface on a Linux operating ...
and
IMAP In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email client Image:Mozilla Thunderbird 78.6.0 screenshot.png, 250px, Mozilla Thunderbird email client user interface on a Linux operating system ...
email protocols the standard.


Operation

The following is a typical sequence of events that takes place when sender
Alice Alice may refer to: * Alice (name) Alice is most often used as a feminine Femininity (also called womanliness or girlishness) is a set of attributes, behaviors, and roles generally associated with women and girls. Although femininity is socia ...
transmits a message using a
mail user agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal sy ...
(MUA) addressed to the
email address An email address identifies an email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_be ...
of the recipient. # The MUA formats the message in email format and uses the submission protocol, a profile of the
Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
(SMTP), to send the message content to the local
mail submission agent A message submission agent (MSA), or mail submission agent, is a computer program A computer program is a collection of instructions that can be executed by a computer to perform a specific task. A computer program is usually written by a co ...
(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 A user is a person who utilizes a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Comp ...

username
of the recipient, and the part after the @ sign is a
domain name A domain name is an identification string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films ...
. The MSA resolves a domain name to determine the
fully qualified domain name A fully qualified domain name (FQDN), sometimes also referred to as an ''absolute domain name'', is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including t ...
of the
mail server Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using Simple Mail Transfer Protocol, SMTP. The terms mail server ...
in the
Domain Name System The Domain Name System (DNS) is the hierarchical and Decentralised system, decentralized naming system used to identify computers, Internet#Applications_and_services, services, and other resources reachable through the internet or other interne ...

Domain Name System
(DNS). # The
DNS server A name server refers to the server component of the Domain Name System The Domain Name System (DNS) is a hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet The Interne ...
for the domain ''b.org'' (''ns.b.org'') responds with any
MX record A mail exchanger record (MX record) specifies the mail server responsible for accepting email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(emai ...
s listing the mail exchange servers for that domain, in this case ''mx.b.org'', a
message transfer agent Within the Internet email File:(at).svg, upThe at sign, a part of every SMTP email address Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as t ...
(MTA) server run by the recipient's ISP. # 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 agentA message delivery agent (MDA), or mail delivery agent, is a computer software Software is a collection of Instruction (computer science), instructions and data (computing), data that tell a computer how to work. This is in contrast to Computer ...
(MDA). # The MDA delivers it to the mailbox of user ''bob''. # Bob's MUA picks up the message using either the
Post Office Protocol In computing, the Post Office Protocol (POP) is an application layer, application-layer Internet standard protocol (computing), protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common us ...
(POP3) or the
Internet Message Access Protocol In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol (computing), protocol used by email clients to retrieve email messages from a mail server over a Internet protocol suite, TCP/IP connection. IMAP is define ...
(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 International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
Lotus Notes HCL Notes (formerly IBM Notes and Lotus Notes; see #Branding, Branding below) and HCL Domino (formerly IBM Domino and Lotus Domino) are the client–server, client and client-server architecture, server, respectively, of a collaborative software, ...
or
Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company with headquarters in Redmond, Washington. It develops, manufactures, licenses, supports, and sells Software, computer software, consumer electroni ...

Microsoft
Exchange Exchange may refer to: Places United States * Exchange, Indiana Exchange is an Unincorporated area, unincorporated community in Green Township, Morgan County, Indiana, Green Township, Morgan County, Indiana, Morgan County, in the U.S. state of In ...
. 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 Webmail (or web-based email) is an Email, email service that can be accessed using a standard web browser. It contrasts with email service accessible through a specialised email client, email client software. Examples of Comparison of webmail provi ...
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 relay An open mail relay is a Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an internet standard communication protocol A communication protocol is a system of rules that allows two or more entities of a communications s ...
s''. This was very important in the early days of the Internet when network connections were unreliable. However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as a consequence open mail relays have become rare, and many MTAs do not accept messages from open mail relays.


Message format

The basic Internet message format used for email is defined by RFC 5322, with encoding of non-ASCII data and multimedia content attachments defined in RFC 2045 through RFC 2049, collectively called ''
Multipurpose Internet Mail Extensions#REDIRECT Mime artist A mime artist or just mime (from Greek , , "imitator, actor") is a person who uses mime as a theatrical medium or as a performance art Performance art is an artwork or art exhibition created through actions executed b ...
'' or ''MIME''. The extensions in
International emailInternational email arises from the combined provision of ''internationalized domain names'' (IDN) and ''email address internationalization'' (EAI).Started with: The result is email that contains international characters (characters which do not ex ...
apply only to email. RFC 5322 replaced the earlier RFC 2822 in 2008, then RFC 2822 in 2001 replaced RFC 822 – the standard for Internet email for decades. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET. Internet email messages consist of two sections, 'header' and 'body'. These are known as 'content'. The header is structured into
fields File:A NASA Delta IV Heavy rocket launches the Parker Solar Probe (29097299447).jpg, FIELDS heads into space in August 2018 as part of the ''Parker Solar Probe'' FIELDS is a science instrument on the ''Parker Solar Probe'' (PSP), designed to mea ...
such as From, To, CC, Subject, Date, and other information about the email. In the process of transporting email messages between systems, SMTP communicates delivery parameters and information using message header fields. The body contains the message, as unstructured text, sometimes containing a
signature block 's signature is the most prominent on the United States Declaration of Independence and the Articles of Confederation The Articles of Confederation and Perpetual Union was an agreement among the U.S. state, 13 original states of the United S ...
at the end. The header is separated from the body by a blank line.


Message header

RFC 5322 specifies the
syntax In linguistics, syntax () is the set of rules, principles, and processes that govern the structure of Sentence (linguistics), sentences (sentence structure) in a given Natural language, language, usually including word order. The term ''syntax'' ...

syntax
of the email header. Each email message has a
header Header may refer to: Computers and engineering * Header (computing)In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the h ...
(the "header section" of the message, according to the specification), comprising a number of
fields File:A NASA Delta IV Heavy rocket launches the Parker Solar Probe (29097299447).jpg, FIELDS heads into space in August 2018 as part of the ''Parker Solar Probe'' FIELDS is a science instrument on the ''Parker Solar Probe'' (PSP), designed to mea ...
("header fields"). Each field has a name ("field name" or "header field name"), followed by the separator character ":", and a value ("field body" or "header field body"). Each field name begins in the first character of a new line in the header section, and begins with a non-
whitespace White space or whitespace may refer to: Technology * Whitespace character, Whitespace characters, characters in computing that represent horizontal or vertical space * White spaces (radio), allocated but locally unused radio frequencies * TV White ...
printable character. It ends with the separator character ":". The separator is followed by the field value (the "field body"). The value can continue onto subsequent lines if those lines have space or tab as their first character. Field names and, without
SMTPUTF8 The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
, field bodies are restricted to 7-bit ASCII characters. Some non-ASCII values may be represented using MIME encoded words.


Header fields

Email header fields can be multi-line, with each line recommended to be no more than 78 characters, although the limit is 998 characters. Header fields defined by RFC 5322 contain only
US-ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the stud ...
characters; for encoding characters in other sets, a syntax specified in RFC 2047 may be used. In some examples, the IETF EAI working group defines some standards track extensions, replacing previous experimental extensions so
UTF-8 UTF-8 is a variable-width character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development ...
encoded
Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requiremen ...

Unicode
characters may be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such addresses are supported by Google and Microsoft products, and promoted by some government agents. The message header must include at least the following fields: * ''From'': The email address, and, optionally, the name of the author(s). Some email clients are changeable through account settings. * ''Date'': The local time and date the message was written. Like the ''From:'' field, many email clients fill this in automatically before sending. The recipient's client may display the time in the format and time zone local to them. RFC 3864 describes registration procedures for message header fields at the
IANA The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, Autonomous system (Internet), autonomous system number allocation, DNS root zone, root zone management in the Domain Name Syst ...
; it provides fo
permanent
an

field names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include: * ''To'': The email address(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below. * ''Subject'': A brief summary of the topic of the message. Certain abbreviations are commonly used in the subject, including "RE:" and "FW:". * ''Cc'':
Carbon copy A copy made with carbon paper Before the development of photographic copiers, a carbon copy—not to be confused with the carbon print family of photographic reproduction processes—was the under-copy of a typed or written document placed over ...
; Many email clients mark email in one's inbox differently depending on whether they are in the To: or Cc: list. * ''Bcc'':
Blind carbon copy Blind carbon copy (abbreviated Bcc) allows the sender of a message to conceal the person entered in the Bcc field from the other recipients. This concept originally applied to paper correspondence and now also applies to email. In some circumstan ...
; addresses are usually only specified during SMTP delivery, and not usually listed in the message header. *
Content-Type A media type (formerly known as MIME type) is a two-part identifier for file formats and format contents transmitted on the Internet The Internet (Capitalization of Internet, or internet) is the global system of interconnected computer ...
: Information about how the message is to be displayed, usually a
MIME Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...
type. * ''Precedence'': commonly with values "bulk", "junk", or "list"; used to indicate automated "vacation" or "out of office" responses should not be returned for this mail, e.g. to prevent vacation notices from sent to all other subscribers of a mailing list.
Sendmail Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet The Interne ...
uses this field to affect prioritization of queued email, with "Precedence: special-delivery" messages delivered sooner. With modern high-bandwidth networks, delivery priority is less of an issue than it was. Microsoft Exchange respects a fine-grained automatic response suppression mechanism, the ''X-Auto-Response-Suppress'' field. * ''
Message-ID Message-ID is a unique identifier A unique identifier (UID) is an identifier that is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. The concept was formalized early in the development of Computer ...
'': Also an automatic-generated field to prevent multiple deliveries and for reference in In-Reply-To: (see below). * ''In-Reply-To'':
Message-ID Message-ID is a unique identifier A unique identifier (UID) is an identifier that is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. The concept was formalized early in the development of Computer ...
of the message this is a reply to. Used to link related messages together. This field only applies to reply messages. * ''References'':
Message-ID Message-ID is a unique identifier A unique identifier (UID) is an identifier that is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. The concept was formalized early in the development of Computer ...
of the message this is a reply to, and the message-id of the message the previous reply was a reply to, etc. * ': Address should be used to reply to the message. * ''Sender'': Address of the sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.). * ''Archived-At'': A direct link to the archived form of an individual email message. The ''To:'' field may be unrelated to the addresses to which the message is delivered. The delivery list is supplied separately to the transport protocol,
SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
, which may be extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter delivered according to the address on the outer envelope. In the same way, the "From:" field may not be the sender. Some mail servers apply
email authenticationEmail authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agent Within the Internet email upThe ...
systems to messages relayed. Data pertaining to the server's activity is also part of the header, as defined below. SMTP defines the ''trace information'' of a message saved in the header using the following two fields: * ''Received'': after an SMTP server accepts a message, it inserts this trace record at the top of the header (last to first). * ''Return-Path'': after the delivery SMTP server makes the ''final delivery'' of a message, it inserts this field at the top of the header. Other fields added on top of the header by the receiving server may be called ''trace fields''. * ''Authentication-Results'': after a server verifies authentication, it can save the results in this field for consumption by downstream agents. * ''Received-SPF'': stores results of checks in more detail than Authentication-Results. * ''DKIM-Signature'': stores results of
DomainKeys Identified Mail DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect forged sender addresses in email ( email spoofing), a technique often used in phishing Phishing is the fraudulent attempt to obtain sensitive informatio ...
(DKIM) decryption to verify the message was not changed after it was sent. * ''Auto-Submitted'': is used to mark automatic-generated messages. * ''VBR-Info'': claims VBR whitelisting


Message body


Content encoding

Internet email was designed for 7-bit ASCII. Most email software is 8-bit clean, but must assume it will communicate with 7-bit servers and mail readers. The
MIME Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...
standard introduced character set specifiers and two content transfer encodings to enable transmission of non-ASCII data: quoted printable for mostly 7-bit content with a few characters outside that range and
base64 In programming, Base64 is a group of binary-to-text encoding schemes that represent binary data (more specifically, a sequence of 8-bit bytes) in an ASCII string format by translating the data into a radix-64 representation. The term ''Base64'' o ...
for arbitrary binary data. The
8BITMIME The Simple Mail Transfer Protocol (SMTP) is an internet standard communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variatio ...
and
BINARY Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics, a binary number is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal ...

BINARY
extensions were introduced to allow transmission of mail without the need for these encodings, but many
mail transport agent Within the Internet email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_peopl ...
s may not support them. In some countries, e-mail software violates by sending rawNot using Internationalized Email or MIME non-ASCII text and several encoding schemes co-exist; as a result, by default, the message in a non-Latin alphabet language appears in non-readable form (the only exception is a coincidence if the sender and receiver use the same encoding scheme). Therefore, for international
character set Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing them to be Data storage, stored, Data communication, transmit ...
s,
Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requiremen ...

Unicode
is growing in popularity.


Plain text and HTML

Most modern graphic
email client Image:Thunderbird 91.4.1 screenshot.png, 250px, Mozilla Thunderbird email client user interface on a Linux operating system An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to ...
s allow the use of either
plain text In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...

plain text
or
HTML The HyperText Markup Language, or HTML is the standard markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntactically distinguishable from the ...
for the message body at the option of the user.
HTML email HTML email is the use of a subset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change ( ...
messages often include an automatic-generated plain text copy for compatibility. Advantages of HTML include the ability to include in-line links and images, set apart previous messages in block quotes, wrap naturally on any display, use emphasis such as
underline 215px, Underscored or underlined text. An underscore, also called an underline, low line, or low dash, is a line drawn under a segment of text. Underscoring/underlining is a proofreading Proofreading is the reading (activity), reading of a g ...
s and
italics In typography File:metal movable type.jpg, 225px, Movable type being assembled on a composing stick using pieces that are stored in the type case shown below it Typography is the art and technique of typesetting, arranging type to make wri ...
, and change
font In metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts Electrical resistivity and cond ...

font
styles. Disadvantages include the increased size of the email, privacy concerns about
web bug A web beacon (also called web bug, tracking bug, tag, web tag, page tag, tracking pixel, pixel tag, 1×1 GIF, or clear GIF) is a technique used on web pages and email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email ...
s, abuse of HTML email as a vector for
phishing Phishing is a type of social engineering where an attacker sends a fraudulent (e.g., spoofed, fake, or otherwise deceptive) message designed to trick a human victim into revealing sensitive information Information sensitivity is the control o ...

phishing
attacks and the spread of
malicious software Malware (a portmanteau for malicious software) is any software intentionally designed to cause damage to a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatical ...

malicious software
. Some e-mail clients interpret the body as HTML even in the absence of a Content-Type: html header field; this may cause various problems. Some web-based
mailing list A mailing list is a collection of names and addresses used by an individual or an organization to send material to multiple recipients. The term is often extended to include the people subscribed to such a list, so the group of subscribers is refe ...
s recommend all posts be made in plain-text, with 72 or 80
characters per lineIn typography File:metal movable type.jpg, 225px, Movable type being assembled on a composing stick using pieces that are stored in the type case shown below it Typography is the art and technique of typesetting, arranging type to make written ...
for all the above reasons, and because they have a significant number of readers using text-based email clients such as Mutt. Some
Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company with headquarters in Redmond, Washington. It develops, manufactures, licenses, supports, and sells Software, computer software, consumer electroni ...

Microsoft
email clients may allow rich formatting using their proprietary
Rich Text Format ) As an example, the following RTF code would be rendered as follows: This is some bold text. Character encoding A standard RTF file can only consist of 7-bit ASCII ASCII ( ), abbreviated from American Standard Code for Information ...
(RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client.


Servers and client applications

Messages are exchanged between hosts using the
Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
with software programs called
mail transfer agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal sy ...
s (MTAs); and delivered to a mail store by programs called
mail delivery agentA message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's Email Mailbox, mailbox., ''Internet Mail Architecture'', D. Crocker (July 2009) It ...
s (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a
bounce message A bounce message or just "bounce" is an automated message from an email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method ...
back to the sender, indicating the problem. Users can retrieve their messages from servers using standard protocols such as
POP Pop or POP may refer to: Places * Gregorio Luperón International Airport (IATA code POP), Puerto Plata, Dominican Republic * Pop, a tributary of the river Jijia in eastern Romania * Poppleton railway station (station code), York, England People ...
or
IMAP In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email client Image:Mozilla Thunderbird 78.6.0 screenshot.png, 250px, Mozilla Thunderbird email client user interface on a Linux operating system ...
, or, as is more likely in a large
corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personality, legal entity representing an association of people, whether Natural person, natural, Legal personality, legal ...

corporate
environment, with a
proprietary {{Short pages monitor {{DEFAULTSORT:Email Email, Internet terminology Electronic documents, Mail History of the Internet Computer-related introductions in 1971