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EMAIL

From Wikipedia, the free encyclopedia



Jump to navigation Jump to search
Mail sent using electronic means
"Reply all" redirects here. For the podcast, see Reply All (podcast).
For the former company, see Email Limited.




This screenshot shows the "Inbox" page of an email client; users can see new
emails and take actions, such as reading, deleting, saving, or responding to
these messages.

The at sign, a part of every SMTP email address[1]

When a "robot" on Wikipedia makes changes to image files, the uploader receives
an email about the changes made.

Electronic mail (email or e-mail) is a method of exchanging messages ("mail")
between people using electronic devices. Email was thus conceived as the
electronic (digital) version of, or counterpart to, mail, at a time when "mail"
meant only physical mail (hence e- + mail). Email later became a ubiquitous
(very widely used) communication medium, to the point that in current use, an
email 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 operates across computer networks, primarily the Internet, and also local
area networks. Today's email systems are based on a store-and-forward model.
Email servers accept, forward, deliver, and store messages. Neither the users
nor their computers are required to be online simultaneously; they need to
connect, typically to a mail server or a webmail interface to send or receive
messages or download it.

Originally an ASCII text-only communications medium, Internet email was extended
by Multipurpose Internet Mail Extensions (MIME) to carry text in other character
sets and multimedia content attachments. International email, with
internationalized email addresses using UTF-8, is standardized but not widely
adopted.[2]


CONTENTS

 * 1 Terminology
 * 2 History
 * 3 Operation
 * 4 Message format
   * 4.1 Message header
     * 4.1.1 Header fields
   * 4.2 Message body
     * 4.2.1 Content encoding
     * 4.2.2 Plain text and HTML
 * 5 Servers and client applications
   * 5.1 Filename extensions
   * 5.2 URI scheme mailto
 * 6 Types
   * 6.1 Web-based email
   * 6.2 POP3 email servers
   * 6.3 IMAP email servers
   * 6.4 MAPI email servers
 * 7 Uses
   * 7.1 Business and organizational use
     * 7.1.1 Email marketing
   * 7.2 Personal use
     * 7.2.1 Personal computer
     * 7.2.2 Mobile
     * 7.2.3 Declining use among young people
 * 8 Issues
   * 8.1 Attachment size limitation
   * 8.2 Information overload
   * 8.3 Spam
   * 8.4 Malware
   * 8.5 Email spoofing
   * 8.6 Email bombing
   * 8.7 Privacy concerns
   * 8.8 Legal contracts
   * 8.9 Flaming
   * 8.10 Email bankruptcy
   * 8.11 Internationalization
   * 8.12 Tracking of sent mail
 * 9 See also
 * 10 Notes
 * 11 References
 * 12 Further reading
 * 13 External links


TERMINOLOGY

Further information: 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:[3][4]

 * email is now the common form, and recommended by style guides.[5][6] It is
   the form required by IETF Requests for Comments (RFC) and working groups.[7]
   This spelling also appears in most
   dictionaries.[8][9][10][11][12][13][14][15]
 * e-mail is the form favored in edited published American English and British
   English writing as reflected in the Corpus of Contemporary American English
   data,[16] but is falling out of favor in some style guides.[6][17]
 * E-mail is sometimes used.[18] The original usage in June 1979 occurred in the
   journal 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.[3][4]
 * Email is also used.
 * EMAIL was used by CompuServe starting in April 1981, which popularized the
   term.[19][20]
 * 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.[21]

An Internet email consists of an envelope and content;[22] the content consists
of a header and a body.[23]


HISTORY



Main article: 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
MIT's CTSS project in 1965.[24] Most developers of early mainframes and
minicomputers developed similar, but generally incompatible, mail applications.
In 1971 the first ARPANET network mail was sent, introducing the now-familiar
address syntax with the '@' symbol designating the user's system address.[25]
Over a series of RFCs, conventions were refined for sending mail messages over
the File Transfer Protocol.

Proprietary electronic mail systems soon began to emerge. IBM, CompuServe and
Xerox used in-house mail systems in the 1970s; CompuServe sold a commercial
intraoffice mail product from 1978 and IBM and Xerox from 1981.[nb
1][26][27][28] DEC's ALL-IN-1 and 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.[29][30]

The Simple Mail Transfer Protocol (SMTP) protocol was implemented on the ARPANET
in 1983. 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,[31][32] a combination of factors made the current Internet suite of SMTP,
POP3 and IMAP email protocols the standard.[nb 2]


OPERATION

The following is a typical sequence of events that takes place when sender Alice
transmits a message using a mail user agent (MUA) addressed to the email address
of the recipient.[33]


Email operation
 1. 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.
 2. The MSA determines the destination address provided in the SMTP protocol
    (not from the message header) — in this case, bob@b.org — which is a fully
    qualified domain address (FQDA). The part before the @ sign is the local
    part of the address, often the username of the recipient, and the part after
    the @ sign is a domain name. The MSA resolves a domain name to determine the
    fully qualified domain name of the mail server in the Domain Name System
    (DNS).
 3. The DNS server for the domain b.org (ns.b.org) responds with any MX records
    listing the mail exchange servers for that domain, in this case mx.b.org, a
    message transfer agent (MTA) server run by the recipient's ISP.[34]
 4. 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).
 5. The MDA delivers it to the mailbox of user bob.
 6. Bob's MUA picks up the message using either the Post Office Protocol (POP3)
    or the Internet Message Access Protocol (IMAP).

In addition to this example, alternatives and complications exist in the email
system:

 * Alice or Bob may use a client connected to a corporate email system, such as
   IBM Lotus Notes or Microsoft Exchange. These systems often have their own
   internal email format and their clients typically communicate with the email
   server using a vendor-specific, proprietary protocol. The server sends or
   receives email via the Internet through the product's Internet mail gateway
   which also does any necessary reformatting. If Alice and Bob work for the
   same company, the entire transaction may happen completely within a single
   corporate email system.
 * Alice may not have an MUA on her computer but instead may connect to a
   webmail service.
 * Alice's computer may run its own MTA, so avoiding the transfer at step 1.
 * Bob may pick up his email in many ways, for example logging into mx.b.org and
   reading it directly, or by using a webmail service.
 * Domains usually have several mail exchange servers so that they can continue
   to accept mail even if the primary is not available.

Many MTAs used to accept messages for any recipient on the Internet and do their
best to deliver them. Such MTAs are called open mail relays. This was very
important in the early days of the Internet when network connections were
unreliable.[35][36] However, this mechanism proved to be exploitable by
originators of unsolicited bulk email and as a consequence open mail relays have
become rare,[37] and many MTAs do not accept messages from open mail relays.


MESSAGE FORMAT

The basic Internet message format used for email[38] 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 or MIME. The extensions in International email 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.[39]

Internet email messages consist of two sections, "header" and "body". These are
known as "content".[40][41] The header is structured into 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

RFC 5322 specifies the syntax of the email header. Each email message has a
header (the "header section" of the message, according to the specification),
comprising a number of 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 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, 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.[42] 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.[43] In some examples,
the IETF EAI working group defines some standards track extensions,[44][45]
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.[46]

The message header must include at least the following fields:[47][48]

 * 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; it provides for permanent and provisional field names, including also
fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs.
Common header fields for email include:[49]

 * 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; 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 respects a fine-grained
   automatic response suppression mechanism, the X-Auto-Response-Suppress
   field.[50]
 * 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.
 * 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.
 * 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,
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:[51]

 * 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.[52]

 * Authentication-Results: after a server verifies authentication, it can save
   the results in this field for consumption by downstream agents.[53]
 * Received-SPF: stores results of SPF checks in more detail than
   Authentication-Results.[54]
 * DKIM-Signature: stores results of DomainKeys Identified Mail (DKIM)
   decryption to verify the message was not changed after it was sent.[55]
 * Auto-Submitted: is used to mark automatic-generated messages.[56]
 * VBR-Info: claims VBR whitelisting[57]


MESSAGE BODY

CONTENT ENCODING

Internet email was designed for 7-bit ASCII.[58] 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 agents may not support them. In some countries, e-mail software
violates RFC 5322 by sending raw[nb 3] 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 sets, Unicode is growing in popularity.[59]

PLAIN TEXT AND HTML

Most modern graphic email clients allow the use of either plain text or 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 quotes, wrap naturally on any display, use
emphasis such as underlines and italics, and change font styles. Disadvantages
include the increased size of the email, privacy concerns about web bugs, abuse
of HTML email as a vector for phishing attacks and the spread of malicious
software.[60] 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 lists recommend all posts be made in plain text, with 72
or 80 characters per line for all the above reasons,[61][62] and because they
have a significant number of readers using text-based email clients such as
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 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.[63]


SERVERS AND CLIENT APPLICATIONS


The interface of an email client, Thunderbird.

Messages are exchanged between hosts using the Simple Mail Transfer Protocol
with software programs called mail transfer agents (MTAs); and delivered to a
mail store by programs called mail delivery agents (MDAs, also sometimes called
local delivery agents, LDAs). Accepting a message obliges an MTA to deliver
it,[64] 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
POP or IMAP, or, as is more likely in a large corporate environment, with a
proprietary protocol specific to Novell Groupwise, Lotus Notes or Microsoft
Exchange Servers. Programs used by users for retrieving, reading, and managing
email are called mail user agents (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.[65]

Mail can be stored on the client, on the 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 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.[66] 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 extensions:

eml 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. emlx
Used by Apple Mail. msg Used by Microsoft Office Outlook and OfficeLogic
Groupware. mbx 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 article: mailto

The URI scheme, as registered with the IANA, defines the mailto: 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.[67][68] Many clients also support query string parameters for the other
email fields, such as its subject line or carbon copy recipients.[69]


TYPES


WEB-BASED EMAIL

Main article: 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 can't be
read without a current Internet connection.


POP3 EMAIL SERVERS

The Post Office Protocol 3 (POP3) is a mail access protocol used by a client
application to read messages from the mail server. Received messages are often
deleted from the server. POP supports simple download-and-delete requirements
for access to remote mailboxes (termed maildrop in the POP RFC's).[70] POP3
allows you to download email messages on your local computer and read them even
when you are offline.[71][72]


IMAP EMAIL SERVERS

The Internet Message Access Protocol (IMAP) provides features to manage a
mailbox from multiple devices. Small portable devices like smartphones are
increasingly used to check email while traveling and to make brief replies,
larger devices with better keyboard access being used to reply at greater
length. IMAP shows the headers of messages, the sender and the subject and the
device needs to request to download specific messages. Usually, the mail is left
in folders in the mail server.


MAPI EMAIL SERVERS

Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook
to communicate to Microsoft Exchange Server - and to a range of other email
server products such as Axigen Mail Server, Kerio Connect, Scalix, Zimbra, HP
OpenMail, IBM Lotus Notes, Zarafa, and Bynari where vendors have added MAPI
support to allow their products to be accessed directly via Outlook.


USES

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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.[73]

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 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
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 calls, 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 and/or addressing the person in charge can
be realized by automated procedures.

EMAIL MARKETING

Email marketing via "opt-in" is often successfully used to send special sales
offerings and new product information.[74] Depending on the recipient's
culture,[75] 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 smartphones 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[update], there were
approximately 1.4 billion email users worldwide and 50 billion non-spam emails
that were sent daily.[68]

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.[76] 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.[77]

DECLINING USE AMONG YOUNG PEOPLE

As of 2010[update], 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 records and film cameras—no longer cool and something older
people do.[78][79]

A 2015 survey of Android users showed that persons 13 to 24 used messaging apps
3.5 times as much as those over 45, and were far less likely to use email.[80]


ISSUES

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ATTACHMENT SIZE LIMITATION

Main article: 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, servers, and Internet service providers implement
various limitations on the size of files, or complete email – typically to 25MB
or less.[81][82][83] Furthermore, due to technical reasons, attachment sizes as
seen by these transport systems can differ from what the user sees,[84] 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
services are available and commonly used.[85][86]


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.[87][88] 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,[89] as efforts to read the many emails could reduce productivity.


SPAM

Main article: 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,[90][91][92] and was
threatening the usefulness of email as a practical tool. The US CAN-SPAM Act of
2003 and similar laws elsewhere[93] had some impact, and a number of effective
anti-spam techniques now largely mitigate the impact of spam by filtering or
rejecting it for most users,[94] but the volume sent is still very high—and
increasingly consists not of advertisements for products, but malicious content
or links.[95] In September 2017, for example, the proportion of spam to
legitimate email rose to 59.56%.[96] The percentage of spam email in 2021 is
estimated to be 85%.[97][better source needed]


MALWARE

A range of malicious email types exist. These range from various types of email
scams, including "social engineering" scams such as advance-fee scam "Nigerian
letters", to phishing, email bombardment and email worms.


EMAIL SPOOFING

Main article: 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 article: Email bomb

Email bombing 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 article: 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 bugs 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 Networks or the Tor network can be used
to encrypt traffic from the user machine to a safer network while GPG, PGP,
SMEmail,[98] or S/MIME can be used for 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 agents do not protect logins and passwords, making
them easy to intercept by an attacker. Encrypted authentication schemes such as
SASL prevent this. Finally, the attached files share many of the same hazards as
those found in peer-to-peer filesharing. Attached files may contain trojans or
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.[99][100] A
signature block on an email may be interpreted as satisfying a signature
requirement for a contract.[101]


FLAMING

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.[102]


EMAIL BANKRUPTCY

Main article: 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.[103]


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[104] have yet
to be widely adopted.[2][105]

Further information: International email and 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 IETF introduced Delivery Status
Notifications (delivery receipts) and Message Disposition Notifications (return
receipts); however, these are not universally deployed in production.[nb 4]

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 from the ISP to the innocent user.

In the absence of standard methods, a range of system based around the use of
web bugs have been developed. However, these are often seen as underhand or
raising privacy concerns,[108][109] and only work with email clients that
support rendering of HTML. Many mail clients now default to not showing "web
content".[110] Webmail providers can also disrupt web bugs by pre-caching
images.[111]


SEE ALSO

 * Anonymous remailer
 * Anti-spam techniques
 * 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 storm
 * Email tracking
 * HTML email
 * Information overload
 * Internet fax
 * Internet mail standards
 * 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
 * Yerkish


NOTES

 1. ^ IBM's system was available on request to customers prior to formal
    release.
 2. ^ See Protocol Wars.
 3. ^ Not using Internationalized Email or MIME
 4. ^ A complete Message Tracking mechanism was also defined, but it never
    gained traction; see RFCs 3885[106] through 3888.[107]


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FURTHER READING

 * Cemil Betanov, Introduction to X.400, Artech House, ISBN 0-89006-597-7.
 * Marsha Egan, "Inbox Detox and The Habit of Email Excellence Archived May 20,
   2016, at the Wayback Machine", 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.
 * Partridge, Craig (April–June 2008). "The Technical Development of Internet
   Email" (PDF). IEEE Annals of the History of Computing. 30 (2): 3–29.
   doi:10.1109/mahc.2008.32. ISSN 1934-1547. S2CID 206442868. Archived from the
   original (PDF) on June 2, 2016.
 * 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.


EXTERNAL LINKS

Look up email or outbox in Wiktionary, the free dictionary.
Wikiversity has learning resources about Email Checklist
 * IANA's list of standard header fields
 * 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.
 * The History of Electronic Mail is a personal memoir by the implementer of an
   early email system
 * A Look at the Origins of Network Email is a short, yet vivid recap of the key
   historical facts
 * Business E-Mail Compromise - An Emerging Global Threat, FBI
 * Explained from first principles, a 2021 article attempting to summarize more
   than 100 RFCs



show
 * v
 * t
 * e

Computer-mediated communication
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