Misplaced Pages

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95-473: The Simple Mail Transfer Protocol ( SMTP ) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC   8314 . For retrieving messages, IMAP (which replaced

190-514: A DNS name). This server will deliver outgoing messages on behalf of the user. Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example spam . Two solutions have been in common use: Under this system, an ISP 's SMTP server will not allow access by users who are outside the ISP's network. More precisely, the server may only allow access to users with an IP address provided by

285-505: A Transmission Control Protocol (TCP) connection. An SMTP session consists of commands originated by an SMTP client (the initiating agent , sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An SMTP transaction consists of three command/reply sequences: Besides

380-447: A mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures. A MUA knows the outgoing mail SMTP server from its configuration. A relay server typically determines which server to connect to by looking up

475-626: A center was "Non-Automated Relay Center" (NARC). In 1948, Western Union introduced Plan 55-A , the first automatic electromechanical store and forward message switching system. All message storage was performed by paper tape punches paired with paper tape readers, with a bin in between. It is very common for an email system using SMTP to accept a message, store it and then forward it on elsewhere. Although fully open mail relays are no longer common, not only does simple server-based forwarding work this way, but also many email filtering and automated electronic mailing lists services. Prior to

570-447: A command is acknowledged by the server with a result code and response message (e.g., 250 Ok ). The transmission of the body of the mail message is initiated with a DATA command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This sequence consists of a new-line ( <CR><LF> ), a single full stop ( . ), followed by another new-line ( <CR><LF> ). Since

665-483: A derivative of SMTP designed for this purpose. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses

760-529: A direct, end-to-end connection is not available. A store-and-forward switching center is a message switching center in which a message is accepted from the originating user , i.e., sender, when it is offered, held in a physical storage , and forwarded to the destination user, i.e., receiver, in accordance with the priority placed upon the message by the originating user and the availability of an outgoing channel . Store and forward switching centers are usually implemented in mobile service stations where

855-406: A domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as

950-521: A fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). Internet Standard Engineering contributions to the IETF start as an Internet Draft , may be promoted to a Request for Comments , and may eventually become an Internet Standard. An Internet Standard is characterized by technical maturity and usefulness. The IETF also defines a Proposed Standard as a less mature but stable and well-reviewed specification. A Draft Standard

1045-523: A general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. Message submission ( RFC   2476 ) and SMTP-AUTH ( RFC   2554 ) were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for

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1140-404: A message body can contain a line with just a period as part of the text, the client sends two periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called dot-stuffing . The server's positive reply to the end-of-data, as exemplified, implies that the server has taken

1235-712: A remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command. The original TURN command was deemed insecure and was extended in RFC   1985 with the ETRN command which operates more securely using an authentication method based on Domain Name System information. An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as

1330-433: A remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see Remote Message Queue Starting below). POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed. Remote Message Queue Starting enables

1425-485: A security protocol with a low adoption rate: DNS Security Extensions (DNSSEC). Essentially, at every stage of the DNS lookup process, DNSSEC adds a signature to data to show it has not been tampered with. Some companies have taken the initiative to secure internet protocols. It is up to the rest to make it more widespread. Store and forward Store and forward is a telecommunications technique in which information

1520-441: A set of clips or hooks. A major relay center in the mid 1900s might have dozens of inbound and outbound teleprinters, scores of operators, and thousands of messages in the queues during peak periods. Operators referred to these centers as "torn- tape relay centers", a reference to removing the received message from the inbound teleprinter by tearing the paper tape to separate one message from the next. The U.S. military term for such

1615-460: A single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host . Each process is an MTA (an SMTP server) in its own right. The boundary MTA uses DNS to look up the MX (mail exchanger) record for

1710-527: A snapshot of the list. Internet standards are a set of rules that devices have to follow when they connect in a network. Since the technology has evolved, the rules of the engagement between computers had to evolve with it. These are the protocols that are in place used today. Most of these were developed long before the Internet Age , going as far back as the 1970s, not long after the creation of personal computers . TCP/IP The official date for when

1805-516: A standard for use in 1979. It was then updated several times and the final version. It took a few years for the protocol to be presented in its final form. ISO 7498 was published in 1984. Lastly in 1995 the OSI model was revised again satisfy the urgent needs of uprising development in the field of computer networking. UDP The goal of User Datagram Protocol was to find a way to communicate between two computers as quickly and efficiently as possible. UDP

1900-587: A time. Normally, the standards used in data communication are called protocols. All Internet Standards are given a number in the STD series. The series was summarized in its first document, STD 1 (RFC 5000), until 2013, but this practice was retired in RFC 7100. The definitive list of Internet Standards is now maintained by the RFC Editor. Documents submitted to the IETF editor and accepted as an RFC are not revised; if

1995-502: A user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical. It is highly desirable to be able to use email client configuration information that does not need to change. Modern SMTP servers typically require authentication of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system

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2090-587: A way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices. As this protocol started out purely ASCII text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions ( MIME ) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8-bit clean , so that

2185-510: Is a statement describing all relevant aspects of a protocol, service, procedure, convention, or format. This includes its scope and its intent for use, or "domain of applicability". However, a TSs use within the Internet is defined by an Applicability Statement. An AS specifies how, and under what circumstances, TSs may be applied to support a particular Internet capability. An AS identifies the ways in which relevant TSs are combined and specifies

2280-501: Is available). The client notifies the receiver of the originating email address of the message in a MAIL FROM command. This is also the return or bounce address in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the To: and Cc: header fields. The corresponding SMTP command is RCPT TO . Each successful reception and execution of

2375-435: Is defined in several "Best Current Practice" documents, notably BCP 9 (currently RFC 2026 and RFC 6410). There were previously three standard maturity levels: Proposed Standard , Draft Standard and Internet Standard . RFC 6410 reduced this to two maturity levels. RFC 2026 originally characterized Proposed Standards as immature specifications, but this stance was annulled by RFC 7127. A Proposed Standard specification

2470-561: Is formally created by official standard-developing organizations. These standards undergo the Internet Standards Process . Common de jure standards include ASCII , SCSI , and Internet protocol suite . Specifications subject to the Internet Standards Process can be categorized into one of the following: Technical Specification (TS) and Applicability Statement (AS). A Technical Specification

2565-759: Is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server. SMTP Authentication , often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication mechanism. Communication between mail servers generally uses the standard TCP port 25 designated for SMTP. Mail clients however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of: Port 2525 and others may be used by some individual providers, but have never been officially supported. Many Internet service providers now block all outgoing port 25 traffic from their customers. Mainly as an anti-spam measure, but also to cure for

2660-570: Is gathered. Many Proposed Standards are actually deployed on the Internet and used extensively, as stable protocols. Actual practice has been that full progression through the sequence of standards levels is typically quite rare, and most popular IETF protocols remain at Proposed Standard. In October 2011, RFC 6410 merged the second and third maturity levels into one Internet Standard . Existing older Draft Standards retain that classification, absent explicit actions. For old Draft Standards two possible actions are available, which must be aproved by

2755-466: Is needed for most non-text data and some text formats). In 2012, the SMTPUTF8 extension was created to support UTF-8 text, allowing international content and addresses in non- Latin scripts like Cyrillic or Chinese . Many people contributed to the core SMTP specifications, among them Jon Postel , Eric Allman , Dave Crocker, Ned Freed , Randall Gellens, John Klensin , and Keith Moore . Email

2850-440: Is not encrypted so in practice HTTPS is used, which stands for HTTP Secure. TLS/SSL TLS stands for Transport Layer Security which is a standard that enables two different endpoints to interconnect sturdy and privately. TLS came as a replacement for SSL. Secure Sockets Layers was first introduced before the creation of HTTPS and it was created by Netscape. As a matter of fact HTTPS was based on SSL when it first came out. It

2945-614: Is recognizably useful in some or all parts of the Internet. An Internet Standard is documented by a Request for Comments (RFC) or a set of RFCs. A specification that is to become a Standard or part of a Standard begins as an Internet Draft , and is later, usually after several revisions, accepted and published by the RFC Editor as an RFC and labeled a Proposed Standard . Later, an RFC is elevated as Internet Standard , with an additional sequence number, when maturity has reached an acceptable level. Collectively, these stages are known as

Simple Mail Transfer Protocol - Misplaced Pages Continue

3040-539: Is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in a networking context, verifies the integrity of the message before forwarding it. In general, this technique is used in networks with intermittent connectivity, especially in the wilderness or environments requiring high mobility. It may also be preferable in situations when there are long delays in transmission and error rates are variable and high, or if

3135-420: Is sent via global networks. IPsec Internet Protocol Security is a collection of protocols that ensure the integrity of encryption in the connection between multiple devices. The purpose of this protocol is to protect public networks. According to IETF Datatracker the group dedicated to its creation was proposed into existence on 25 November 1992. Half a year later the group was created and not long after in

3230-608: Is stable, has resolved known design choices, has received significant community review, and appears to enjoy enough community interest to be considered valuable. Usually, neither implementation nor operational experience is required for the designation of a specification as a Proposed Standard. Proposed Standards are of such quality that implementations can be deployed in the Internet. However, as with all technical specifications, Proposed Standards may be revised if problems are found or better solutions are identified, when experiences with deploying implementations of such technologies at scale

3325-417: Is submitted by a mail client ( mail user agent , MUA) to a mail server ( mail submission agent , MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on

3420-668: Is that connecting to an MSA requires SMTP Authentication . SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users retrieving messages and managing mailboxes . To permit an intermittently-connected mail server to pull messages from

3515-519: Is the existing BGP safeguard called Routing Public Key Infrastructure (RPKI). It is a database of routes that are known to be safe and have been cryptographically signed. Users and companies submit routes and check other users' routes for safety. If it were more widely adopted, more routes could be added and confirmed. However, RPKI is picking up momentum. As of December 2020, tech giant Google registered 99% of its routes with RPKI. They are making it easier for businesses to adopt BGP safeguards. DNS also has

3610-435: The MX (Mail eXchange) DNS resource record for each recipient's domain name . If no MX record is found, a conformant relaying server (not all are) instead looks up the A record . Relay servers can also be configured to use a smart host . A relay server initiates a TCP connection to the server on the " well-known port " for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA

3705-835: The Standards Track , and are defined in RFC 2026 and RFC 6410. The label Historic is applied to deprecated Standards Track documents or obsolete RFCs that were published before the Standards Track was established. Only the IETF , represented by the Internet Engineering Steering Group (IESG), can approve Standards Track RFCs. The definitive list of Internet Standards is maintained in the Official Internet Protocol Standards . Previously, STD 1 used to maintain

3800-588: The Transmission Control Protocol on port number 25 (between servers) and 587 (for submission from authenticated clients), both with or without encryption. Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers . As more computers were interconnected, especially in the U.S. Government's ARPANET , standards were developed to permit exchange of messages between different operating systems. Mail on

3895-633: The World Wide Web . They allow for the building and rendering of websites. The three key standards used by the World Wide Web are Hypertext Transfer Protocol , HTML , and URL . Respectively, they specify the transfer of data between a browser and a web server, the content and layout of a web page, and what web page identifiers mean. Network standards are a type of internet standard which defines rules for data communication in networking technologies and processes. Internet standards allow for

Simple Mail Transfer Protocol - Misplaced Pages Continue

3990-727: The ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in RFC   196 ; and the SNDMSG program, which Ray Tomlinson of BBN adapted that year to send messages across two computers on the ARPANET. A further proposal for a Mail Protocol was made in RFC 524 in June 1973, which was not implemented. The use of the File Transfer Protocol (FTP) for "network mail" on

4085-791: The ARPANET was proposed in RFC 469 in March 1973. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed. SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the International Network Working Group in INWG Protocol note 2 , written in September 1974. INWG discussed protocols for electronic mail in 1979, which

4180-515: The Border Gateway Protocol (BGP) and Domain Name System (DNS).   This reflects common practices that focus more on innovation than security.  Companies have the power to improve these issues.  With the Internet in the hands of the industry, users must depend on businesses to protect vulnerabilities present in these standards. Ways to make BGP and DNS safer already exist but they are not widespread. For example, there

4275-563: The IESG: A Draft Standard may be reclassified as an Internet Standard as soon as the criteria in RFC 6410 are satisfied; or, after two years since RFC 6410 was aproved as BCP (October 2013), the IESG can choose to reclassify an old Draft Standard as Proposed Standard . An Internet Standard is characterized by a high degree of technical maturity and by a generally held belief that the specified protocol or service provides significant benefit to

4370-411: The IETF offers include RFCs, internet-drafts, IANA functions, intellectual property rights, standards process, and publishing and accessing RFCs. There are two ways in which an Internet Standard is formed and can be categorized as one of the following: "de jure" standards and "de facto" standards. A de facto standard becomes a standard through widespread use within the tech community. A de jure standard

4465-408: The ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP. A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible. This system has several variations. For example, an organisation's SMTP server may only provide service to users on

4560-768: The Internet Engineering Task Force (IETF). It is the leading Internet standards association that uses well-documented procedures for creating these standards. Once circulated, those standards are made easily accessible without any cost. Till 1993, the United States federal government was supporting the IETF. Now, the Internet Society's Internet Architecture Board (IAB) supervises it. It is a bottom-up organization that has no formal necessities for affiliation and does not have an official membership procedure either. It watchfully works with

4655-438: The Internet community. Generally Internet Standards cover interoperability of systems on the Internet through defining protocols, message formats, schemas, and languages. An Internet Standard ensures that hardware and software produced by different vendors can work together. Having a standard makes it much easier to develop software and hardware that link different networks because software and hardware can be developed one layer at

4750-518: The Internet language in order to remain competitive in the current Internet phase. Some basic aims of the Internet Standards Process are; ensure technical excellence; earlier implementation and testing; perfect, succinct as well as easily understood records. Creating and improving the Internet Standards is an ongoing effort and Internet Engineering Task Force plays a significant role in this regard. These standards are shaped and available by

4845-561: The World Wide Web Consortium (W3C) and other standard development organizations. Moreover, it heavily relies on working groups that are constituted and proposed to an Area Director. IETF relies on its working groups for expansion of IETF conditions and strategies with a goal to make the Internet work superior. The working group then operates under the direction of the Area Director and progress an agreement. After

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4940-523: The alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP. Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII . 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding

5035-522: The body of the message itself. STD 10 and RFC   5321 define SMTP (the envelope), while STD 11 and RFC   5322 define the message (header and body), formally referred to as the Internet Message Format . SMTP is a connection-oriented , text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically

5130-607: The circulation of the proposed charter to the IESG and IAB mailing lists and its approval then it is further forwarded to the public IETF. It is not essential to have the complete agreement of all working groups and adopt the proposal. IETF working groups are only required to recourse to check if the accord is strong. Likewise, the Working Group produce documents in the arrangement of RFCs which are memorandum containing approaches, deeds, examination as well as innovations suitable to

5225-414: The common consideration of the necessities that the effort should discourse. Then an IETF Working Group is formed and necessities are ventilated in the influential Birds of a Feather (BoF) assemblies at IETF conferences. The Internet Engineering Task Force (IETF) is the premier internet standards organization. It follows an open and well-documented processes for setting internet standards. The resources that

5320-524: The communication procedure of a device to or from other devices. In reference to the TCP/IP Model, common standards and protocols in each layer are as follows: The Internet has been viewed as an open playground, free for people to use and communities to monitor. However, large companies have shaped and molded it to best fit their needs. The future of internet standards will be no different. Currently, there are widely used but insecure protocols such as

5415-498: The concluding form. This process is followed in every area to generate unanimous views about a problem related to the internet and develop internet standards as a solution to different glitches. There are eight common areas on which IETF focus and uses various working groups along with an area director. In the "general" area it works and develops the Internet standards. In "Application" area it concentrates on internet applications such as Web-related protocols. Furthermore, it also works on

5510-467: The corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web , meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email ( spam ). Work on message submission ( RFC   2476 ) was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding

5605-467: The current destination(s) had been queued. The information that the client sends in the HELO and MAIL FROM commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a Received and Return-Path header field, respectively. Some clients are implemented to close the connection after the message is accepted ( 250 Ok: queued as 12345 ), so

5700-503: The deployment of the Internet , computers were connected via a variety of point-to-point techniques, with many smaller computers using dial-up connections . The UUCP store-and-forward protocols allowed a message (typically e-mail) to move across the collection of computers and eventually reach its destination. Late in the 20th century, store and forward techniques evolved into packet switching which replaced it for most purposes. FidoNet

5795-486: The development of internet infrastructure in the form of PPP extensions. IETF also establish principles and description standards that encompass the Internet protocol suite (TCP/IP). The Internet Architecture Board (IAB) along with the Internet Research Task Force (IRTF) counterpart the exertion of the IETF using innovative technologies. The IETF is the standards making organization concentrate on

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5890-553: The document has to be changed, it is submitted again and assigned a new RFC number. When an RFC becomes an Internet Standard (STD), it is assigned an STD number but retains its RFC number. When an Internet Standard is updated, its number is unchanged but refers to a different RFC or set of RFCs. For example, in 2007 RFC 3700 was an Internet Standard (STD 1) and in May 2008 it was replaced with RFC 5000. RFC 3700 received Historic status, and RFC 5000 became STD 1. The list of Internet standards

5985-448: The exchange.) After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its fully qualified domain name (FQDN), in this case smtp.example.com . The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN (or an address literal if none

6080-610: The first internet went live is January 1, 1983. The Transmission Control Protocol/Internet Protocol (TCP/IP) went into effect. ARPANET (Advanced Research Projects Agency Network) and the Defense Data Network were the networks to implement the Protocols. These protocols are considered to be the essential part of how the Internet works because they define the rules by which the connections between servers operate. They are still used today by implementing various ways data

6175-457: The functioning of the Internet and Internet-linked arrangements. In other words, Requests for Comments (RFCs) are primarily used to mature a standard network protocol that is correlated with network statements. Some RFCs are aimed to produce information while others are required to publish Internet standards. The ultimate form of the RFC converts to the standard and is issued with a numeral. After that, no more comments or variations are acceptable for

6270-528: The generation of "standard" stipulations of expertise and their envisioned usage. The IETF concentrates on matters associated with the progress of current Internet and TCP/IP know-how. It is alienated into numerous working groups (WGs), every one of which is accountable for evolving standards and skills in a specific zone, for example routing or security. People in working groups are volunteers and work in fields such as equipment vendors, network operators and different research institutions. Firstly, it works on getting

6365-452: The higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open. A typical example of sending a message via SMTP to two mailboxes ( alice and theboss ) located in the same mail domain ( example.com ) is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with S: and C: , for server and client , respectively; these labels are not part of

6460-511: The incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP),

6555-484: The intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A reject is a permanent failure and the client should send a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than delivery. The initiating host, the SMTP client, can be either an end-user's email client , functionally identified as

6650-510: The last two lines may actually be omitted. This causes an error on the server when trying to send the 221 Bye reply. Clients learn a server's supported options by using the EHLO greeting, as exemplified below, instead of the original HELO . Clients fall back to HELO only if the server does not support EHLO greeting. Modern clients may use the ESMTP extension keyword SIZE to query

6745-494: The message that they will try to deliver. The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes. The QUIT command ends the session. If the email has other recipients located elsewhere, the client would QUIT and connect to an appropriate SMTP server for subsequent recipients after

6840-446: The messages that are sent from the sender is first sent to these centers. If the destination address isn't available, then the center stores this message and tries sending it later. This improves the probability of the message to be delivered. In the other case, if the destination is available at that time, then the message is immediately sent. Store and forward networks predate the use of computers. Point-to-point teleprinter equipment

6935-540: The mid 1993 the first draft was published. HTTP HyperText Transfer Protocol is one of the most commonly used protocols today in the context of the World Wide Web. HTTP is a simple protocol to govern how documents, that are written in HyperText Mark Language(HTML) , are exchanged via networks. This protocol is the backbone of the Web allowing for the whole hypertext system to exist practically. It

7030-458: The most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent). The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack , spoofing , and spamming , and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server

7125-439: The network all the time. Both used a store and forward mechanism and are examples of push technology . Though Usenet's newsgroups were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the " bang paths " it used as message routing headers. Sendmail , released with 4.1cBSD in 1983, was one of the first mail transfer agents to implement SMTP. Over time, as BSD Unix became

7220-549: The older POP3 ) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync . SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication , encryption , binary data transfer, and internationalized email addresses . SMTP servers commonly use

7315-403: The organization from the outside , and relaying messages from the organization to the outside . But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become message submission agents for mail user agents , some of which were now relaying mail from the outside of an organization. (e.g. a company executive wishes to send email while on a trip using

7410-459: The parameters or sub-functions of TS protocols. An AS also describes the domains of applicability of TSs, such as Internet routers, terminal server, or datagram-based database servers. An AS also applies one of the following "requirement levels" to each of the TSs to which it refers: TCP/ IP Model & associated Internet Standards Web standards are a type of internet standard which define aspects of

7505-421: The process is called the Standards Track . If an RFC is part of a proposal that is on the Standards Track, then at the first stage, the standard is proposed and subsequently organizations decide whether to implement this Proposed Standard. After the criteria in RFC 6410 is met (two separate implementations, widespread use, no errata etc.), the RFC can advance to Internet Standard. The Internet Standards Process

7600-422: The recipient's domain (the part of the email address on the right of @ ). The MX record contains the name of the target MTA. Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange. Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be

7695-436: The responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power outage: Until the sender has received that 250 Ok reply, it must assume the message was not delivered. On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of

7790-444: The same network, enforcing this by firewalling to block access by users on the wider Internet. Or the server may perform range checks on the client's IP address. These methods were typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation. However, most of these bodies now use client authentication methods, as described below. Where

7885-469: The server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute. Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the HELO command with the EHLO command. Thus smtp2.example.com declares that it can accept

7980-429: The traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes / Domino . Webmail clients may use either method, but the retrieval protocol is often not a formal standard. SMTP defines message transport , not the message content . Thus, it defines the mail envelope and its parameters, such as the envelope sender , but not the header (except trace information ) nor

8075-418: The ultimate destination, an intermediate "relay" (that is, it stores and forwards the message) or a "gateway" (that is, it may forward the message using some protocol other than SMTP). Per RFC   5321 section 2.1, each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so. Once the final hop accepts

8170-412: Was an open mail relay . The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002. Because of spam concerns most email providers blocklist open relays, making original SMTP essentially impractical for general use on the Internet. In November 1995, RFC   1869 defined Extended Simple Mail Transfer Protocol (ESMTP), which established

8265-438: Was an email store-and-forward system for bulletin board systems that peaked at 45,000 systems with millions of users across the world. The system was highly efficient, using the latest file compression and file transfer systems to aggressively drive down the cost of transmission on what was largely a hobby network. The system was later modified to support public messages ( forums ) called EchoMail, which grew to about 8 MB

8360-446: Was an intermediate level, discontinued in 2011. A Draft Standard was an intermediary step that occurred after a Proposed Standard but prior to an Internet Standard. As put in RFC 2026: In general, an Internet Standard is a specification that is stable and well-understood, is technically competent, has multiple, independent, and interoperable implementations with substantial operational experience, enjoys significant public support, and

8455-599: Was apparent that one common way of encrypting data was needed so the IETF specified TLS 1.0 in RFC 2246 in January, 1999. It has been upgraded since. Last version of TLS is 1.3 from RFC 8446 in August 2018. OSI Model The Open Systems Interconnection model began its development in 1977. It was created by the International Organization for Standardization . It was officially published and adopted as

8550-589: Was conceived and realized by David P. Reed in 1980. Essentially the way it works is using compression to send information. Data would be compressed into a datagram and sent point to point. This proved to be a secure way to transmit information and despite the drawback of losing quality of data UDP is still in use. Becoming a standard is a two-step process within the Internet Standards Process: Proposed Standard and Internet Standard . These are called maturity levels and

8645-491: Was created by the team of developers spearheaded by Tim Berners-Lee . Berners-Lee is responsible for the proposal of its creation, which he did in 1989. August 6, 1991 is the date he published the first complete version of HTTP on a public forum. This date subsequently is considered by some to be the official birth of the World Wide Web. HTTP has been continually evolving since its creation, becoming more complicated with time and progression of networking technology. By default HTTP

8740-483: Was developed around the same time as Usenet , a one-to-many communication network with some similarities. SMTP became widely used in the early 1980s. At the time, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to

8835-558: Was originally published as STD 1 but this practice has been abandoned in favor of an online list maintained by the RFC Editor. The standardization process is divided into three steps: There are five Internet standards organizations: the Internet Engineering Task Force (IETF), Internet Society (ISOC), Internet Architecture Board (IAB), Internet Research Task Force (IRTF), World Wide Web Consortium (W3C). All organizations are required to use and express

8930-808: Was referenced by Jon Postel in his early work on Internet email. Postel first proposed an Internet Message Protocol in 1979 as part of the Internet Experiment Note (IEN) series. In 1980, Postel and Suzanne Sluizer published RFC   772 which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. RFC   780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP , an allocation that has since been removed by IANA . In November 1981, Postel published RFC   788 "Simple Mail Transfer Protocol". The SMTP standard

9025-437: Was used to send messages which were stored at the receiving end on punched paper tape at a relay center. A human operator at the center removed the message tape from the receiving machine, read the addressing information, and then sent it toward its destination on appropriate outbound point-to-point teleprinter link. If the outbound link was in use, the operator placed the message in tape in a physical queue, usually consisting of

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