View source for Email

{{Short description|Mail sent using electronic means}}
{{For|the former company|Email Limited}}
{{redirect|Reply all|the podcast|Reply All (podcast){{!}}''Reply All'' (podcast)}}
{{pp-vandalism|small=yes}}
{{Use American English|date=October 2020}}
{{Use mdy dates|date=October 2020}}
<!--Before adding {{lowercase title}} again, please see talk page.-->
[[File: Evolution 36 mail.png|thumb|right|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.]]
[[File:E-post från Wikipedia - 2019.jpg|thumb|When a "robot" on [[Wikipedia]] makes changes to image files, the uploader receives an email about the changes made.]]
<!--Top-level synopsis of what it is-->
'''Electronic mail''' (usually shortened to '''email'''; alternatively hyphenated '''e-mail''') is a method of transmitting and receiving [[Digital media|digital messages]] using [[electronics|electronic]] devices over a [[computer network]]. It was conceived in the late–20th century as the digital version of, or counterpart to, [[mail]] (hence ''[[wikt:e-#Etymology 2|e-]] + mail''). Email is a ubiquitous and very widely used communication medium; in current use, an [[email address]] (commonly ''local-part'' + [[@]] + [[domain name]]) 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.

<!--Synopsis of how it works, in basic high-level terms, including asynchronous nature-->
Email operates across computer networks, primarily the [[Internet access|Internet]], and also [[local area network]]s. Today's email systems are based on a [[store-and-forward]] model. Email [[Server (computing)|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 [[Message transfer agent|mail server]] or a [[webmail]] interface to send or receive messages or download it.

Originally a text-only [[ASCII]] communications medium, Internet email was extended by [[Multipurpose Internet Mail Extensions|MIME]] to carry text in expanded character sets and multimedia content such as images. [[International email]], with internationalized email addresses using [[UTF-8]], is standardized but not widely adopted.<ref name=first/>
<!--end of lede-->

==Terminology==
{{Further|History of email#Terminology and usage}}
The term ''electronic mail'' has been in use with its modern meaning since 1975, and variations of the shorter ''E-mail'' have been in use since 1979:<ref name="Oxford English Dictionary 2012">{{cite web | title=email noun earlier than 1979 | website=Oxford English Dictionary | date=2012-10-25 | url=https://public.oed.com/appeals/email/ | access-date=2020-05-14 | archive-date=April 6, 2023 | archive-url=https://web.archive.org/web/20230406100025/https://public.oed.com/appeals/email/ | url-status=live }}</ref><ref name="Ohlheiser 2015">{{cite news | last=Ohlheiser | first=Abby | title=Why the first use of the word 'e-mail' may be lost forever | newspaper=Washington Post | date=2015-07-28 | url=https://www.washingtonpost.com/news/the-intersect/wp/2015/07/28/why-the-first-use-of-the-word-e-mail-may-be-lost-forever/ | access-date=2020-05-14 | archive-date=April 7, 2023 | archive-url=https://web.archive.org/web/20230407131904/https://www.washingtonpost.com/news/the-intersect/wp/2015/07/28/why-the-first-use-of-the-word-e-mail-may-be-lost-forever/ | url-status=live }}</ref>
* '''''email''''' is now the common form, and recommended by [[style guide]]s.<ref name="cmos"/><ref>{{cite web|url=https://styleguide.yahoo.com/word-list/e|title=Yahoo style guide|publisher=Styleguide.yahoo.com|archive-url=https://web.archive.org/web/20130509154006/https://styleguide.yahoo.com/word-list/e|archive-date=May 9, 2013|access-date=2014-01-09}}</ref><ref>{{cite web|url=https://www.huffingtonpost.com/2011/03/18/ap-removes-hyphen-from-em_n_837833.html|title=AP Removes Hyphen From 'Email' In Style Guide|website=[[Huffington Post]]|location=New York City|date=March 18, 2011|url-status=live|archive-url=https://web.archive.org/web/20150512055628/https://www.huffingtonpost.com/2011/03/18/ap-removes-hyphen-from-em_n_837833.html |archive-date=May 12, 2015}}</ref> It is the form required by [[IETF]] [[Request for Comments|Requests for Comments]] (RFC) and working groups.<ref>{{cite web|url=https://www.rfc-editor.org/rfc-style-guide/terms-online.txt|publisher=IETF|title=RFC Editor Terms List|url-status=live|archive-url=https://web.archive.org/web/20131228152111/https://www.rfc-editor.org/rfc-style-guide/terms-online.txt|archive-date=2013-12-28}} This is suggested by the [https://www.rfc-editor.org/rfc-style-guide/rfc-style-manual-08.txt RFC Document Style Guide]  {{webarchive|url=https://web.archive.org/web/20150424002009/https://www.rfc-editor.org/rfc-style-guide/rfc-style-manual-08.txt |date=2015-04-24 }}</ref>  This spelling also appears in most dictionaries.<ref>{{cite web | url=https://www.askoxford.com/asktheexperts/faq/aboutspelling/email | title=What is the correct way to spell 'e' words such as 'email', 'ecommerce', 'egovernment'? | publisher=[[Oxford University Press]] | work=FAQ | access-date=4 September 2009 | author=AskOxford Language Query team | archive-url=https://web.archive.org/web/20080701194047/https://www.askoxford.com/asktheexperts/faq/aboutspelling/email?view=uk | quote=We recommend email, this is the common form | url-status=dead | archive-date=July 1, 2008}}</ref><ref>{{cite web |url=https://dictionary.reference.com/browse/email |title=Reference.com |publisher=Dictionary.reference.com |access-date=2014-01-09 |url-status=live |archive-url=https://web.archive.org/web/20131216094405/https://dictionary.reference.com/browse/email |archive-date=2013-12-16 }}</ref><ref name="cmos"/><ref>{{cite web |title=''"RFC Style Guide"'', Table of decisions on consistent use in RFC |url=https://www.rfc-editor.org/rfc-style-guide/terms-online.txt |url-status=live |archive-url=https://web.archive.org/web/20131228152111/https://www.rfc-editor.org/rfc-style-guide/terms-online.txt |archive-date=2013-12-28 |access-date=2014-01-09}}</ref>
* '''''e-mail''''' was originally the form favored in edited published American English and British English writing, and was formerly preferred by some style guides.<ref name="cmos">{{cite web |title=From E-mail to Email: Is the Sky Falling? |url=https://cmosshoptalk.com/2017/10/11/from-e-mail-to-email-is-the-sky-falling/ |website=CMOS Shop Talk |publisher=[[Chicago Manual of Style]] |access-date=18 September 2024 |date=11 October 2017}}</ref>
* '''''E-mail''''' is sometimes used.<ref>{{cite web |url=https://alt-usage-english.org/excerpts/fxhowdoy.html |title=How do you spell "e-mail"? |first1=Mark |last1=Israel |publisher=Alt-usage-english.org |access-date=2014-01-09 |url-status=live |archive-url=https://web.archive.org/web/20120403084841/https://www.alt-usage-english.org/excerpts/fxhowdoy.html |archive-date=2012-04-03 }}</ref> The original usage in June 1979 occurred in the journal ''[[Electronics (magazine)|Electronics]]'' in reference to the [[United States Postal Service]] initiative called [[E-COM]], which was developed in the late 1970s and operated in the early 1980s.<ref name="Oxford English Dictionary 2012" /><ref name="Ohlheiser 2015" />
* '''''EMAIL''''' was used by [[CompuServe]] starting in April 1981, which popularized the term.<ref>{{Cite web |title=Did V.A. Shiva Ayyadurai Invent Email? |website= SIGCIS |author1=thaigh |date=2015 |url=https://www.sigcis.org/ayyadurai |access-date=2020-09-05 |archive-date=April 17, 2022 |archive-url=https://web.archive.org/web/20220417130400/https://www.sigcis.org/Ayyadurai |url-status=live }}</ref><ref>{{Cite web |last=Masnick |first=Mike |title=Laying Out All The Evidence: Shiva Ayyadurai Did Not Invent Email |url=https://www.techdirt.com/articles/20190518/23370542236/laying-out-all-evidence-shiva-ayyadurai-did-not-invent-email.shtml |access-date=2020-09-05 |website=Techdirt. |date=May 22, 2019 |archive-date=January 27, 2022 |archive-url=https://web.archive.org/web/20220127173452/https://www.techdirt.com/articles/20190518/23370542236/laying-out-all-evidence-shiva-ayyadurai-did-not-invent-email.shtml |url-status=live }}</ref>
* '''''EMail''''' is a traditional form used in RFCs for the "Author's Address".

The service is often simply referred to as ''mail'', and a single piece of electronic mail is called a ''message''. The conventions for fields within emails—the "To", "From", "CC", "BCC" etc.—began with RFC-680 in 1975.<ref>{{Cite news |last=Pexton |first=Patrick B. |date=2012-03-01 |title=Origins of e-mail: My mea culpa |url=https://www.washingtonpost.com/blogs/omblog/post/origins-of-e-mail-my-mea-culpa/2012/03/01/gIQAiOD5kR_blog.html |url-access=subscription |access-date=2022-04-18 |newspaper=Washington Post |language=en-US |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519192857/https://www.washingtonpost.com/blogs/omblog/post/origins-of-e-mail-my-mea-culpa/2012/03/01/gIQAiOD5kR_blog.html |url-status=live }}</ref>

An Internet email consists of an ''envelope'' and ''content'';<ref>{{cite ietf
 |       title = Simple Mail Transfer Protocol
 |         rfc = 5321
 |     section = 2.3.1
 | sectionname = Mail Objects
 |       quote = SMTP transports a mail object.  A mail object contains an envelope and content.
 |   publisher = [[Internet Engineering Task Force|IETF]]
}}</ref> the content consists of a ''header'' and a ''body''.<ref>{{cite ietf
 |       title = Simple Mail Transfer Protocol
 |         rfc = 5321
 |     section = 2.3.1
 | sectionname = Mail Objects
 |       quote = The SMTP content is sent in the SMTP DATA protocol unit, and has two parts: the header section and the body.  If the content conforms to other contemporary standards, the header section is a collection of header fields, each consisting of a header name, a colon, and data, structured as in the message format specification
 |   publisher = [[Internet Engineering Task Force|IETF]]
}}</ref>

== History ==
{{anchor|Origin}}
{{Main|History of email}}
Computer-based messaging between users of the same system became possible after the advent of [[time-sharing]] in the early 1960s, with a notable implementation by [[Massachusetts Institute of Technology|MIT]]'s [[Compatible Time-Sharing System|CTSS]] project in 1965.<ref>{{cite web|url=http://www.multicians.org/thvv/mail-history.html|title=The History of Electronic Mail|author=Tom Van Vleck|access-date=March 23, 2005|archive-date=December 2, 2017|archive-url=https://web.archive.org/web/20171202025034/http://www.multicians.org/thvv/mail-history.html|url-status=live}}</ref> Most developers of early [[mainframe]]s and [[minicomputer]]s developed similar, but generally incompatible, mail applications. In 1971 the first [[ARPANET]] network mail was sent, introducing the now-familiar address syntax with the '[[At sign|@]]' symbol designating the user's system address.<ref>{{cite web |author=Ray Tomlinson |url=https://openmap.bbn.com/~tomlinso/ray/firstemailframe.html |title=The First Network Email |publisher=Openmap.bbn.com |access-date=2019-10-05 |archive-date=May 6, 2006 |archive-url=https://web.archive.org/web/20060506003539/https://openmap.bbn.com/~tomlinso/ray/firstemailframe.html |url-status=dead }}</ref> Over a series of [[Request for Comments|RFCs]], conventions were refined for sending mail messages over the [[File Transfer Protocol]].

Proprietary electronic mail systems soon began to emerge. [[IBM OfficeVision#Earlier PROFS, DISOSS and Office/36|IBM]], [[CompuServe]] and [[Xerox Star|Xerox]] used in-house mail systems in the 1970s; CompuServe sold a commercial intraoffice mail product in 1978 to IBM and to Xerox from 1981.{{refn|group=nb|IBM's system was available on request to customers prior to formal release.}}<ref>{{cite journal |last1=Gardner |first1=P. C. |year=1981 |title=A system for the automated office environment |journal=IBM Systems Journal |volume=20 |issue=3 |pages=321–345 |doi=10.1147/sj.203.0321 |issn=0018-8670 |postscript=none}}; {{Cite web |date=2020-08-02 |title=IBM100 - The Networked Business Place |website=[[IBM]] |url=https://www.ibm.com/ibm/history/ibm100/us/en/icons/networkbus/ |url-status=live |archive-url=https://web.archive.org/web/20200802211021/https://www.ibm.com/ibm/history/ibm100/us/en/icons/networkbus/ |archive-date=2020-08-02 |access-date=2020-09-07}}</ref><ref>{{cite magazine |author=Connie Winkler |date=October 22, 1979 |title=CompuServe pins hopes on MicroNET, InfoPlex |url=https://books.google.com/books?id=ChMAmfS1nEkC&dq=compuserve+Infoplex+1979&pg=PA69 |magazine=[[Computerworld]] |volume=13 |issue=42 |page=69 |postscript=none}}; {{cite magazine |author=Dylan Tweney |date=September 24, 1979 |title=Sept. 24, 1979: First Online Service for Consumers Debuts |url=https://www.wired.com/2009/09/0924compuserve-launches/ |magazine=[[Wired (magazine)|Wired]]}}</ref><ref>{{cite news |last=Ollig |first=Mark |date=October 31, 2011 |title=They could have owned the computer industry |work=Herald Journal |url=http://www.herald-journal.com/archives/2011/columns/mo103111.html |access-date=2021-02-26 |postscript=none |archive-date=February 27, 2021 |archive-url=https://web.archive.org/web/20210227062954/http://www.herald-journal.com/archives/2011/columns/mo103111.html |url-status=live }}; {{Cite web |last= |date=15 February 2012 |title=Tech before its time: Xerox's shooting Star computer |url=https://www.newscientist.com/article/mg21328521-800-tech-before-its-time-xeroxs-shooting-star-computer/ |access-date=2022-04-18 |website=New Scientist |language=en-US |postscript=none |archive-date=April 18, 2022 |archive-url=https://web.archive.org/web/20220418181102/https://www.newscientist.com/article/mg21328521-800-tech-before-its-time-xeroxs-shooting-star-computer/ |url-status=live }}; {{Cite web |title=The Xerox Star |url=http://toastytech.com/guis/star.html |access-date=2022-04-18 |website=toastytech.com |archive-date=July 18, 2011 |archive-url=https://web.archive.org/web/20110718030555/http://toastytech.com/guis/star.html |url-status=live }}</ref> DEC's [[ALL-IN-1]] and [[Hewlett-Packard|Hewlett-Packard's]] HPMAIL (later HP DeskManager) were released in 1982; development work on the former began in the late 1970s and the latter became the world's largest selling email system.<ref>{{cite web |date=1998-01-30 |title=ALL-IN-1 |url=https://research.microsoft.com/en-us/um/people/gbell/Digital/timeline/1982-4.htm |work=DIGITAL Computing Timeline |access-date=April 20, 2022 |archive-date=January 3, 2017 |archive-url=https://web.archive.org/web/20170103002807/http://research.microsoft.com/en-us/um/people/gbell/Digital/timeline/1982-4.htm |url-status=live }}</ref><ref>{{cite web |title=HP Computer Museum |url=http://www.hpmuseum.net/divisions.php?did=10 |access-date=November 10, 2016 |archive-date=September 9, 2016 |archive-url=https://web.archive.org/web/20160909011243/http://www.hpmuseum.net/divisions.php?did=10 |url-status=live }}</ref>

The [[Simple Mail Transfer Protocol]] (SMTP) was implemented on the ARPANET in 1983. [[Local area network|LAN]] email systems emerged in the mid-1980s. 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. However, once the final restrictions on carrying commercial traffic over the Internet ended in 1995,<ref>[https://merit.edu/research/nsfnet_article.php "Retiring the NSFNET Backbone Service: Chronicling the End of an Era"] {{webarchive|url=https://web.archive.org/web/20160101025735/https://merit.edu/research/nsfnet_article.php |date=2016-01-01 }}, Susan R. Harris, Ph.D., and Elise Gerich, ''ConneXions'', Vol. 10, No. 4, April 1996</ref><ref>{{cite web | url= https://www.walthowe.com/navnet/history.html | title= A Brief History of the Internet | url-status= live | archive-url= https://web.archive.org/web/20150811053448/https://www.walthowe.com/navnet/history.html | archive-date= 2015-08-11 | bibcode= 1999cs........1011L | last1= Leiner | first1= Barry M. | last2= Cerf | first2= Vinton G. | last3= Clark | first3= David D. | last4= Kahn | first4= Robert E. | last5= Kleinrock | first5= Leonard | last6= Lynch | first6= Daniel C. | last7= Postel | first7= Jon | last8= Roberts | first8= Larry G. | last9= Wolf | first9= Stephen | year= 1999 | arxiv= cs/9901011 }}</ref> a combination of factors made the current Internet suite of SMTP, [[POP3]] and [[IMAP]] email protocols the standard (see [[Protocol Wars]]).<ref>{{Cite thesis |last=Rutter |first=Dorian |title=From Diversity to Convergence: British Computer Networks and the Internet, 1970-1995 |date=2005 |degree=Computer Science |publisher=The University of Warwick |url=http://wrap.warwick.ac.uk/1197/1/WRAP_THESIS_Rutter_2005.pdf |access-date=December 23, 2022 |archive-date=October 10, 2022 |archive-url=https://ghostarchive.org/archive/20221010/http://wrap.warwick.ac.uk/1197/1/WRAP_THESIS_Rutter_2005.pdf |url-status=live }}</ref><ref>{{Cite journal |last1=Campbell-Kelly |first1=Martin |last2=Garcia-Swartz |first2=Daniel D |date=2013 |title=The History of the Internet: The Missing Narratives |url=https://papers.ssrn.com/abstract=867087 |journal=Journal of Information Technology |language=en |volume=28 |issue=1 |pages=18–33 |doi=10.1057/jit.2013.4 |ssrn=867087 |s2cid=41013 |issn=0268-3962|url-access=subscription }}</ref>

==Operation==
The following is a typical sequence of events that takes place when sender [[Placeholder names in cryptography|Alice]] transmits a message using a [[E-mail client|mail user agent]] (MUA) addressed to the [[email address]] of the recipient.<ref>{{cite video|title=How E-mail Works|publisher=howstuffworks.com|year=2008|url=https://computer.howstuffworks.com/e-mail-messaging/email.htm|url-status=live|archive-url=https://web.archive.org/web/20170611181852/https://computer.howstuffworks.com/e-mail-messaging/email.htm|archive-date=2017-06-11}}</ref>

[[File:email.svg|thumb|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|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 [[Message transfer agent|mail server]] in the [[Domain Name System]] (DNS).
# The [[DNS server]] for the domain ''b.org'' (''ns.b.org'') responds with any [[MX record]]s 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.<ref>[https://dnsdb.cit.cornell.edu/explain_mx.html "MX Record Explanation"] {{webarchive|url=https://web.archive.org/web/20150117204846/https://dnsdb.cit.cornell.edu/explain_mx.html |date=2015-01-17 }}, it.cornell.edu</ref>
# 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 [[Email Mailbox|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]] [[Microsoft Exchange Server|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 relay]]s''. This was very important in the early days of the Internet when network connections were unreliable.<ref>{{cite web|url=https://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci782509,00.html |title=What is open relay? |access-date=2008-04-07 |date=2004-07-19 |work=WhatIs.com |publisher=[[Indiana University]] |archive-url=https://web.archive.org/web/20070824005337/https://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci782509,00.html |archive-date=2007-08-24 }}</ref><ref>{{cite book|author=Ch Seetha Ram|title=Information Technology for Management|url=https://books.google.com/books?id=0bHAGrUxqRsC&pg=PA164|year=2010|publisher=Deep & Deep Publications|isbn=978-81-8450-267-1|page=164}}</ref>  However, this mechanism proved to be exploitable by originators of [[email spam|unsolicited bulk email]] and as a consequence open mail relays have become rare,<ref>{{cite web|url=https://www.imc.org/ube-relay.html |title=Allowing Relaying in SMTP: A Series of Surveys |access-date=2008-04-13 |last=Hoffman |first=Paul |date=2002-08-20 |work=IMC Reports |publisher=[[Internet Mail Consortium]] |archive-url=https://web.archive.org/web/20070118121843/https://www.imc.org/ube-relay.html |archive-date=2007-01-18 }}</ref> and many MTAs do not accept messages from open mail relays.

Email pre-dates [[instant messaging]], and transmission favors reliability over speed, in order to be able to cope with unreliable network links and busy servers (more common in the early days of the Internet). Reasons for slower delivery include:<ref name="luxsci">{{Cite web|url=https://luxsci.com/blog/why-email-is-not-instantaneous-and-not-supposed-to-be.html|title=Why Email is Not Instantaneous -- and Not Supposed to Be|date=October 15, 2013}}</ref>
* Messages going to a large number of recipients require more processing
* Large messages (e.g. with large attachments) require more time to transmit over the network
* Messages need to pass through multiple servers (sometimes multiple servers inside the same organization)
* One or more mail servers are overloaded (possibly due to [[email spam|spam]] or [[denial-of-service attack]]) and queuing incoming mail or temporarily refusing incoming connections
* [[SMTP]] requires multiple back-and-forths, which can amplify the impact of a slow network or dropped packets
* Sender or recipient temporarily disconnected from the network (e.g. a laptop out of wifi range)
* Slow DNS response
* Server down for maintenance or malfunction

Mail can be queued and retried for up to five days before senders are notified of a permanent delivery failure.<ref name="luxsci" /> Messages are timestamped as they pass through each server, allowing for diagnosis of slow delivery, though analysis is complicated by [[time zone]]s and computer clocks that are inaccurately set.<ref name="luxsci" />

Email messages classified as [[email spam|spam]] by a [[spam filter]] may be sorted into a separate folder which the recipient must check manually, or may be dropped entirely.

==Message format {{anchor|Internet Message Format}}==
The basic Internet message format used for email<ref>The Internet message format is also used for [[usenet|network news]]</ref> is defined by {{IETF 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]]'' or ''MIME''. The extensions in [[International email]] apply only to email. RFC 5322 replaced RFC 2822 in 2008.  Earlier, in 2001, RFC 2822 had in turn replaced RFC 822, which had been the standard for Internet email for decades. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET.<ref>{{cite web|first=Ken|last=Simpson|title=An update to the email standards|date=October 3, 2008|publisher=MailChannels Blog Entry|url=https://blog.mailchannels.com/2008/10/update-to-email-standards.html|url-status=live|archive-url=https://web.archive.org/web/20081006080735/https://blog.mailchannels.com/2008/10/update-to-email-standards.html|archive-date=October 6, 2008}}</ref>

Internet email messages consist of two sections, "header" and "body". These are known as "content".<ref>{{cite ietf
| rfc = 5321
| title = Simple Mail Transfer Protocol
| date = October 2008
| author = J. Klensin
| section = 2.3.1
| sectionname = Mail Objects
| quote = SMTP transports a mail object.  A mail object contains an envelope and content. ... The SMTP content is sent in the SMTP DATA protocol unit, and has two parts: the header section and the body.
| mode = cs2
}}</ref><ref>{{cite ietf
| rfc = 5598
| title = Internet Mail Architecture
| date = July 2009
| author = D. Crocker
| section = 4.1
| sectionname = Message Data
| quote = A message comprises a transit-handling envelope and the message content.  The envelope contains information used by the MHS.  The content is divided into a structured header and the body.
| mode = cs2
}}</ref>  The header is structured into [[Field (computer science)|fields]] 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]] at the end. The header is separated from the body by a blank line.

===Message header===
<!-- This section is linked from [[Bracket]] -->
RFC 5322 specifies the [[syntax]] of the email header. Each email message has a [[Header (computing)|header]] (the "header section" of the message, according to the specification), comprising a number of [[Field (computer science)|fields]] ("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 character|whitespace]] [[Printable characters|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 [[Simple Mail Transfer Protocol#SMTPUTF8|SMTPUTF8]], field bodies are restricted to 7-bit ASCII characters. Some non-ASCII values may be represented using MIME [[MIME#Encoded-Word|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.{{ref RFC|5322}} Header fields defined by RFC 5322 contain only [[US-ASCII]] characters; for encoding characters in other sets, a syntax specified in RFC 2047 may be used.{{ref RFC|2047}} In some examples, the IETF EAI working group defines some standards track extensions,{{ref RFC|6532}}{{ref RFC|6531}} replacing previous experimental extensions so [[UTF-8]] encoded [[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.<ref>{{Cite news|url=https://economictimes.indiatimes.com/tech/internet/now-get-your-email-address-in-hindi/articleshow/53830034.cms|title=Now, get your email address in Hindi - The Economic Times|newspaper=The Economic Times|access-date=2016-10-17|url-status=live|archive-url=https://web.archive.org/web/20160828044631/https://economictimes.indiatimes.com/tech/internet/now-get-your-email-address-in-hindi/articleshow/53830034.cms|archive-date=2016-08-28}}</ref>

The message header must include at least the following fields:<ref>{{cite IETF|rfc=5322|section=3.6 |title=Internet Message Format|sectionname=Field Definitions |date=October 2008 |access-date=2014-01-09  |editor-first1=P |editor-last1=Resnick  }}</ref><ref>{{cite IETF|rfc=5322|section=3.6.4 |title=Internet Message Format|sectionname=Identification Fields  |date=October 2008 |access-date=2014-01-09  |editor-first1=P |editor-last1=Resnick  }}</ref>
* ''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 [[Internet Assigned Numbers Authority|IANA]]; it provides for [https://www.iana.org/assignments/message-headers/perm-headers.html permanent] and [https://www.iana.org/assignments/message-headers/prov-headers.html provisional] field names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include:{{ref RFC|5064}}
* ''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. [[E-mail subject abbreviations|Certain abbreviations]] are commonly used in the subject, including [[E-mail subject abbreviations|"RE:" and "FW:"]].
* ''Cc'': [[Carbon copy]]; 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]]; addresses are usually only specified during SMTP delivery, and not usually listed in the message header.
* [[Content-Type]]: Information about how the message is to be displayed, usually a [[MIME]] 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]] 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 Server|Microsoft Exchange]] respects a fine-grained automatic response suppression mechanism, the ''X-Auto-Response-Suppress'' field.<ref>Microsoft, Auto Response Suppress, 2010, [https://msdn.microsoft.com/en-us/library/ee219609(v=EXCHG.80).aspx Microsoft reference] {{webarchive|url=https://web.archive.org/web/20110407033540/https://msdn.microsoft.com/en-us/library/ee219609(v=EXCHG.80).aspx |date=2011-04-07 }}, 2010 Sep 22</ref>
* ''[[Message-ID]]'': Also an automatic-generated field to prevent multiple deliveries and for reference in In-Reply-To: (see below).
* ''In-Reply-To'': Message-ID of the message this is a reply to. Used to link related messages together. This field only applies to reply messages.
* ''List-Unsubscribe'': HTTP link to unsubscribe from a mailing list.
* ''References'': Message-ID of the message this is a reply to, and the message-id of the message the previous reply was a reply to, etc.
* ''{{vanchor|Reply-To}}'': 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, [[Simple Mail Transfer Protocol|SMTP]], 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 authentication]] 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:<ref>{{cite IETF|title=Simple Mail Transfer Protocol|rfc=5321|author=[[John Klensin]]|sectionname=Trace Information|section=4.4| date=October 2008 |publisher=[[Internet Engineering Task Force|IETF]]}}</ref>
* ''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''.<ref>{{cite web|url=https://www.ietf.org/mail-archive/web/apps-discuss/current/msg04115.html|title=Trace headers|author=John Levine|date=14 January 2012|work=email message|publisher=[[Internet Engineering Task Force|IETF]]|access-date=16 January 2012|quote=there are many more trace fields than those two|url-status=live|archive-url=https://web.archive.org/web/20120811135249/https://www.ietf.org/mail-archive/web/apps-discuss/current/msg04115.html|archive-date=11 August 2012}}</ref>
* ''Authentication-Results'': after a server verifies authentication, it can save the results in this field for consumption by downstream agents.<ref>This extensible field is defined by {{IETF RFC|7001}}, this also defines an [[Internet Assigned Numbers Authority|IANA]] registry of [https://www.iana.org/assignments/email-auth/ Email Authentication Parameters].</ref>
* ''Received-SPF'': stores results of [[Sender Policy Framework|SPF]] checks in more detail than Authentication-Results.<ref>{{IETF RFC|7208}}.</ref>
* ''DKIM-Signature'': stores results of [[DomainKeys Identified Mail]] (DKIM) decryption to verify the message was not changed after it was sent.{{ref RFC|6376}}
* ''Auto-Submitted'': is used to mark automatic-generated messages.<ref>Defined in {{IETF RFC|3834}}, and updated by {{IETF RFC|5436}}.</ref>
* ''VBR-Info'': claims [[Vouch by Reference|VBR]] whitelisting<ref>{{IETF RFC| 5518}}.</ref>

===Message body===

====Content encoding====
Internet email was designed for 7-bit ASCII.<ref>{{cite book|title=TCP/IP Network Administration|year=2002|isbn=978-0-596-00297-8|author=Craig Hunt|publisher=[[O'Reilly Media]]|page=70}}</ref> Most email software is [[8-bit clean]], but must assume it will communicate with 7-bit servers and mail readers. The [[MIME]] 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]] for arbitrary binary data. The [[8BITMIME]] and [[BINARY]] extensions were introduced to allow transmission of mail without the need for these encodings, but many [[mail transport agent]]s may not support them. In some countries, e-mail software violates {{IETF RFC|5322}} by sending raw<ref group=nb>Not using Internationalized Email or MIME</ref> 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]]s, [[Unicode]] is growing in popularity.<ref>{{Cite web|title=What is unicode? |url=https://www.konfinity.com/what-is-unicode|access-date=2022-01-31|website=Konfinity |archive-date=January 31, 2022|archive-url=https://web.archive.org/web/20220131083432/https://www.konfinity.com/what-is-unicode|url-status=live}}</ref>

====Plain text and HTML====
Most modern graphic [[email client]]s allow the use of either [[plain text]] or [[HTML email|HTML]] for the message body at the option of the user. HTML email 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 quote]]s, wrap naturally on any display, use emphasis such as [[underline]]s and [[italics]], and change [[font]] styles. Disadvantages include the increased size of the email, privacy concerns about [[web bug]]s, abuse of HTML email as a vector for [[phishing]] attacks and the spread of [[malware|malicious software]].<ref>{{cite web|url=https://advosys.ca/papers/mail-policies.html|title=Email policies that prevent viruses|archive-url=https://web.archive.org/web/20070512053927/https://advosys.ca/papers/mail-policies.html |website=Advosys Consulting | archive-date=2007-05-12|url-status=dead}}</ref>
Some e-mail clients interpret the body as HTML even in the absence of a <code>Content-Type: html</code> header field; this may cause various problems.

Some web-based [[mailing list]]s recommend all posts be made in plain text, with 72 or 80 [[characters per line]] for all the above reasons,<ref>{{cite web |url=https://helpdesk.rootsweb.com/listadmins/plaintext.html |quote=When posting to a RootsWeb mailing list, your message should be sent as "plain  text." |publisher=RootsWeb HelpDesk |title=Problem Solving: Sending Messages in Plain Text |access-date=2014-01-09 |url-status=dead |archive-url=https://web.archive.org/web/20140219024856/https://helpdesk.rootsweb.com/listadmins/plaintext.html |archive-date=2014-02-19 }}</ref><ref>{{cite web |url=https://www.openbsd.org/mail.html |title= ''Open''BSD Mailing Lists |quote=Plain text, 72 characters per line |publisher=OpenBSD |access-date=2014-01-09 |url-status=live |archive-url=https://web.archive.org/web/20140208005706/https://openbsd.org/mail.html |archive-date=2014-02-08 }}</ref> and because they have a significant number of readers using [[Comparison of email clients#Text-based|text-based email clients]] such as [[Mutt (email client)|Mutt]].
Various informal conventions evolved for marking up plain text in email and [[usenet]] posts, which later led to the development of formal languages like [[setext]] ''(c. 1992)'' and [[Lightweight markup language|many others]], the most popular of them being [[markdown]].

Some [[Microsoft]] email clients may allow rich formatting using their proprietary [[Rich Text Format]] (RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client.<ref>{{cite web |url=https://support.microsoft.com/kb/138053 |title=Verhindern, dass die Datei "Winmail.dat" an Internetbenutzer gesendet wird |trans-title=How to Prevent the Winmail.dat File from Being Sent to Internet Users |publisher=Microsoft Support |date=2010-07-02 |access-date=2014-01-09 |url-status=dead |archive-url=https://web.archive.org/web/20140109193922/https://support.microsoft.com/kb/138053 |archive-date=2014-01-09 }}</ref>

==Servers and client applications==
[[File: Mozilla Thunderbird 3.1.png|thumb|The interface of an [[email client]], [[Mozilla Thunderbird|Thunderbird]]]]
<!-- This section is linked from [[Catch-all (Mail)]]. See [[WP:MOS#Section management]] -->
Messages are exchanged between hosts using the [[Simple Mail Transfer Protocol]] with software programs called [[mail transfer agent]]s (MTAs); and delivered to a mail store by programs called [[mail delivery agent]]s (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it,<ref>In practice, some accepted messages may nowadays not be delivered to the recipient's InBox, but instead to a Spam or Junk folder which, especially in a corporate environment, may be inaccessible to the recipient</ref> and when a message cannot be delivered, that MTA must send a [[bounce message]] back to the sender, indicating the problem.

Users can retrieve their messages from servers using standard protocols such as [[Post Office Protocol|POP]] or [[IMAP]], or, as is more likely in a large [[corporation|corporate]] environment, with a [[Proprietary software|proprietary]] protocol specific to [[Novell Groupwise]], [[Lotus Notes]] or [[Microsoft Exchange Server]]s.  Programs used by users for retrieving, reading, and managing email are called [[mail user agent]]s (MUAs).

When opening an email, it is marked as "read", which typically visibly distinguishes it from "unread" messages on clients' user interfaces. Email clients may allow hiding read emails from the inbox so the user can focus on the unread.<ref>{{cite web |title=View only unread messages |url=https://support.microsoft.com/en-us/office/view-only-unread-messages-f2c8450c-9cd0-4037-a5d3-26f6946727ca |website=support.microsoft.com |access-date=November 13, 2021 |archive-date=November 13, 2021 |archive-url=https://web.archive.org/web/20211113164358/https://support.microsoft.com/en-us/office/view-only-unread-messages-f2c8450c-9cd0-4037-a5d3-26f6946727ca |url-status=live }}</ref>

Mail can be stored on the [[client (computing)|client]], on the [[Server (computing)|server]] side, or in both places. Standard formats for mailboxes include [[Maildir]] and [[mbox]]. Several prominent email clients use their own proprietary format and require conversion software to transfer email between them. Server-side storage is often in a proprietary format but since access is through a standard protocol such as IMAP, moving email from one server to another can be done with any [[Mail user agent|MUA]] supporting the protocol.

Many current email users do not run MTA, MDA or MUA programs themselves, but use a web-based email platform, such as [[Gmail]] or [[Yahoo! Mail]], that performs the same tasks.<ref>{{Cite web|url=https://dir.yahoo.com/business_and_economy/business_to_business/communications_and_networking/internet_and_world_wide_web/email_providers/free_email/|title=Free Email Providers in the Yahoo! Directory|website=dir.yahoo.com|archive-url=https://web.archive.org/web/20140704154604/https://dir.yahoo.com/business_and_economy/business_to_business/communications_and_networking/internet_and_world_wide_web/email_providers/free_email/|archive-date=2014-07-04|url-status=dead}}</ref> Such [[webmail]] interfaces allow users to access their mail with any standard [[web browser]], from any computer, rather than relying on a local email client.

===Filename extensions===
Upon reception of email messages, email client applications save messages in operating system files in the file system. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the ''mbox'' format. The specific format used is often indicated by special [[filename extension]]s:
;<code>eml</code>
:Used by many email clients including [[Novell GroupWise]], [[Microsoft Outlook Express]], [[Lotus notes]], [[Windows Mail]], [[Mozilla Thunderbird]], and Postbox. The files contain the email contents as [[plain text]] in [[MIME]] format, containing the email header and body, including attachments in one or more of several formats.
;<code>emlx</code>
:Used by [[Apple Mail]].
;<code>msg</code>
:Used by [[Microsoft Outlook|Microsoft Office Outlook]] and [[OfficeLogic|OfficeLogic Groupware]].
;<code>mbx</code>
:Used by Opera Mail, [[KMail]], and [[Apple Mail]] based on the mbox format.

Some applications (like [[Apple Mail]]) leave attachments encoded in messages for searching while also saving separate copies of the attachments. Others separate attachments from messages and save them in a specific directory.

===URI scheme mailto===
{{main|mailto}}
The [[URI scheme]], as registered with the IANA, defines the <code>mailto:</code> scheme for SMTP email addresses. Though its use is not strictly defined, URLs of this form are intended to be used to open the new message window of the user's mail client when the URL is activated, with the address as defined by the URL in the ''To:'' field.<ref>RFC 2368 section 3 : by Paul Hoffman in 1998 discusses operation of the "mailto" URL.</ref><ref name="Barnett 2011 p. 245" /> Many clients also support query string parameters for the other email fields, such as its subject line or carbon copy recipients.<ref>{{Cite web|url=https://developer.mozilla.org/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks|title=Creating hyperlinks § E-mail links|website=MDN Web Docs|language=en|access-date=2019-09-30|archive-date=August 18, 2019|archive-url=https://web.archive.org/web/20190818224233/https://developer.mozilla.org/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks|url-status=live}}</ref>

==Types==

===Web-based email===
{{main|Webmail}}
Many email providers have a web-based email client. 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 it cannot 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 (computing)|server]]. POP supports simple download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC's).<ref>{{cite book | last = Allen | first = David | title = Windows to Linux | publisher = Prentice Hall | year = 2004 | page = 192 | url = https://books.google.com/books?id=UD0h_GqgbHgC&q=network%2B+guide+to+networks | url-status = live | archive-url = https://web.archive.org/web/20161226051835/https://books.google.com/books?id=UD0h_GqgbHgC&printsec=frontcover&dq=network%2B+guide+to+networks&ct=result&resnum=1 | archive-date = 2016-12-26 | isbn = 978-1423902454 }}</ref> POP3 allows downloading messages on a local computer and reading them even when offline.<ref>{{cite IETF
 |         rfc = 1733
 |       title = DISTRIBUTED ELECTRONIC MAIL MODELS IN IMAP4
 |     section = 4.5. 
 | sectionname = Implementation and Operation
  }}
</ref><ref>{{cite IETF
 |         rfc = 5598
 |       title = Internet Mail Architecture
 |     section = 4.2.2.
 | sectionname = Message Store (MS)
  }}
</ref>

===IMAP email servers===
The [[Internet Message Access Protocol]] (IMAP) provides features to manage a mailbox from multiple devices. Small portable devices like [[smartphone]]s 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===
[[MAPI|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|Axigen Mail Server]], [[Kerio Connect]], [[Scalix]], [[Zimbra]], [[HP OpenMail]], [[IBM Lotus Notes]], [[Zarafa (software)|Zarafa]], and [[Bynari]] where vendors have added MAPI support to allow their products to be accessed directly via Outlook.

==Uses==
{{More citations needed section|date=November 2007}}

===Business and organizational use===
Email has been widely accepted by businesses, governments and non-governmental organizations in the developed world, and it is one of the key parts of an 'e-revolution' in workplace communication (with the other key plank being widespread adoption of highspeed [[Internet]]). A sponsored 2010 study on workplace communication found 83% of U.S. knowledge workers felt email was critical to their success and productivity at work.<ref>By Om Malik, GigaOm. "[https://gigaom.com/collaboration/is-email-a-curse-or-a-boon/ Is Email a Curse or a Boon?] {{webarchive|url=https://web.archive.org/web/20101204012735/https://gigaom.com/collaboration/is-email-a-curse-or-a-boon/ |date=2010-12-04 }}" September 22, 2010. Retrieved October 11, 2010.</ref>

It has some key benefits to business and other organizations, including:
; Facilitating logistics
: Much of the business world relies on communications between people who are not physically in the same building, area, or even country; setting up and attending an in-person meeting, [[telephone call]], or [[conference call]] can be inconvenient, time-consuming, and costly. Email provides a method of exchanging information between two or more people with no set-up costs and that is generally far less expensive than a physical meeting or phone call.
; Helping with synchronization
: With [[Real-time computing|real time]] communication by meetings or phone calls, participants must work on the same schedule, and each participant must spend the same amount of time in the meeting or call. Email allows [[wikt:asynchrony|asynchrony]]: each participant may control their schedule independently. Batch processing of incoming emails can improve workflow compared to interrupting calls.
; Reducing cost
: Sending an email is much less expensive than sending postal mail, or [[long distance telephone call]]s, [[telex]] or [[telegrams]].
; Increasing speed
: Much faster than most of the alternatives.
; Creating a "written" record
: Unlike a telephone or in-person conversation, email by its nature creates a detailed written record of the communication, the identity of the sender(s) and recipient(s) and the date and time the message was sent. In the event of a [[contract]] or [[legal dispute]], saved emails can be used to prove that an individual was advised of certain issues, as each email has the date and time recorded on it.
; Possibility of auto-processing and improved distribution
: As well pre-processing of customer's orders or addressing the person in charge can be realized by automated procedures.

====Email marketing====
[[Email marketing]] via "[[opt-in email|opt-in]]" is often successfully used to send special sales offerings and new product information.<ref>{{cite journal | last1 = Martin | first1 = Brett A. S. | last2 = Van Durme | first2 = Joel | last3 = Raulas | first3 = Mika | last4 = Merisavo | first4 = Marko | year = 2003 | title = E-mail Marketing: Exploratory Insights from Finland | url = https://www.basmartin.com/wp-content/uploads/2010/08/Martin-et-al-2003.pdf | journal = Journal of Advertising Research | volume = 43 | issue = 3 | pages = 293–300 | doi = 10.1017/s0021849903030265 | url-status = live | archive-url = https://web.archive.org/web/20121021150626/https://www.basmartin.com/wp-content/uploads/2010/08/Martin-et-al-2003.pdf | archive-date = 2012-10-21 | issn=0021-8499}}</ref> Depending on the recipient's culture,<ref>{{cite web|url=https://www.computerworld.com/article/2467778/endpoint-security/spam-culture--part-1--china.html|title=Spam culture, part 1: China|first=Amir|last=Lev|url-status=live|archive-url=https://web.archive.org/web/20161110174233/https://www.computerworld.com/article/2467778/endpoint-security/spam-culture--part-1--china.html|archive-date=2016-11-10|date=2009-10-02}}</ref> email sent without permission—such as an "opt-in"—is likely to be viewed as unwelcome "[[email spam]]".

===Personal use===

====Personal computer====
Many users access their personal emails from friends and family members using a [[personal computer]] in their house or apartment.

====Mobile====
Email has become used on [[smartphone]]s and on all types of computers. Mobile "apps" for email increase accessibility to the medium for users who are out of their homes. While in the earliest years of email, users could only access email on desktop computers, in the 2010s, it is possible for users to check their email when they are away from home, whether they are across town or across the world. Alerts can also be sent to the smartphone or other devices to notify them immediately of new messages. This has given email the ability to be used for more frequent communication between users and allowed them to check their email and write messages throughout the day. {{As of|2011}}, there were approximately 1.4 billion email users worldwide and 50 billion non-spam emails that were sent daily.<ref name="Barnett 2011 p. 245">{{cite book |last1=Hansen |first1=Derek |last2=Smith |first2=Marc A. |last3=Heer |first3=Jeffrey |author-link3=Jeffrey Heer |chapter=E-Mail |chapter-url=https://books.google.com/books?id=fCfrCgAAQBAJ&pg=PA245 |editor1-last=Barnett |editor1-first=George A |title=Encyclopedia of social networks |publisher=Sage |location=Thousand Oaks, Calif |year=2011 |isbn=9781412994170 |page=245 |oclc=959670912}}</ref>

Individuals often check emails on smartphones for both personal and work-related messages. It was found that US adults check their email more than they browse the web or check their [[Facebook]] accounts, making email the most popular activity for users to do on their smartphones. 78% of the respondents in the study revealed that they check their email on their phone.<ref>{{cite web|url=https://marketingland.com/smartphone-activities-study-email-web-facebook-37954|title=Email Is Top Activity On Smartphones, Ahead Of Web Browsing & Facebook [Study]|date=28 March 2013|url-status=live|archive-url=https://web.archive.org/web/20140429184034/https://marketingland.com/smartphone-activities-study-email-web-facebook-37954|archive-date=29 April 2014}}</ref> It was also found that 30% of consumers use only their smartphone to check their email, and 91% were likely to check their email at least once per day on their smartphone. However, the percentage of consumers using email on a smartphone ranges and differs dramatically across different countries. For example, in comparison to 75% of those consumers in the US who used it, only 17% in India did.<ref>{{cite web|url=https://www.emailmonday.com/mobile-email-usage-statistics|title=The ultimate mobile email statistics overview|url-status=live|archive-url=https://web.archive.org/web/20140711160527/https://www.emailmonday.com/mobile-email-usage-statistics|archive-date=2014-07-11}}</ref>

====Declining use among young people====
{{as of|2010}}, the number of Americans visiting email web sites had fallen 6 percent after peaking in November 2009. For persons 12 to 17, the number was down 18 percent. Young people preferred [[instant messaging]], [[texting]] and [[social media]]. Technology writer Matt Richtel said in ''[[The New York Times]]'' that email was like the [[VCR]], [[vinyl record]]s and [[Still camera|film cameras]]—no longer cool and something older people do.<ref>{{cite news|url=https://www.nytimes.com/2010/12/21/technology/21email.html |url-access=subscription |title=E-Mail Gets an Instant Makeover|last=Richtel|first=Matt|work=[[The New York Times]]|date=2010-12-20|access-date=2018-04-04|archive-date=April 5, 2018|archive-url=https://web.archive.org/web/20180405033137/https://www.nytimes.com/2010/12/21/technology/21email.html|url-status=live}}</ref><ref>{{cite news|url=https://www.theatlantic.com/technology/archive/2010/12/why-are-young-people-abandoning-email/339329/ |url-access=subscription |title=Why Are Young People Abandoning Email?|last=Gustini|first=Ray|work=[[The Atlantic]]|date=2010-12-21|access-date=2018-04-04|archive-date=April 5, 2018|archive-url=https://web.archive.org/web/20180405024822/https://www.theatlantic.com/technology/archive/2010/12/why-are-young-people-abandoning-email/339329/|url-status=live}}</ref>

A 2015 survey of [[Android (operating system)|Android]] users showed that persons 13 to 24 used messaging [[Mobile app|app]]s 3.5 times as much as those over 45, and were far less likely to use email.<ref>{{cite news|url=https://techcrunch.com/2016/03/24/email-is-dying-among-mobiles-youngest-users/|title=Email is dying among mobile's youngest users|last=Perez|first=Sarah|work=TechCrunch |date=2016-03-24|access-date=2018-04-04|archive-date=April 5, 2018|archive-url=https://web.archive.org/web/20180405025214/https://techcrunch.com/2016/03/24/email-is-dying-among-mobiles-youngest-users/|url-status=live}}</ref>

==Issues==
{{More citations needed section|date=October 2016}}

===Attachment size limitation===
{{Main|Email attachment}}
Email messages may have one or more attachments, which are additional files that are appended to the email. Typical attachments include [[Microsoft Word]] documents, [[PDF]] documents, and scanned images of paper documents. In principle, there is no technical restriction on the size or number of attachments. However, in practice, email clients, [[server (computing)|server]]s, and Internet service providers implement various limitations on the size of files, or complete email – typically to 25MB or less.<ref>{{cite web |url-status=live |url=https://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html |title=Set Message Size Limits in Exchange 2010 and Exchange 2007 |archive-url=https://web.archive.org/web/20130212114611/https://exchangepedia.com/2007/09/exchange-server-2007-setting-message-size-limits.html |archive-date=2013-02-12 |first1=Bharat |last1=Suneja |website=Exchangepedia |date=September 10, 2007 }}</ref><ref>{{Cite web |last=Humphries |first=Matthew |date=Jun 29, 2009 |title=Google updates file size limits for Gmail and YouTube |url=https://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629 |url-status=dead |archive-url=https://web.archive.org/web/20111219141547/https://www.geek.com/articles/news/google-updates-file-size-limits-for-gmail-and-youtube-20090629 |archive-date=Dec 19, 2011 |website=Geek.com}}</ref><ref>[https://mail.google.com/support/bin/answer.py?answer=8770&topic=1517 "Maximum attachment size", Gmail Help.] {{Webarchive|url=https://web.archive.org/web/20111015232245/http://mail.google.com/support/bin/answer.py?answer=8770&topic=1517 |date=October 15, 2011 }}.</ref> Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ from what the user sees,<ref>{{cite magazine |last=Walther |first=Henrik |date=January 2009 |url=https://docs.microsoft.com/en-us/previous-versions/technet-magazine/dd314394(v=msdn.10) |title=Mysterious Attachment Size Increases, Replicating Public Folders, and More |magazine=[[TechNet Magazine]] |via=[[Microsoft Docs]] |access-date=2021-11-07}} [https://docs.microsoft.com/en-us/previous-versions/technet-magazine/cc135877(v=msdn.10) Exchange Queue & A].</ref> which can be confusing to senders when trying to assess whether they can safely send a file by email. Where larger files need to be shared, various [[file hosting service]]s are available and commonly used.<ref>[https://support.office.com/en-us/article/Send-large-files-to-other-people-7005da19-607a-47d5-b2c5-8f3982c6cc83 "Send large files to other people"] {{webarchive|url=https://web.archive.org/web/20160807110751/https://support.office.com/en-us/article/Send-large-files-to-other-people-7005da19-607a-47d5-b2c5-8f3982c6cc83 |date=2016-08-07 }}, Microsoft.com</ref><ref>[https://www.makeuseof.com/tag/8-ways-to-email-large-attachments/ "8 ways to email large attachments"] {{webarchive|url=https://web.archive.org/web/20160702171053/https://www.makeuseof.com/tag/8-ways-to-email-large-attachments/ |date=2016-07-02 }}, Chris Hoffman, December 21, 2012, makeuseof.com</ref>

===Information overload===
The ubiquity of email for knowledge workers and "white collar" employees has led to concerns that recipients face an "[[information overload]]" in dealing with increasing volumes of email.<ref>{{cite web|last=Radicati|first=Sara|title=Email Statistics Report, 2010|url=https://www.radicati.com/wp/wp-content/uploads/2010/04/Email-Statistics-Report-2010-2014-Executive-Summary2.pdf|url-status=live|archive-url=https://web.archive.org/web/20110901222039/https://www.radicati.com/wp/wp-content/uploads/2010/04/Email-Statistics-Report-2010-2014-Executive-Summary2.pdf|archive-date=2011-09-01}}</ref><ref>{{cite news|last=Gross|first=Doug|title=Happy Information Overload Day!|url=https://edition.cnn.com/2010/TECH/web/10/20/information.overload.day/index.html|work=CNN|date=October 20, 2010|url-status=live|archive-url=https://web.archive.org/web/20151023041454/https://edition.cnn.com/2010/TECH/web/10/20/information.overload.day/index.html|archive-date=October 23, 2015|access-date=March 24, 2019}}</ref> With the growth in mobile devices, by default employees may also receive work-related emails outside of their working day. This can lead to increased stress and decreased satisfaction with work. Some observers even argue it could have a significant negative economic effect,<ref>{{cite news|url=https://www.nytimes.com/2008/04/20/technology/20digi.html|title=Struggling to Evade the E-Mail Tsunami|date=2008-04-20|newspaper=The New York Times|first=Randall|last=Stross|access-date=May 1, 2010|url-status=live|archive-url=https://web.archive.org/web/20090417094850/https://www.nytimes.com/2008/04/20/technology/20digi.html|archive-date=April 17, 2009}}</ref> as efforts to read the many emails could reduce [[productivity]].

===Spam===
{{Main|Email spam}}
Email "spam" is unsolicited bulk email. The low cost of sending such email meant that, by 2003, up to 30% of total email traffic was spam,<ref>{{cite web|url=https://www.sitepronews.com/2015/05/04/seeing-spam-how-to-take-care-of-your-google-analytics-data/|title=Seeing Spam? How To Take Care of Your Google Analytics Data|website=sitepronews.com|access-date=5 September 2017|url-status=live|archive-url=https://web.archive.org/web/20171107005139/https://www.sitepronews.com/2015/05/04/seeing-spam-how-to-take-care-of-your-google-analytics-data/|archive-date=7 November 2017|date=2015-05-04}}</ref><ref>Rich Kawanagh. The top ten email spam list of 2005. ITVibe news, 2006, January 02, [https://itvibe.com/news/3837/ ITvibe.com] {{webarchive|url=https://web.archive.org/web/20080720071624/https://itvibe.com/news/3837/ |date=2008-07-20 }}</ref><ref>How Microsoft is losing the war on spam [https://dir.salon.com/story/tech/feature/2005/01/19/microsoft_spam/index.html Salon.com] {{webarchive|url=https://web.archive.org/web/20080629050141/https://dir.salon.com/story/tech/feature/2005/01/19/microsoft_spam/index.html |date=2008-06-29 }}</ref> and was threatening the usefulness of email as a practical tool. The US [[CAN-SPAM Act of 2003]] and similar laws elsewhere<ref>Spam Bill 2003 ([https://www.aph.gov.au/library/pubs/bd/2003-04/04bd045.pdf PDF] {{webarchive|url=https://web.archive.org/web/20060911062331/https://www.aph.gov.au/library/pubs/bd/2003-04/04bd045.pdf |date=2006-09-11 }})</ref> had some impact, and a number of effective [[anti-spam techniques (email)|anti-spam techniques]] now largely mitigate the impact of spam by filtering or rejecting it for most users,<ref>[https://www.wired.com/2015/07/google-says-ai-catches-99-9-percent-gmail-spam/ "Google Says Its AI Catches 99.9 Percent of Gmail Spam"] {{webarchive|url=https://web.archive.org/web/20160916054313/https://www.wired.com/2015/07/google-says-ai-catches-99-9-percent-gmail-spam/ |date=2016-09-16 }}, Cade Metz, July 09 2015, wired.com</ref> but the volume sent is still very high—and increasingly consists not of advertisements for products, but malicious content or links.<ref>[https://securelist.com/analysis/quarterly-spam-reports/74682/spam-and-phishing-in-q1-2016/ "Spam and phishing in Q1 2016"] {{webarchive|url=https://web.archive.org/web/20160809154745/https://securelist.com/analysis/quarterly-spam-reports/74682/spam-and-phishing-in-q1-2016/ |date=2016-08-09 }},  May 12, 2016, securelist.com</ref> In September 2017, for example, the proportion of spam to legitimate email rose to 59.56%.<ref>{{Cite web|url=https://usa.kaspersky.com/about/press-releases/2018_fifa-2018-and-bitcoin-among-2017-most-luring-topics|title=Kaspersky Lab Spam and Phishing report|date=May 26, 2021|access-date=July 17, 2018|archive-date=July 17, 2018|archive-url=https://web.archive.org/web/20180717183438/https://usa.kaspersky.com/about/press-releases/2018_fifa-2018-and-bitcoin-among-2017-most-luring-topics|url-status=live}}</ref> The percentage of spam email in 2021 is estimated to be 85%.<ref>{{Cite web|url=https://www.thexyz.com/blog/2021-email-usage-statistics/|title=2021 Email Usage Statistics|date=October 5, 2021|access-date=October 5, 2021|archive-date=October 5, 2021|archive-url=https://web.archive.org/web/20211005234608/https://www.thexyz.com/blog/2021-email-usage-statistics/|url-status=live}}</ref>{{better source needed|date=October 2021}}

===Malware===
Emails are a major vector for the distribution of [[malware]].<ref>{{Cite web |last=Waichulis |first=Arin |date=2024-04-05 |title=Security Bite: iCloud Mail, Gmail, others shockingly bad at detecting malware, study finds |url=https://9to5mac.com/2024/04/05/security-bite-icloud-mail-gmail-others-shockingly-bad-at-detecting-malware-study-finds/ |access-date=2024-05-27 |website=9to5Mac |language=en-US}}</ref> This is often achieved by attaching malicious programs to the message and persuading potential victims to open the file.<ref>{{Cite web |title=When are email attachments safe to open? |url=https://www.cloudflare.com/en-gb/learning/email-security/email-attachments/ |access-date=2024-05-27 |website=[[Cloudflare]]}}</ref> Types of malware distributed via email include [[computer worm]]s<ref>{{Cite web |last=Griffiths |first=James |date=2020-05-02 |title=How a badly-coded computer virus caused billions in damage {{!}} CNN Business |url=https://www.cnn.com/2020/05/01/tech/iloveyou-virus-computer-security-intl-hnk/index.html |access-date=2024-05-27 |website=[[CNN]] |language=en}}</ref> and [[ransomware]].<ref>{{Cite web |last=French |first=Laura |date=2024-05-14 |title=LockBit ransomware spread in millions of emails via Phorpiex botnet |url=https://www.scmagazine.com/news/lockbit-ransomware-spread-in-millions-of-emails-via-phorpiex-botnet |access-date=2024-05-27 |website=SC Media |language=en}}</ref>

===Email spoofing===
{{Main|Email spoofing}}
[[Email spoofing]] occurs when the email message header is designed to make the message appear to come from a known or trusted source. [[Email spam]] and [[phishing]] methods typically use spoofing to mislead the recipient about the true message origin. Email spoofing may be done as a prank, or as part of a criminal effort to defraud an individual or organization. An example of a potentially fraudulent email spoofing is if an individual creates an email that appears to be an invoice from a major company, and then sends it to one or more recipients. In some cases, these fraudulent emails incorporate the logo of the purported organization and even the email address may appear legitimate.

===Email bombing===
{{main|Email bomb}}

[[Email bomb]]ing is the intentional sending of large volumes of messages to a target address. The overloading of the target email address can render it unusable and can even cause the mail server to crash.

===Privacy concerns===
{{Main|Email privacy}}

Today it can be important to distinguish between the Internet and internal email systems. Internet email may travel and be stored on networks and computers without the sender's or the recipient's control. During the transit time it is possible that third parties read or even modify the content. Internal mail systems, in which the information never leaves the organizational network, may be more secure, although [[information technology]] personnel and others whose function may involve monitoring or managing may be accessing the email of other employees.

Email privacy, without some security precautions, can be compromised because:
* email messages are generally not encrypted.
* email messages have to go through intermediate computers before reaching their destination, meaning it is relatively easy for others to intercept and read messages.
* many Internet Service Providers (ISP) store copies of email messages on their mail servers before they are delivered. The backups of these can remain for up to several months on their server, despite deletion from the mailbox.
* the "Received:"-fields and other information in the email can often identify the sender, preventing anonymous communication.
* [[web bug]]s invisibly embedded in HTML content can alert the sender of any email whenever an email is rendered as HTML (some e-mail clients do this when the user reads, or re-reads the e-mail) and from which IP address. It can also reveal whether an email was read on a smartphone or a PC, or Apple Mac device via the [[user agent string]].

There are [[cryptography]] applications that can serve as a remedy to one or more of the above. For example, [[Virtual Private Network]]s or the [[Tor (network)|Tor network]] can be used to encrypt traffic from the user machine to a safer network while [[GNU Privacy Guard|GPG]], [[Pretty Good Privacy|PGP]], SMEmail,<ref>[https://www.arxiv.org/pdf/1002.3176 SMEmail – A New Protocol for the Secure E-mail in Mobile Environments], Proceedings of the Australian Telecommunications Networks and Applications Conference (ATNAC'08), pp. 39–44, Adelaide, Australia, Dec. 2008.</ref> or [[S/MIME]] can be used for [[end-to-end principle|end-to-end]] message encryption, and SMTP STARTTLS or SMTP over [[Transport Layer Security]]/Secure Sockets Layer can be used to encrypt communications for a single mail hop between the SMTP client and the SMTP server.

Additionally, many [[mail user agent]]s do not protect logins and passwords, making them easy to intercept by an attacker. Encrypted authentication schemes such as [[Simple Authentication and Security Layer|SASL]] prevent this. Finally, the attached files share many of the same hazards as those found in [[Peer-to-peer|peer-to-peer filesharing]]. Attached files may contain [[Trojan horse (computing)|trojans]] or [[Computer virus|viruses]].

===Legal contracts===
It is possible for an exchange of emails to form a binding contract, so users must be careful about what they send through email correspondence.<ref>{{Cite web|url=https://www.law.com/newyorklawjournal/almID/1202794877741/When-Email-Exchanges-Become-Binding-Contracts/|title=When Email Exchanges Become Binding Contracts|website=law.com|access-date=December 6, 2019|archive-date=June 19, 2019|archive-url=https://web.archive.org/web/20190619151252/https://www.law.com/newyorklawjournal/almID/1202794877741/When-Email-Exchanges-Become-Binding-Contracts/|url-status=live}}</ref><ref>{{cite journal |last1=Catarina |first1=Jessica |last2=Feitel |first2=Jesse |title=Inadvertent Contract Formation via Email under New York Law: An Update |journal=[[Syracuse Law Review]] |date=2019 |volume=69 |url=https://lawreview.syr.edu/wp-content/uploads/2020/01/K-Contracts-Article.pdf}}</ref> A signature block on an email may be interpreted as satisfying a signature requirement for a contract.<ref>{{cite news|last1=Corfield|first1=Gareth|title=UK court ruling says email signature blocks can sign binding contracts|url=https://www.theregister.co.uk/2019/09/30/email_signature_legally_binding_contract/|access-date=6 December 2019|work=[[The Register]]|archive-date=October 17, 2019|archive-url=https://web.archive.org/web/20191017063749/https://www.theregister.co.uk/2019/09/30/email_signature_legally_binding_contract/|url-status=live|date=30 September 2019}}</ref>

===Flaming===
[[Flaming (Internet)|Flaming]] occurs when a person sends a message (or many messages) with angry or antagonistic content. The term is derived from the use of the word ''incendiary'' to describe particularly heated email discussions. The ease and impersonality of email communications mean that the [[social norms]] that encourage civility in person or via telephone do not exist and civility may be forgotten.<ref>{{cite journal|author1=S. Kiesler |author2=D. Zubrow |author3=A.M. Moses |author4=V. Geller |title=Affect in computer-mediated communication: an experiment in synchronous terminal-to-terminal discussion|journal=Human-Computer Interaction|volume=1|pages=77–104|year=1985|doi=10.1207/s15327051hci0101_3}}</ref>

===Email bankruptcy===
{{main|Email bankruptcy}}
Also known as "email fatigue", email bankruptcy is when a user ignores a large number of email messages after falling behind in reading and answering them. The reason for falling behind is often due to information overload and a general sense there is so much information that it is not possible to read it all. As a solution, people occasionally send a "boilerplate" message explaining that their email inbox is full, and that they are in the process of clearing out all the messages. [[Harvard University]] law professor [[Lawrence Lessig]] is credited with coining this term, but he may only have popularized it.<ref>{{cite news|title=All We Are Saying.|url=https://www.nytimes.com/2007/12/23/weekinreview/23buzzwords.html|newspaper=The New York Times|date=December 23, 2007|access-date=2007-12-24|first=Grant|last=Barrett|url-status=live|archive-url=https://web.archive.org/web/20090417094849/https://www.nytimes.com/2007/12/23/weekinreview/23buzzwords.html|archive-date=April 17, 2009}}</ref>

===Internationalization===
Originally Internet email was completely ASCII text-based. MIME now allows body content text and some header content text in international character sets, but other headers and email addresses using UTF-8, while standardized<ref>{{Cite web|url=https://registry.in/Internationalized_Domain_Names_IDNs|title=Internationalized Domain Names (IDNs) {{!}} Registry.In|website=registry.in|access-date=2016-10-17|url-status=live|archive-url=https://web.archive.org/web/20160513012539/https://registry.in/Internationalized_Domain_Names_IDNs|archive-date=2016-05-13}}</ref> have yet to be widely adopted.<ref name=first>{{cite web|url=https://economictimes.indiatimes.com/tech/internet/datamail-worlds-first-free-linguistic-email-service-supports-eight-india-languages/articleshow/54923001.cms|title=DataMail: World's first free linguistic email service supports eight India languages|work=The Economic Times |url-status=live|archive-url=https://web.archive.org/web/20161022080739/https://economictimes.indiatimes.com/tech/internet/datamail-worlds-first-free-linguistic-email-service-supports-eight-india-languages/articleshow/54923001.cms|archive-date=2016-10-22}}</ref><ref>{{cite web|url=https://digitalconqurer.com/gadgets/made-india-datamail-empowers-russia-email-address-russian-language/|title=Made In India 'Datamail' Empowers Russia With Email Address In Russian Language - Digital Conqueror|date=7 December 2016|url-status=live|archive-url=https://web.archive.org/web/20170305005327/https://digitalconqurer.com/gadgets/made-india-datamail-empowers-russia-email-address-russian-language/|archive-date=5 March 2017}}</ref>
{{further|International email|Email address#Internationalization}}

===Tracking of sent mail===
The original SMTP mail service provides limited mechanisms for tracking a transmitted message, and none for verifying that it has been delivered or read. It requires that each mail server must either deliver it onward or return a failure notice (bounce message), but both software bugs and system failures can cause messages to be lost. To remedy this, the [[Internet Engineering Task Force|IETF]] introduced [[Delivery Status Notification]]s (delivery receipts) and [[Return receipt#Email|Message Disposition Notifications]] (return receipts); however, these are not universally deployed in production.{{Refn|group=nb|A complete Message Tracking mechanism was also defined, but it never gained traction; see RFCs 3885<ref>RFC 3885, ''SMTP Service Extension for Message Tracking''</ref> through 3888.<ref>RFC 3888, ''Message Tracking Model and Requirements''</ref>}}

Many ISPs now deliberately disable non-delivery reports (NDRs) and delivery receipts due to the activities of spammers:
* Delivery Reports can be used to verify whether an address exists and if so, this indicates to a spammer that it is available to be spammed.
* If the spammer uses a forged sender email address ([[email spoofing]]), then the innocent email address that was used can be flooded with NDRs from the many invalid email addresses the spammer may have attempted to mail. These NDRs then constitute spam ([[backscatter (email)|backscatter]]) from the ISP to the innocent user.

In the absence of standard methods, a range of system based around the use of [[web bug]]s have been developed. However, these are often seen as underhand or raising privacy concerns,<ref>{{cite news|url=https://query.nytimes.com/gst/fullpage.html?res=940CE0D9143AF931A15752C1A9669C8B63|title=Software That Tracks E-Mail Is Raising Privacy Concerns|author=Amy Harmon|newspaper=The New York Times|date=2000-11-22|access-date=2012-01-13}}</ref><ref>{{cite web |url=https://email.about.com/od/emailbehindthescenes/a/html_return_rcp.htm |title=About.com |publisher=Email.about.com |date=2013-12-19 |access-date=2014-01-09 |url-status=live |archive-url=https://web.archive.org/web/20160827203219/https://email.about.com/od/emailbehindthescenes/a/html_return_rcp.htm |archive-date=2016-08-27 }}</ref> and only work with email clients that support rendering of HTML. Many mail clients now default to not showing "web content".<ref>[https://www.slipstick.com/outlook/email/microsoft-outlook-web-bugs-blocked-html-images "Outlook: Web Bugs & Blocked HTML Images"] {{webarchive|url=https://web.archive.org/web/20150218074718/https://www.slipstick.com/outlook/email/microsoft-outlook-web-bugs-blocked-html-images/ |date=2015-02-18 }}, slipstick.com</ref> [[Webmail]] providers can also disrupt web bugs by pre-caching images.<ref>[https://arstechnica.com/information-technology/2013/12/gmail-blows-up-e-mail-marketing-by-caching-all-images-on-google-servers/ "Gmail blows up e-mail marketing..."] {{webarchive|url=https://web.archive.org/web/20170607090403/https://arstechnica.com/information-technology/2013/12/gmail-blows-up-e-mail-marketing-by-caching-all-images-on-google-servers/ |date=2017-06-07 }}, Ron Amadeo, Dec 13 2013, Ars Technica</ref>

==See also==
{{Div col|colwidth=18em}}
* [[Anonymous remailer]]
* [[Anti-spam techniques]]
* [[Biff (Unix)|biff]]
* [[Bounce message]]
* [[Comparison of email clients]]
* [[Dark Mail Alliance]]
* [[Disposable email address]]
* [[E-card]]
* [[Electronic mailing list]]
* [[Email art]]
* [[Email authentication]]
* [[Email digest]]
* [[Email encryption]]
* [[Email hosting service]]
* [[Email hub]]
* [[Email storm]]
* [[Email tracking]]
* [[HTML email]]
* [[Information overload]]
* [[Internet fax]]
* [[List of email subject abbreviations]]
* [[MCI Mail]]
* [[Netiquette]]
* [[Posting style]]
* [[Privacy-enhanced Electronic Mail]]
* [[Push email]]
* [[RSS]]
* [[Telegraphy]]
* [[Unicode and email]]
* [[Usenet quoting]]
* [[Webmail]], [[Comparison of webmail providers]]
* [[X-Originating-IP]]
* [[X.400]]
{{Div col end}}

== Notes ==
{{Reflist|group=nb}}

==References==
{{Reflist}}

==Further reading==
{{refbegin}}
* Cemil Betanov, ''Introduction to X.400'', Artech House, {{ISBN|0-89006-597-7}}.
* Marsha Egan, "[https://www.inboxdetox.com Inbox Detox and The Habit of Email Excellence] {{Webarchive|url=https://web.archive.org/web/20160520091235/http://www.inboxdetox.com/ |date=May 20, 2016 }}", Acanthus Publishing {{ISBN|978-0-9815589-8-1}}
* Lawrence Hughes, ''Internet e-mail Protocols, Standards and Implementation'', Artech House Publishers, {{ISBN|0-89006-939-5}}.
* Kevin Johnson, ''Internet Email Protocols: A Developer's Guide'', Addison-Wesley Professional, {{ISBN|0-201-43288-9}}.
* Pete Loshin, ''Essential Email Standards: RFCs and Protocols Made Practical'', John Wiley & Sons, {{ISBN|0-471-34597-0}}.
* {{Cite journal|last=Partridge|first=Craig|title=The Technical Development of Internet Email|journal=IEEE Annals of the History of Computing|volume=30|issue=2|date=April–June 2008|url=https://www.ir.bbn.com/~craig/papers/email.pdf|issn=1934-1547|doi=10.1109/mahc.2008.32|pages=3–29|bibcode=2008IAHC...30b...3P |s2cid=206442868|url-status=dead|archive-url=https://web.archive.org/web/20160602161643/https://www.ir.bbn.com/~craig/papers/email.pdf|archive-date=2016-06-02}}
* Sara Radicati, ''Electronic Mail: An Introduction to the X.400 Message Handling Standards'', Mcgraw-Hill, {{ISBN|0-07-051104-7}}.
* John Rhoton, ''Programmer's Guide to Internet Mail: SMTP, POP, IMAP, and LDAP'', Elsevier, {{ISBN|1-55558-212-5}}.
* John Rhoton, ''X.400 and SMTP: Battle of the E-mail Protocols'', Elsevier, {{ISBN|1-55558-165-X}}.
* David Wood, ''Programming Internet Mail'', O'Reilly, {{ISBN|1-56592-479-7}}.
{{refend}}

==External links==
{{Wiktionary|email|outbox}}
{{Wikiversity|Email Checklist}}
* [https://www.iana.org/assignments/message-headers/perm-headers.html IANA's list of standard header fields]
* [https://web.archive.org/web/20121118062543/https://emailhistory.org/ The History of Email] is Dave Crocker's attempt at capturing the sequence of 'significant' occurrences in the evolution of email; a collaborative effort that also cites this page.
* [https://www.multicians.org/thvv/mail-history.html The History of Electronic Mail] is a personal memoir by the implementer of an early email system
* [https://www.circleid.com/posts/20140903_a_look_at_the_origins_of_network_email/ A Look at the Origins of Network Email] is a short, yet vivid recap of the key historical facts
* [https://www.fbi.gov/news/stories/2015/august/business-e-mail-compromise Business E-Mail Compromise - An Emerging Global Threat], [[FBI]]
* [https://explained-from-first-principles.com/email/ Explained from first principles], a 2021 article attempting to summarize more than 100 RFCs
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{{E-mail clients}}

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[[Category:Email| ]]
[[Category:Internet terminology]]
[[Category:Electronic documents|Mail]]
[[Category:History of the Internet]]
[[Category:Computer-related introductions in 1971]]
[[Category:Fediverse]]