Mailing

Email

This article is about the communications medium. For the former manufacturing conglomerate, see Email Limited.
"Inbox" redirects here. For the Google product, seeInbox by Gmail.
This screenshot shows the "Inbox" page of an email system, where 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 SMTPemail address[1]
Electronic Mail (email or e-mail) is a method of exchanging messages between people using electronic devices. Email first entered substantial 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 theInternet. Some early email systems required the author and the recipient to both be onlineat the same time, in common with instant messaging. Today's email systems are based on a store-and-forward model. Email serversaccept, 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 byMultipurpose 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.

TerminologyEdit

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 faxdocument 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:
  • email is the most common form used online, and is required by IETF Requests for Comments (RFC) and working groups[7] and increasingly by style guides.[8][9] This spelling also appears in most dictionaries.[10][11][12][13][14][15][16]
  • e-mail is the format that sometimes appears in edited, published American English and British English writing as reflected in the Corpus of Contemporary American English data,[17] but is falling out of favor in style guides.[9][18]
  • mail was the form used in the original protocol standard, RFC 524.[19] The service is referred to as mail, and a single piece of electronic mail is called amessage.[20][21]
  • EMail is a traditional form that has been used in RFCs for the "Author's Address"[20][21] and is expressly required "for historical reasons".[22]
  • E-mail is sometimes used, capitalizing the initial E as in similar abbreviations like E-piano, E-guitar, A-bomb, and H-bomb.[23]
An Internet e-mail consists[24] of an envelope and content; the content in turn consists[25] of a header and a body.

OriginEdit

Main article: History of email
Computer-based mail and messaging became possible with the advent of time-sharingcomputers 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 theARPANET (created in the late-1960s), which aimed at software portability between its systems. That portability helped make theSimple 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.

OperationEdit

The diagram to the right shows a typical sequence of events[28] that takes place when sender Alice transmits a message using amail user agent (MUA) addressed to the email address of the recipient.
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 casebob@b.org. The part before the @ sign is thelocal part of the address, often the usernameof 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 recordslisting 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 userbob.
  6. Bob's MUA picks up the message using either the Post Office Protocol (POP3) or theInternet 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 IBMLotus 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 webmailservice.
  • 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.[citation needed]However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as a consequence open mail relays have become rare,[30] and many MTAs do not accept messages from open mail relays.

Message formatEdit

The Internet email message format is now defined by RFC 5322, with encoding of non-ASCII data and multimedia content attachments being defined in RFC 2045through RFC 2049, collectively calledMultipurpose Internet Mail Extensions orMIME. 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 theARPANET.[31]
Internet email messages consist of two major sections, the message header and the message body, collectively known as content.[32][33] The header is structured intofields 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 headerEdit

Each message has exactly one header, which is structured into fields. Each field has a name and a value. RFC 5322 specifies the precise syntax.
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. Some non-ASCII values may be represented using MIME encoded words.

Header fieldsEdit

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.[34] Header fields defined byRFC 5322 can only contain US-ASCIIcharacters; for encoding characters in other sets, a syntax specified in RFC 2047 can be used.[35] Recently the IETF EAI working group has defined some standards track extensions,[36][37] replacing previous experimental extensions, to allow UTF-8encoded 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.[38]
The message header must include at least the following fields:[39][40]
  • 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 him/her.
RFC 3864 describes registration procedures for message header fields at the IANA; it provides for permanent and provisionalfield names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include:[41]
  • 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 MIMEtype.
  • 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.[42]
  • 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:[43]
  • 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 calledtrace fields, in a broader sense.[44]
  • Authentication-Results: when a server carries out authentication checks, it can save the results in this field for consumption by downstream agents.[45]
  • Received-SPF: stores results of SPF checks in more detail than Authentication-Results.[46]
  • Auto-Submitted: is used to mark automatically generated messages.[47]
  • VBR-Info: claims VBR whitelisting[48]

Message bodyEdit

Content encodingEdit

Internet email was originally designed for 7-bitASCII.[49] Most email software is 8-bit cleanbut must assume it will communicate with 7-bit servers and mail readers. The MIMEstandard 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 andBINARY 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.[citation needed]

Plain text and HTMLEdit

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 inblock 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.[50]
Some web-based mailing lists recommend that all posts be made in plain-text, with 72 or 80 characters per line[51][52] for all the above reasons, but also because they have a significant number of readers using text-based email clients such as Mutt. SomeMicrosoft 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 compatibleemail client.[53]

Servers and client applicationsEdit

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,[54] and when a message cannot be delivered, that MTA must send abounce message back to the sender, indicating the problem.
Users can retrieve their messages from servers using standard protocols such as POPor IMAP, or, as is more likely in a largecorporate environment, with a proprietaryprotocol 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 serverside, 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 anyMUA 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.[55] Such webmail interfaces allow users to access their mail with any standardweb browser, from any computer, rather than relying on an email client.

Filename extensionsEdit

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 specialfilename 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 MIMEformat, 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 andOfficeLogic Groupware.
mbx
Used by Opera Mail, KMail, and Apple Mailbased 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 mailtoEdit

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 theTo: field.[56]

Comments

Post a Comment

Popular posts from this blog

CA

Image
Edit Watch this page Read in another language Institute of Chartered Accountants of India The Institute of Chartered Accountants of India  ( ICAI ) is the national  professional accounting body  of  India . It was established on 1 July 1949 as a  statutory body  under the Chartered Accountants Act, 1949  enacted by the  Parliament  (acting as the provisional Parliament of India ) to regulate the profession of  Chartered Accountancy  in India. ICAI is the second largest professional Accounting & Finance body in the world. However, the rapid growth and autonomy of the ACCA in the early 2000s, soon made ICAI the third largest in terms of membership, the first being the ACCA. ICAI is the only licensing cum regulating body  of the  financial audit  and accountancy  profession in India. It recommends the  accounting standards  to be followed by companies in India to  National Advisor...
Read more

Telecommunication

Image
Edit Watch this page Read in another language Telecommunication Not to be confused with   Teleconnection . Earth station   at the satellite communication facility in Raisting, Bavaria, Germany Visualization from the   Opte Project   of the various routes through a portion of the Internet Telecommunication   is the   transmission   of signs, signals, messages, words, writings, images and sounds or information of any nature by   wire ,   radio , optical or other electromagnetic   systems. [1] [2] Telecommunication occurs when the exchange of   information   between communication   participants includes the use of   technology . It is transmitted either electrically over physical media, such as cables , or via   electromagnetic radiation . [3] [4] [5] [6] [7] [8]   Such transmission paths are often divided into   communication channels   which afford the advantages of mult...
Read more

Local aria network

LAN A conceptual diagram of a local area network. "LAN" redirects here. For other uses, see   LAN (disambiguation) . A   local area network   ( LAN ) is a   computer network   that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. [1]   By contrast, a   wide area network (WAN) not only covers a larger geographic distance, but also generally involves   leased telecommunication circuits . Ethernet   and   Wi-Fi   are the two most common technologies in use for local area networks. Historical technologies include   ARCNET , Token ring , and   AppleTalk . History Cabling Edit Early LAN cabling had generally been based on various grades of   coaxial cable . Shielded twisted pair   was used in IBM's   Token Ring LAN implementation, but in 1984,   StarLAN showed the potential of simple   unshielded twis...
Read more