Email

Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Invented by Ray Tomlinson, email first entered limited use in the 1960s and by the mid-1970s had taken the form now recognized as email. Email operates across computer networks, which today is primarily the Internet. Some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging. 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 only briefly, typically to a mail server or a webmail interface for as long as it takes to send or receive messages.

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, has been standardized, but as of 2017 it has not been widely adopted.[2]

The history of modern Internet email services reaches back to the early ARPANET, with standards for encoding email messages published as early as 1973 (RFC 561). An email message sent in the early 1970s looks very similar to a basic email sent today. Email had an important role in creating the Internet,[3] and the conversion from ARPANET to the Internet in the early 1980s produced the core of the current services.

Evolution 36 mail
This screenshot shows the "Inbox" page of an email client, where users can see new emails and take actions, such as reading, deleting, saving, or responding to these messages.
(at)
The at sign, a part of every SMTP email address[1]

Terminology

Historically, the term electronic mail was used generically for any electronic document transmission. For example, several writers in the early 1970s used the term to describe fax document transmission.[4][5] As a result, it is difficult to find the first citation for the use of the term with the more specific meaning it has today.

Electronic mail has been most commonly called email or e-mail since around 1993,[6] but variations of the spelling have been used:

An Internet e-mail consists of an envelope and content;[24] the content in turn consists of a header and a body.[25]

Origin

Computer-based mail and messaging became possible with the advent of time-sharing computers in the early 1960s, and informal methods of using shared files to pass messages were soon expanded into the first mail systems. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. Over time, a complex web of gateways and routing systems linked many of them. Many US universities were part of the ARPANET (created in the late 1960s), which aimed at software portability between its systems. That portability helped make the Simple Mail Transfer Protocol (SMTP) increasingly influential.

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,[26][27] a combination of factors made the current Internet suite of SMTP, POP3 and IMAP email protocols the standard.

Operation

The diagram to the right shows 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.[28]

Email
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.[29]
  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 a 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.[30][31] However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as a consequence open mail relays have become rare,[32] and many MTAs do not accept messages from open mail relays.

Message format

The basic Internet email message format is now defined by RFC 5322, with encoding of non-ASCII data and multimedia content attachments being defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME. RFC 5322 replaced the earlier RFC 2822 in 2008, and in turn RFC 2822 in 2001 replaced RFC 822 – which had been the standard for Internet email for nearly 20 years. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET.[33]

Internet email messages consist of two major sections, the message header and the message body, collectively known as content.[34][35] 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

Each message has exactly one header (the "header section" of the message, according to the specification), which is structured into fields ("header fields"). Each field has a name ("field name" or "header field name") and a value ("field body" or "header field body"). RFC 5322 specifies the precise syntax.

Informally, each line of text in the header that begins with a non-whitespace printable character begins a separate field. The field name starts in the first character of the line and ends before the separator character ":". The separator is then followed by the field value (the "body" of the field). The value is continued onto subsequent lines if those lines have a 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 technical limit is 998 characters.[36] Header fields defined by RFC 5322 can only contain US-ASCII characters; for encoding characters in other sets, a syntax specified in RFC 2047 can be used.[37] Recently the IETF EAI working group has defined some standards track extensions,[38][39] replacing previous experimental extensions, to allow UTF-8 encoded Unicode characters to 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 governments.[40]

The message header must include at least the following fields:[41][42]

  • From: The email address, and optionally the name of the author(s). In many email clients not changeable except through changing account settings.
  • Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically when sending. The recipient's client may then 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:[43]

  • 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 will 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 that automated "vacation" or "out of office" responses should not be returned for this mail, e.g. to prevent vacation notices from being 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 once was. Microsoft Exchange respects a fine-grained automatic response suppression mechanism, the X-Auto-Response-Suppress field.[44]
  • Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To: (see below).
  • In-Reply-To: Message-ID of the message that this is a reply to. Used to link related messages together. This field only applies for reply messages.
  • References: Message-ID of the message that this is a reply to, and the message-id of the message the previous reply was a reply to, etc.
  • Reply-To: Address that should be used to reply to the message.
  • Sender: Address of the actual 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.

Note that the To: field is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied separately to the transport protocol, SMTP, which may or may not originally have been extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter which is delivered according to the address on the outer envelope. In the same way, the "From:" field does not have to be the real sender of the email message. Some mail servers apply email authentication systems to messages being relayed. Data pertaining to server's activity is also part of the header, as defined below.

SMTP defines the trace information of a message, which is also saved in the header using the following two fields:[45]

  • Received: when an SMTP server accepts a message it inserts this trace record at the top of the header (last to first).
  • Return-Path: when the delivery SMTP server makes the final delivery of a message, it inserts this field at the top of the header.

Other fields that are added on top of the header by the receiving server may be called trace fields, in a broader sense.[46]

  • Authentication-Results: when a server carries out authentication checks, it can save the results in this field for consumption by downstream agents.[47]
  • Received-SPF: stores results of SPF checks in more detail than Authentication-Results.[48]
  • Auto-Submitted: is used to mark automatically generated messages.[49]
  • VBR-Info: claims VBR whitelisting[50]

Message body

Content encoding

Internet email was originally designed for 7-bit ASCII.[51] 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 still do not support them fully. In some countries, several encoding schemes coexist; as the result, by default, the message in a non-Latin alphabet language appears in non-readable form (the only exception is coincidence, when the sender and receiver use the same encoding scheme). Therefore, for international character sets, Unicode is growing in popularity.

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 automatically generated plain text copy as well, for compatibility reasons. 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.[52]

Some web-based mailing lists recommend that all posts be made in plain-text, with 72 or 80 characters per line for all the above reasons,[53][54] but also because they have a significant number of readers using text-based email clients such as Mutt. Some Microsoft email clients 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.[55]

Servers and client applications

Mozilla Thunderbird 3.1
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,[56] 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).

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.[57] Such webmail interfaces allow users to access their mail with any standard web browser, from any computer, rather than relying on an 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

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.[58][59]

Types

Web-based email

Many email providers have a web-based email client (e.g. AOL Mail, Gmail, Outlook.com, Hotmail and Yahoo! Mail). This allows users to log into the email account by using any compatible web browser to send and receive their email. Mail is typically not downloaded to the 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).[60]

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 travelling, 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 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

Business and organizational use

Email has been widely accepted by business, 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.[61]

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 synchronisation
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.
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.

Email marketing

Email marketing via "opt-in" is often successfully used to send special sales offerings and new product information.[62] Depending on the recipient's culture,[63] 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 email 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 home. 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 device 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.[59]

Individuals often check email 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.[64] 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 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.[65]

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 records and film cameras—no longer cool and something older people do.[66][67]

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.[68]

Issues

Attachment size limitation

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, but 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.[69][70][71] Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ to what the user sees,[72] 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, file hosting services of various sorts are available; and generally suggested.[73][74]

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.[75][76] 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, decreased satisfaction with work, and some observers even argue it could have a significant negative economic effect,[77] as efforts to read the many emails could reduce productivity.

Spam

Email "spam" is the term used to describe unsolicited bulk email. The low cost of sending such email meant that by 2003 up to 30% of total email traffic was already spam,[78][79][80] and was threatening the usefulness of email as a practical tool. The US CAN-SPAM Act of 2003 and similar laws elsewhere[81] 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,[82] but the volume sent is still very high—and increasingly consists not of advertisements for products, but malicious content or links.[83] In September 2017, for example, the proportion of spam to legitimate email rose to 59.56%.[84]

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

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 which 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

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

Today it can be important to distinguish between 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 email content can alert the sender of any email whenever an email is read, or re-read, 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 anonymity network can be used to encrypt traffic from the user machine to a safer network while GPG, PGP, SMEmail,[85] 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, attached files share many of the same hazards as those found in peer-to-peer filesharing. Attached files may contain trojans or viruses.

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.[86]

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.[87]

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[88] have yet to be widely adopted.[2][89]

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. (A complete Message Tracking mechanism was also defined, but it never gained traction; see RFCs 3885[90] through 3888.[91])

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,[92][93][94] and only work with email clients that support rendering of HTML. Many mail clients now default to not showing "web content".[95] Webmail providers can also disrupt web bugs by pre-caching images.[96]

See also

References

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  35. ^ D. Crocker (July 2009). "Message Data". Internet Mail Architecture. sec. 4.1.. doi:10.17487/RFC5598. RFC 5598. https://tools.ietf.org/html/rfc5598#section-4.1. "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."
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  47. ^ This extensible field is defined by RFC 7001, that also defines an IANA registry of Email Authentication Parameters.
  48. ^ RFC 7208.
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  50. ^ RFC 5518.
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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", 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. Archived from the original (PDF) on 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.

External links

Climatic Research Unit email controversy

The Climatic Research Unit email controversy (also known as "Climategate") began in November 2009 with the hacking of a server at the Climatic Research Unit (CRU) at the University of East Anglia (UEA) by an external attacker, copying thousands of emails and computer files, the Climatic Research Unit documents, to various internet locations several weeks before the Copenhagen Summit on climate change.

The story was first broken by climate change denialists, with columnist James Delingpole popularising the term "Climategate" to describe the controversy. Several climate-change "skeptics" argued that the emails showed that global warming was a scientific conspiracy and that scientists manipulated climate data and attempted to suppress critics. The CRU rejected this, saying that the emails had been taken out of context and merely reflected an honest exchange of ideas.The mainstream media picked up the story, as negotiations over climate change mitigation began in Copenhagen on 7 December 2009. Because of the timing, scientists, policy makers and public-relations experts said that the release of emails was a smear campaign intended to undermine the climate conference. In response to the controversy, the American Association for the Advancement of Science (AAAS), the American Meteorological Society (AMS) and the Union of Concerned Scientists (UCS) released statements supporting the scientific consensus that the Earth's mean surface temperature had been rising for decades, with the AAAS concluding: "based on multiple lines of scientific evidence that global climate change caused by human activities is now underway... it is a growing threat to society".Eight committees investigated the allegations and published reports, finding no evidence of fraud or scientific misconduct. The scientific consensus that global warming is occurring as a result of human activity remained unchanged throughout the investigations.

Electronic mailing list

An electronic mailing list or email list is a special use of email that allows for widespread distribution of information to many Internet users. It is similar to a traditional mailing list – a list of names and addresses – as might be kept by an organization for sending publications to its members or customers, but typically refers to four things:

a list of email addresses,

the people ("subscribers") receiving mail at those addresses, thus defining a community gathered around a topic of interest.

the publications (email messages) sent to those addresses, and

a reflector, which is a single email address that, when designated as the recipient of a message, will send a copy of that message to all of the subscribers.

Email address

An email address identifies an email box to which email messages are delivered. A wide variety of formats were used in early email systems, but only a single format is used today, following the standards developed for Internet mail systems since the 1980s. This article uses the term email address to refer to the addr-spec defined in RFC 5322, not to the address that is commonly used; the difference is that an address may contain a display name, a comment, or both.

An email address such as John.Smith@example.com is made up of a local-part, an @ symbol, then a case-insensitive domain. Although the standard requires

the local part to be case-sensitive, it also urges that receiving hosts deliver messages in a case-independent fashion,

e.g., that the mail system at example.com treat John.Smith as equivalent to john.smith; some mail systems even treat them as equivalent to johnsmith. Mail systems often limit their users' choice of name to a subset of the technically valid characters, and in some cases also limit which addresses it is possible to send mail to.

With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses.

Email client

An email client, email reader or more formally mail user agent (MUA) is a computer program used to access and manage a user's email.

A web application which provides message management, composition, and reception functions may act as an email client, and "email client" may also refer to a piece of computer hardware or software whose primary or most visible role is to work as an email client.

Email marketing

Email marketing is the act of sending a commercial message, typically to a group of people, using email. In its broadest sense, every email sent to a potential or current customer could be considered email marketing. It usually involves using email to send advertisements, request business, or solicit sales or donations, and is meant to build loyalty, trust, or brand awareness. Marketing emails can be sent to a purchased lead list or a current customer database. The term usually refers to sending email messages with the purpose of enhancing a merchant's relationship with current or previous customers, encouraging customer loyalty and repeat business, acquiring new customers or convincing current customers to purchase something immediately, and sharing third-party ads.

Email spam

Email spam, also known as junk email, is unsolicited messages sent in bulk by email (spamming).

The name comes from Spam luncheon meat by way of a Monty Python sketch in which Spam is ubiquitous, unavoidable and repetitive. Email spam has steadily grown since the early 1990s, and by 2014 was estimated that it made up around 90% of email messages sent.Since the expense of the spam is borne mostly by the recipient, it is effectively postage due advertising. This makes it an excellent example of a negative externality.The legal definition and status of spam varies from one jurisdiction to another, but laws and lawsuits have nowhere been particularly successful in stemming spam.

Most email spam messages are commercial in nature. Whether commercial or not, many are not only annoying, but also dangerous because they may contain links that lead to phishing web sites or sites that are hosting malware - or include malware as file attachments.

Spammers collect email addresses from chat rooms, websites, customer lists, newsgroups, and viruses that harvest users' address books. These collected email addresses are sometimes also sold to other spammers.

Gmail

Gmail is a free email service developed by Google. Users can access Gmail on the web and using third-party programs that synchronize email content through POP or IMAP protocols. Gmail started as a limited beta release on April 1, 2004 and ended its testing phase on July 7, 2009.

At launch, Gmail had an initial storage capacity offer of one gigabyte per user, a significantly higher amount than competitors offered at the time. Today, the service comes with 15 gigabytes of storage. Users can receive emails up to 50 megabytes in size, including attachments, while they can send emails up to 25 megabytes. In order to send larger files, users can insert files from Google Drive into the message. Gmail has a search-oriented interface and a "conversation view" similar to an Internet forum. The service is notable among website developers for its early adoption of Ajax.

Google's mail servers automatically scan emails for multiple purposes, including to filter spam and malware, and to add context-sensitive advertisements next to emails. This advertising practice has been significantly criticized by privacy advocates due to concerns over unlimited data retention, ease of monitoring by third parties, users of other email providers not having agreed to the policy upon sending emails to Gmail addresses, and the potential for Google to change its policies to further decrease privacy by combining information with other Google data usage. The company has been the subject of lawsuits concerning the issues. Google has stated that email users must "necessarily expect" their emails to be subject to automated processing and claims that the service refrains from displaying ads next to potentially sensitive messages, such as those mentioning race, religion, sexual orientation, health, or financial statements. In June 2017, Google announced the upcoming end to the use of contextual Gmail content for advertising purposes, relying instead on data gathered from the use of its other services.By February 2016, Gmail had one billion active users worldwide.

Hillary Clinton email controversy

During her tenure as United States Secretary of State, Hillary Clinton drew controversy by using her family's private email server for official communications rather than using official State Department email accounts maintained on secure federal servers. An FBI examination of Clinton's server found over 100 emails containing classified information, including 65 emails deemed "Secret" and 22 deemed "Top Secret". An additional 2,093 emails not marked classified were retroactively classified by the State Department.

Some experts, officials, and members of Congress contended that Clinton's use of a private messaging system and a private server violated State Department protocols and procedures, as well as federal laws and regulations governing recordkeeping. Clinton responded that her use complied with federal laws and State Department regulations, and that former secretaries of state had also maintained personal email accounts. News reports indicated that the emails discussed "innocuous" matters already available in the public domain. For example, the CIA drone program has been widely discussed in the public domain since the early 2000s; however, the very existence of the program is technically classified, so even sharing a newspaper article that mentions it would constitute a security breach as far as the CIA is concerned.The controversy was a major point of discussion during the 2016 presidential election, in which Clinton was the Democratic nominee. In May, the State Department's Office of the Inspector General released a report about the State Department's email practices, including Clinton's. In July, FBI director James Comey announced that the FBI investigation had concluded that Clinton had been "extremely careless" but recommended that no charges be filed. Clinton's opponent, presidential candidate Donald Trump, used the nickname "Crooked Hillary" to criticize Clinton primarily for the email controversy. Trump supporters adopted the chant "lock her up" to imply Clinton had committed a crime.On October 28, 2016, days before the election, Comey notified Congress that the FBI had started looking into newly discovered emails. On November 6, Comey notified Congress that the FBI had not changed its original conclusion. Comey's timing was contentious, with critics saying that he had violated Department of Justice guidelines and precedent and prejudiced the public against Clinton. The controversy received more media coverage than any other topic during the presidential campaign. Clinton and other observers argue that the reopening of the investigation contributed to her loss in the election. In April 2018, Comey said his announcement had been influenced by the fact that he thought it extremely likely that Clinton would become the next President. On June 14, 2018, the Department of Justice's Office of the Inspector General released its report on the FBI's and DOJ's handling of Clinton's investigation, finding no evidence of political bias and lending support for the decision to not prosecute Clinton.

Microsoft Exchange Server

Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.

The first version was called Exchange Server 4.0, to position it as the successor to the unrelated Microsoft Mail 3.5. Exchange initially used the X.400 directory service but switched to Active Directory later. Until version 5.0 it came bundled with an email client called Microsoft Exchange Client. This was discontinued in favor of Microsoft Outlook.

Exchange Server primarily uses a proprietary protocol called MAPI to talk to email clients, but subsequently added support for POP3, IMAP, and EAS. The standard SMTP protocol is used to communicate to other Internet mail servers.

Exchange Server is licensed both as on-premises software and software as a service (SaaS). In the on-premises form, customers purchase client access licenses (CALs); as SaaS, Microsoft charges a monthly service fee instead.

Microsoft Outlook

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note taking, journal, and web browsing.

It can be used as a stand-alone application, or can work with Microsoft Exchange Server and Microsoft SharePoint Server for multiple users in an organization, such as shared mailboxes and calendars, Exchange public folders, SharePoint lists, and meeting schedules. Microsoft has also released mobile applications for most mobile platforms, including iOS and Android. Developers can also create their own custom software that works with Outlook and Office components using Microsoft Visual Studio. In addition, Windows Phone devices can synchronize almost all Outlook data to Outlook Mobile.

Mozilla Thunderbird

Mozilla Thunderbird is a free and open-source cross-platform email client, news client, RSS and chat client developed by the Mozilla Foundation. The project strategy was modeled after that of the Mozilla Firefox web browser. It is installed by default on Ubuntu desktop systems.

On December 7, 2004, version 1.0 was released, and received more than 500,000 downloads in its first three days of release, and 1,000,000 in 10 days.On July 6, 2012, Mozilla announced the company was dropping the priority of Thunderbird development because the continuous effort to extend Thunderbird's feature set was mostly fruitless. The new development model shifted to Mozilla offering only "Extended Support Releases", which deliver security and maintenance updates, while allowing the community to take over the development of new features.On December 1, 2015, Mozilla Executive Chair Mitchell Baker announced in a company-wide memo that Thunderbird development needs to be uncoupled from Firefox. She referred to Thunderbird developers spending large efforts responding to changes to Mozilla technologies, while Firefox was paying a tax to support Thunderbird development. She also said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does. At the same time, it was announced that Mozilla Foundation will provide at least a temporary legal and financial home for the Thunderbird project.

Outlook.com

Outlook.com is a web-based suite of webmail, contacts, tasks, and calendaring services from Microsoft. One of the world's first webmail services, it was founded in 1996 as Hotmail (stylized as HoTMaiL) by Sabeer Bhatia and Jack Smith in Mountain View, California, and headquartered in Sunnyvale. Microsoft acquired Hotmail in 1997 for an estimated $400 million and launched it as MSN Hotmail, later rebranded to Windows Live Hotmail as part of the Windows Live suite of products. Microsoft released the final version of Hotmail in October 2011 and it was replaced by Outlook.com in 2013.

Outlook Express

Outlook Express, formerly known as Microsoft Internet Mail and News, is a discontinued email and news client included with Internet Explorer versions 3.0 through to 6.0. As such, it was bundled with several versions of Microsoft Windows, from Windows 98 to Windows Server 2003, and was available for Windows 3.x, Windows NT 3.51, Windows 95, Mac System 7, Mac OS 8, and Mac OS 9. In Windows Vista, Outlook Express was superseded by Windows Mail.

Outlook Express is a different application from Microsoft Outlook. The two apps do not share a common codebase, but they do share a common architectural philosophy. The similar names lead many people to conclude incorrectly that Outlook Express is a stripped-down version of Microsoft Outlook. Outlook Express uses the Windows Address Book to store contact information and integrates tightly with it. On Windows XP, it also integrates with Windows Messenger.

Post Office Protocol

In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server.POP version 3 (POP3) is the version in common use.

RSS

RSS (originally RDF Site Summary; later, two competing approaches emerged, which used the backronyms Rich Site Summary and Really Simple Syndication respectively) is a type of web feed which allows users and applications to access updates to online content in a standardized, computer-readable format. These feeds can, for example, allow a user to keep track of many different websites in a single news aggregator. The news aggregator will automatically check the RSS feed for new content, allowing the content to be automatically passed from website to website or from website to user. This passing of content is called web syndication. Websites usually use RSS feeds to publish frequently updated information, such as blog entries, news headlines, or episodes of audio and video series. RSS is also used to distribute podcasts. An RSS document (called "feed", "web feed", or "channel") includes full or summarized text, and metadata, like publishing date and author's name.

A standard XML file format ensures compatibility with many different machines/programs. RSS feeds also benefit users who want to receive timely updates from favourite websites or to aggregate data from many sites.

Subscribing to a website RSS removes the need for the user to manually check the website for new content. Instead, their browser constantly monitors the site and informs the user of any updates. The browser can also be commanded to automatically download the new data for the user.

RSS feed data is presented to users using software called a news aggregator. This aggregator can be built into a website, installed on a desktop computer, or installed on a mobile device. Users subscribe to feeds either by entering a feed's URI into the reader or by clicking on the browser's feed icon. The RSS reader checks the user's feeds regularly for new information and can automatically download it, if that function is enabled. The reader also provides a user interface.

Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is an Internet standard for email transmission. First defined by RFC 821 in 1982, it was updated in 2008 with Extended SMTP additions by RFC 5321; which is the protocol in widespread use today.

Mail servers and other mail transfer agents use SMTP to send and receive mail messages on TCP port 25. Although proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as Outlook.com, Gmail and Yahoo! Mail may use their own non-standard protocols internally, all use SMTP when sending or receiving email from outside their own systems.

User-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For this, mail clients typically submit their outgoing emails to a mail server on port 587 or 465 as per RFC 8314. For retrieving messages, IMAP and POP3 are standard, but proprietary servers also often prefer their own protocols, such as Exchange ActiveSync.

Web feed

On the World Wide Web, a web feed (or news feed) is a data format used for providing users with frequently updated content. Content distributors syndicate a web feed, thereby allowing users to subscribe a channel to it. Making a collection of web feeds accessible in one spot is known as aggregation, which is performed by a news aggregator. A web feed is also sometimes referred to as a syndicated feed.

A typical scenario of web-feed use might involve the following: a content provider publishes a feed link on its site which end users can register with an aggregator program (also called a feed reader or a news reader) running on their own machines; doing this is usually as simple as dragging the link from the web browser to the aggregator. When instructed, the aggregator asks all the servers in its feed list if they have new content; if so, the aggregator either makes a note of the new content or downloads it. One can schedule aggregators to check for new content periodically.

Web feeds exemplify pull technology, although they may appear to push content to the user.

The kinds of content delivered by a web feed are typically HTML (webpage content) or links to webpages and other kinds of digital media. Often when websites provide web feeds to notify users of content updates, they only include summaries in the web feed rather than the full content itself.

Many news websites, weblogs, schools, and podcasters operate web feeds.

Work

Web feeds have some advantages compared to receiving frequently published content via an email:

Users do not disclose their email address when subscribing to a feed and so are not increasing their exposure to threats associated with email: spam, viruses, phishing, and identity theft.

Users do not have to send an unsubscribe request to stop receiving news. They simply remove the feed from their aggregator.

The feed items are automatically sorted in that each feed URL has its own sets of entries (unlike an email box where messages must be sorted by user-defined rules and pattern matching).In its explanation "What is a web feed?", the publishing group of Nature describes two benefits of web feeds:

It makes it easier for users to keep track of our content...This is a very convenient way of staying up to date with the content of a large number of sites.

It makes it easier for other websites to link to our content. Because RSS feeds can easily be read by computers, it's also easy for webmasters to configure their sites so that the latest headlines from another site's RSS feed are embedded into their own pages, and updated automatically.

Webmail

Webmail (or web-based email) is any email client implemented as a web application running on a web server. Examples of webmail software are Roundcube and SquirrelMail. Examples of webmail providers are AOL Mail, Gmail, Outlook.com/Hotmail.com, Rackspace Email, Yahoo! Mail and IceWarp Mail Server. Many webmail providers also offer email access by a desktop email client using standard email protocols, while many internet service providers provide a webmail client as part of the email service included in their internet service package.

As with any web application, webmail's main advantage over the use of a desktop email client is the ability to send and receive email anywhere from a web browser. Its main disadvantage is the need to be connected to the Internet while using it.

Yahoo! Mail

Yahoo! Mail is an email service launched in 1997 through the American parent company Yahoo!. Yahoo Mail provides four different email plans: three for personal use (Basic, Plus, and Ad Free) and another for businesses. By December 2011, Yahoo! Mail had 281 million users, making it the third largest web-based email service in the world. Since 2015 its webmail client also supports managing non-Yahoo e-mail accounts.As many as three web interfaces were available at any given time. The traditional "Yahoo! Mail Classic" preserved the availability of their original 1997 interface until July 2013 in North America. A 2005 version included a new Ajax interface, drag-and-drop, improved search, keyboard shortcuts, address auto-completion, and tabs. However, other features were removed, such as column widths and one click delete-move-to-next. In October 2010, Yahoo! released a beta version of Yahoo! Mail, which included improvements to performance, search, and Facebook integration. In May 2011, this became the default interface. Their current Webmail interface was introduced in 2017.

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