Electronic mail is a method of exchanging digital messages between computer users; such messaging first entered substantial use in the 1960s and by the 1970s had taken the form now recognised as email. Email operates across computer networks, now 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 connect only briefly, typically to a mail server, 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 multi-media content attachments. International email, with internationalized email addresses using UTF-8, has been standardized, but as of 2016 not widely adopted.
The history of modern, global Internet email services reaches back to the early ARPANET, with standards for encoding email messages proposed as early as 1973 (RFC 561). An email message sent in the early 1970s looks very similar to a basic text email sent today. Email played an important part in creating the Internet, and the conversion from ARPANET to the Internet in the early 1980s produced the core of the current services. The ARPANET initially used extensions to the File Transfer Protocol (FTP) to exchange network email, but this is now done with the Simple Mail Transfer Protocol (SMTP), first published as Internet standard 10 (RFC 821) in 1982.
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. 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.
The AUTODIN network, first operational in 1962, provided a message service between 1,350 terminals, handling 30 million messages per month, with an average message length of approximately 3,000 characters. Autodin was supported by 18 large computerized switches, and was connected to the United States General Services Administration Advanced Record System, which provided similar services to roughly 2,500 terminals.
With the introduction of MIT's Compatible Time-Sharing System (CTSS) in 1961 multiple users could log in to a central system from remote dial-up terminals, and to store and share files on the central disk. Informal methods of using this to pass messages were developed and expanded:
Developers of other early systems developed similar email applications:
These original messaging systems had widely different features and ran on systems that were incompatible with each other. Most of them only allowed communication between users logged into the same host or "mainframe", although there might be hundreds or thousands of users within an organization.
In the early 1980s, networked personal computers on LANs became increasingly important. Server-based systems similar to the earlier mainframe systems were developed. Again, these systems initially allowed communication only between users logged into the same server infrastructure. Examples include:
Eventually these systems too could link different organizations as long as they ran the same email system and proprietary protocol.
To facilitate electronic mail exchange between remote sites and with other organizations, telecommunication links, such as dialup modems or leased lines, provided means to transport email globally, creating local and global networks. This was challenging for a number of reasons, including the widely different email address formats in use.
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Early interoperability among independent systems included:
In the early 1970s, Ray Tomlinson updated an existing utility called SNDMSG so that it could copy messages (as files) over the network. Lawrence Roberts, the project manager for the ARPANET development, took the idea of READMAIL, which dumped all "recent" messages onto the user's terminal, and wrote a programme for TENEX in TECO macros called RD, which permitted access to individual messages. Barry Wessler then updated RD and called it NRD.
Marty Yonke rewrote NRD to include reading, access to SNDMSG for sending, and a help system, and called the utility WRD, which was later known as BANANARD. John Vittal then updated this version to include three important commands: Move (combined save/delete command), Answer (determined to whom a reply should be sent) and Forward (sent an email to a person who was not already a recipient). The system was called MSG. With inclusion of these features, MSG is considered to be the first integrated modern email programme, from which many other applications have descended.
Experimental email transfers between separate computer systems began shortly after the creation of the ARPANET in 1969. Ray Tomlinson is generally credited as having sent the first email across a network, initiating the use of the "@" sign to separate the names of the user and the user's machine in 1971, when he sent a message from one Digital Equipment Corporation DEC-10 computer to another DEC-10. The two machines were placed next to each other. Tomlinson's work was quickly adopted across the ARPANET, which significantly increased the popularity of email.
As the influence of the ARPANET spread across academic communities, gateways were developed to pass mail to and from other networks such as CSNET, JANET, BITNET, X.400, and FidoNet. This often involved addresses such as:
which routes mail to a user with a "bang path" address at a UUCP host.
In addition to this example, alternatives and complications exist in the email system:
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. However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as a consequence open mail relays have become rare, and many MTAs do not accept messages from open mail relays.
The Internet email message format is now defined by RFC 5322, with multi-media 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.
Internet email messages consist of two major sections, the message header and the message body. 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.
Informally, each line of text in the header that begins with a 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 values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words.
Email header fields can be multi-line, and each line should be at most 78 characters long and in no event more than 998 characters long. 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. Recently the IETF EAI working group has defined some standards track extensions, 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 characters must only be used by servers that support these extensions.
The message header must include at least the following fields:
The message header should include at least the following fields:
RFC 3864 describes registration procedures for message header fields at the IANA; it provides for permanent and provisional message header field names, including also fields defined for MIME, netnews, and http, and referencing relevant RFCs. Common header fields for email include:
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:
Other header fields that are added on top of the header by the receiving server may be called trace fields, in a broader sense.
Email was originally designed for 7-bit ASCII. Most email software is 8-bit clean but must assume it will communicate with 7-bit servers and mail readers. The MIME 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.
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.
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, but also because they have a significant number of readers using text-based email clients such as Mutt.
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, 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, Hotmail, or Yahoo! Mail, that performs the same tasks. Such webmail interfaces allow users to access their mail with any standard web browser, from any computer, rather than relying on an email client.
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:
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.
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.
Many email providers have a web-based email client (e.g. AOL Mail, Gmail, Outlook.com and Yahoo! Mail). This allows users to log in to 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.
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).
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.
Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook to communicate to Microsoft Exchange Server - and to a range of other e-mail 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.
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Email has been widely accepted by business as one of the key parts of an 'e-revolution' in business communication. It is now the most widely used medium of communication within the business world. A 2010 study on workplace communication by Paytronics found 83% of U.S. knowledge workers felt email was critical to their success and productivity at work.
It has some key benefits to business, including:
Email marketing via "opt-in" is often successfully used to send special sales offerings and new product information, but offering hyperlinks or generic information on consumer trends is less useful - and email sent without permission such as "opt-in" is likely to be viewed as unwelcome "email spam".
Email has become widely used on smart phones. Mobile apps for email increase accessibility to the medium. While before users could only access email on computers, it is now possible for users to check their email out of the home and out of the library while on the go. Alerts can also be sent to the phone 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. Today, there are an estimated 1.4 billion email users worldwide and 50 billion non-spam emails that are sent daily.
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 smart phones. 78% of the respondents in the study revealed that they check their email on their phone. 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.
Flaming occurs when a person sends a message 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 don't exist and civility may be forgotten.
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 the email inbox is being cleared out. Harvard University law professor Lawrence Lessig is credited with coining this term, but he may only have popularized it.
Email messages may have one or more attachments. In principle there is no technical restriction on the size or number of attachments, but in practice email clients, servers and service providers implement various limitations on the size of files, or complete email - typically to 25MB or less. Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ to what the user sees, 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.
The ubiquity of email for knowledge workers has led many to suggest that people face an "information overload" in dealing with increasing volumes of email, and even suggested that this has a significant negative economic effect.
Email "spam" is the term used to describe unsolicited commercial, or bulk, email. The low cost of sending such email meant that by 2003 up to 30% of total email traffic was already spam. and was threatening the usefulness of email as a practical tool. The US CAN-SPAM Act of 2003 and similar laws elsewhere 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, but the volume sent is still very high - and increasingly consists not of advertisements for products, but malicious content or links.
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 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 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.
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:
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, 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.
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 through 3888.)
Many ISPs now deliberately disable non-delivery reports (NDRs) and delivery receipts due to the activities of spammers:
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, and only work with e-mail clients that support rendering of HTML. Many mail clients now default to not showing "web content". Webmail providers can also disrupt web bugs by pre-caching images.
The U.S. state and federal governments have been involved in electronic messaging and the development of email in several different ways.
Starting in 1977, the U.S. Postal Service (USPS) recognized that electronic messaging and electronic transactions posed a significant threat to First Class mail volumes and revenue. The USPS explored an electronic messaging initiative in 1977 and later disbanded it. Twenty years later, in 1997, when email volume overtook postal mail volume, the USPS was again urged to embrace email, and the USPS declined to provide email as a service. The USPS initiated an experimental email service known as E-COM. E-COM provided a method for the simple exchange of text messages. In 2011, shortly after the USPS reported its state of financial bankruptcy, the USPS Office of Inspector General (OIG) began exploring the possibilities of generating revenue through email servicing. Electronic messages were transmitted to a post office, printed out, and delivered as hard copy. To take advantage of the service, an individual had to transmit at least 200 messages. The delivery time of the messages was the same as First Class mail and cost 26 cents. Both the Postal Regulatory Commission and the Federal Communications Commission opposed E-COM. The FCC concluded that E-COM constituted common carriage under its jurisdiction and the USPS would have to file a tariff. Three years after initiating the service, USPS canceled E-COM and attempted to sell it off.
The early ARPANET dealt with multiple email clients that had various, and at times incompatible, formats. For example, in the Multics, the "@" sign meant "kill line" and anything before the "@" sign was ignored, so Multics users had to use a command-line option to specify the destination system. The Department of Defense DARPA desired to have uniformity and interoperability for email and therefore funded efforts to drive towards unified inter-operable standards. This led to David Crocker, John Vittal, Kenneth Pogran, and Austin Henderson publishing RFC 733, "Standard for the Format of ARPA Network Text Message" (November 21, 1977), a subset of which provided a stable base for common use on the ARPANET, but which was not fully effective, and in 1979, a meeting was held at BBN to resolve incompatibility issues. Jon Postel recounted the meeting in RFC 808, "Summary of Computer Mail Services Meeting Held at BBN on 10 January 1979" (March 1, 1982), which includes an appendix listing the varying email systems at the time. This, in turn, led to the release of David Crocker's RFC 822, "Standard for the Format of ARPA Internet Text Messages" (August 13, 1982). RFC 822 is a small adaptation of RFC 733's details, notably enhancing the host portion, to use Domain Names, that were being developed at the same time.
The National Science Foundation took over operations of the ARPANET and Internet from the Department of Defense, and initiated NSFNet, a new backbone for the network. A part of the NSFNet AUP forbade commercial traffic. In 1988, Vint Cerf arranged for an interconnection of MCI Mail with NSFNET on an experimental basis. The following year Compuserve email interconnected with NSFNET. Within a few years the commercial traffic restriction was removed from NSFNETs AUP, and NSFNET was privatised.
In the late 1990s, the Federal Trade Commission grew concerned with fraud transpiring in email, and initiated a series of procedures on spam, fraud, and phishing. In 2004, FTC jurisdiction over spam was codified into law in the form of the CAN SPAM Act. Several other U.S. federal agencies have also exercised jurisdiction including the Department of Justice and the Secret Service.
NASA has provided email capabilities to astronauts aboard the Space Shuttle and International Space Station since 1991 when a Macintosh Portable was used aboard Space Shuttle mission STS-43 to send the first email via AppleLink. Today astronauts aboard the International Space Station have email capabilities via the wireless networking throughout the station and are connected to the ground at 10 Mbit/s Earth to station and 3 Mbit/s station to Earth, comparable to home DSL connection speeds.
We recommend email, as this is now by far the most common form
there are many more trace headers than those two
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