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Encyclopedia > Simple Mail Transfer Protocol
The five-layer TCP/IP model
5. Application layer

DHCPDNSFTPGopherHTTPIMAP4IRCNNTPXMPPPOP3SIPSMTPSNMPSSHTELNETRPC • RTP • RTCPRTSPTLS/SSLSDPSOAPBGP • PPTP • L2TPGTPSTUNNTP • ... The TCP/IP model or Internet reference model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model, is a layered abstract description for communications and computer network protocol design. ... The application layer is the seventh level of the seven-layer OSI model. ... (DHCP) is a set of rules used by a communications device such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment. ... It has been suggested that this article be split into multiple articles. ... This article is about the File Transfer Protocol standardised by the IETF. For other file transfer protocols, see File transfer protocol (disambiguation). ... Gopher is a distributed document search and retrieval network protocol designed for the Internet. ... Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. ... The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on... “IRC” redirects here. ... The Network News Transfer Protocol or NNTP is an Internet application protocol used primarily for reading and posting Usenet articles, as well as transferring news among news servers. ... Extensible Messaging and Presence Protocol, or XMPP, is an open, XML-based protocol for near real-time extensible messaging and presence events. ... In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. ... The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ... The simple network management protocol (SNMP) forms part of the internet protocol suite as defined by the Internet Engineering Task Force (IETF). ... Secure Shell or SSH is a network protocol that allows data to be exchanged over a secure channel between two computers. ... This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Remote procedure call (RPC) is a protocol that allows a computer program running on one computer to cause a subroutine on another computer to be executed without the programmer explicitly coding the details for this interaction. ... The Real-time Transport Protocol (or RTP) defines a standardized packet format for delivering audio and video over the Internet. ... RTP Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP). ... The Real Time Streaming Protocol (RTSP), developed by the IETF and created in 1998 as RFC 2326, is a protocol for use in streaming media systems which allows a client to remotely control a streaming media server, issuing VCR-like commands such as play and pause, and allowing time-based... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... Session Description Protocol (SDP), is a format for describing streaming media initialization parameters. ... This article is about the computer protocol. ... The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. ... The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks. ... In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). ... GPRS Tunneling Protocol (or GTP) is an IP based protocol used within GSM and UMTS networks. ... This article is about the Internet protocol. ... The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. ...

4. Transport layer

TCPUDPDCCPSCTPRSVP • ... In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ...

3. Network/Internet Layer

IP (IPv4IPv6) • IGMPICMPOSPFISISIPsecARPRARPRIP • ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... It has been suggested that IPv6 internet be merged into this article or section. ... The Internet Group Management Protocol is a communications protocol used to manage the membership of Internet Protocol multicast groups. ... The (ICMP) is one of the core protocols of the Internet protocol suite. ... The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy. ... Intermediate system to intermediate system (IS-IS), is an IGP routing protocol originally designed for CLNS as part of the OSI protocol stack and described in ISO 10589 . ... IPsec (IP security) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. ... In computer networking, the Address Resolution Protocol (ARP) is the standard method for finding a hosts hardware address when only its network layer address is known. ... Reverse Address Resolution Protocol (RARP) is a network layer protocol used to obtain an IP address for a given hardware address (such as an Ethernet address). ... This article is chiefly about the Routing Information Protocol for IPv4 and IPv6. ...

2. Data link layer

802.11WiFiWiMAXATMDTMToken RingEthernetFDDIFrame RelayGPRS • EVDO • HSPA • HDLC • PPP • ... This article does not cite any references or sources. ... IEEE 802. ... Wi-Fi (or Wi-fi, WiFi, Wifi, wifi), short for Wireless Fidelity, is a set of standards for wireless local area networks (WLAN) currently based on the IEEE 802. ... WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Dynamic synchronous Transfer Mode , or DTM for short, is a network protocol. ... Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). ... In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. ... Evolution-Data Optimized or Evolution-Data only, abbreviated as EV-DO or EVDO and often EV, is one telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. ... High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing UMTS protocols. ... High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ...

1. Physical layer

Ethernet physical layerISDNModemsPLCSONET/SDHG.709OFDMOptical FiberCoaxial CableTwisted Pair • ... This article does not cite any references or sources. ... IEEE photograph of a diagram with the original terms for describing Ethernet drawn by Robert M. Metcalfe around 1976. ... // Integrated Services Digital Network (ISDN) is a circuit-switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than that is available with the PSTN system. ... A modem (from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. ... For other uses, see Power band. ... It has been suggested that this article be split into articles entitled Synchronous optical networking, SONET and Synchronous digital hierarchy. ... There are very few or no other articles that link to this one. ... Orthogonal frequency division modulation (OFDM, also called orthogonal frequency division multiplexing) is a technique for the modulation of digital information onto an analog carrier electromagnetic (e. ... Optical fibers An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. ... Coaxial Cable For the weapon, see coaxial weapon. ... 25 Pair Color Code Chart 10BASE-T UTP Cable Twisted pair cabling is a common form of wiring in which two conductors are wound around each other for the purposes of cancelling out electromagnetic interference known as crosstalk. ...

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Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821. De facto is a Latin expression that means in fact or in practice. It is commonly used as opposed to de jure (meaning by law) when referring to matters of law or governance or technique (such as standards), that are found in the common experience as created or developed without... Wikipedia does not yet have an article with this exact name. ... Extended SMTP (ESMTP) is a definition of protocol extensions to the Simple Mail Transfer Protocol standard. ...

Contents

Description

SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is a client-server protocol, where the client transmits an email message to the server. Either an end-user's email client, a.k.a. MUA (Mail User Agent), or a relaying server's MTA (Mail Transfer Agents) can act as an SMTP client. Usually used in reference to a computer application, especially a computer game, a text-based application is one whose primary input and output are based on text rather than graphics. ... Client/Server is a network application architecture which separates the client (usually the graphical user interface) from the server. ... In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ... This article does not cite any references or sources. ... An email client (or mail user agent [MUA]) is a computer program that is used to read and send e-mail. ... A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. ...


An email client knows the outgoing mail SMTP server from its configuration. A relaying server typically determines which SMTP server to connect to by looking up the MX (Mail eXchange) DNS record for each recipient's domain name (the part of the email address to the right of the at (@) sign). Conformant MTAs (not all) fall back to a simple A record in the case of no MX. Some current mail transfer agents will also use SRV records, a more general form of MX, though these are not widely adopted. (Relaying servers can also be configured to use a smart host.) An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... It has been suggested that this article be split into multiple articles. ... The term domain name has multiple related meanings: A name that identifies a computer or computers on the internet. ... An e-mail address identifies a location to which e-mail can be delivered. ... The Domain Name System or DNS is a system that stores information about host names and domain names in a kind of distributed database on networks, such as the Internet. ... An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. ... An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... A smart host is a type of mail relay server which allows an SMTP server to route e-mail to an intermediate mail server rather than directly to the recipient’s server. ...


The SMTP client initiates a TCP connection to server's port 25 (unless overridden by configuration). It is quite easy to test an SMTP server using the telnet program (see below). The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... It has been suggested that this article or section be merged into Computer port (software). ... This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ...


SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN. This article does not cite any references or sources. ... In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. ... The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on... ETRN (Extended Turn) is an extension to the SMTP e-mail protocol. ...


History

Forms of one-to-one electronic messaging were used in the 1960s. People communicated with one another using systems developed for a particular mainframe computer. As more computers began to be interconnected with others, especially in the US Government's ARPANET, standards were developed to allow users using different systems to be able to email one another. SMTP grew out of these standards developed during the 1970s. Electronic messaging may refer to: One to One communication Instant message (on a computer network) Personal message (on a computer network) Text message (on a cellular phone network) SMTP (on a computer network) email (on a computer Network) Voicemail (using the PSTN) Fax (using the PSTN) Pager (using the PSTN... The 1960s decade refers to the years from January 1, 1960 to December 31, 1969, inclusive. ... Mainframe may refer to one of the following: Mainframe computer, large data processing systems Mainframe Entertainment, a Canadian computer animation and design company. ... ARPANET logical map, March 1977. ... E-mail, or email, is short for electronic mail and is a method of composing, sending, and receiving messages over electronic communication systems. ... The 1970s decade refers to the years from 1970 to 1979, also called The Seventies. ...


SMTP can trace its roots to the Mail Box Protocol (ca. 1971), FTP Mail (ca. 1973),[1] and Mail Protocol.[2] The work continued throughout the 1970s, until the ARPANET converted into the modern Internet around 1980. Jon Postel then proposed a Mail Transfer Protocol in 1980 that began to remove the mail's reliance on FTP.[3] SMTP was published as RFC 821 in 1982, also by Jonathan Postel. Year 1971 (MCMLXXI) was a common year starting on Friday (link will display full calendar) of the 1971 Gregorian calendar. ... The 1970s decade refers to the years from 1970 to 1979, also called The Seventies. ... ARPANET logical map, March 1977. ... Year 1980 (MCMLXXX) was a leap year starting on Tuesday (link displays the 1980 Gregorian calendar). ... Jon Postel (Photo by Irene Fertik, USC News Service. ... The abbreviation FTP can refer to: The File Transfer Protocol used on the Internet. ... Year 1982 (MCMLXXXII) was a common year starting on Friday (link displays the 1982 Gregorian calendar). ...


The SMTP standard was developed around the same time the Usenet was, a one-to-many communication network with some similarities. Usenet (USEr NETwork) is a global, decentralized, distributed Internet discussion system that evolved from a general purpose UUCP architecture of the same name. ...


SMTP became widely used in the early 1980s. At the time, it was a complement to UUCP (Unix to Unix CoPy) mail, which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both use a store and forward mechanism and are examples of push technology. Usenet's newsgroups are still propagated with UUCP between servers[4], but UUCP mail has virtually disappeared[5] along with the "bang paths" it used as message routing headers. The 1980s refers to the years from 1980 to 1989. ... UUCP stands for Unix to Unix CoPy. ... Store and forward is a communications technique in which messages are sent to a intermediate station where they are kept and sent at a later time to the final destination or to another intermediate station. ... It has been suggested that this article or section be merged with Push media. ... A newsgroup is a repository, usually within the Usenet system, for messages posted from many users at different locations. ... UUCP stands for Unix to Unix CoPy. ... UUCP stands for Unix to Unix Copy Protocol, and is a computer program and protocol allowing remote execution of commands and transfer of files, email and netnews between Unix computers not connected to the Internet proper. ...


The article about sender rewriting contains technical background info about the early SMTP history and source routing before RFC 1123. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...


Sendmail was one of the first (if not the first) mail transfer agents to implement SMTP. Some other popular SMTP server programs include Postfix, qmail, Novell GroupWise, Exim, Novell NetMail, Microsoft Exchange Server and Sun Java System Messaging Server. As of 2001 there were at least 50 programs that implemented SMTP either as clients (senders of messages) or as servers (receivers of messages). Sendmail is a mail transfer agent (MTA) that is a well known project of the open source, free software and Unix communities, which is distributed both as free software and proprietary software. ... Postfix is a free software / open source mail transfer agent (MTA), a computer program for the routing and delivery of email. ... qmail is a mail transfer agent that runs on Unix. ... GroupWise is a cross-platform collaborative software product from Novell, Inc. ... Exim is a mail transfer agent (MTA) used in Unix-like operating systems. ... Novell NetMail is an ISP-grade E-Mail package by Novell, Inc. ... Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. ... The Sun Java Messaging Server is Suns highly scalable messaging (e-mail) server. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ...


Message Submission (RFC 2576) 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 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 Mail 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 the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web, meant that the SMTP protocol had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as unsolicited email (spam) relaying. A mail submission agent or MSA is a computer program or software agent which receives electronic mail messages from a mail user agent (MUA) and contacts an mail transfer agent (MTA) for delivery of the mail. ... SMTP-AUTH extends SMTP (the Internet e-mail transmission protocol) to include an authentication step through which the client effectively logs in to the mail server during the process of sending mail. ... A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. ... A mail submission agent or MSA is a computer program or software agent which receives electronic mail messages from a mail user agent (MUA) and contacts an mail transfer agent (MTA) for delivery of the mail. ... An email client (or mail user agent [MUA]) is a computer program that is used to read and send email. ... WWWs historical logo designed by Robert Cailliau The World Wide Web (commonly shortened to the Web) is a system of interlinked, hypertext documents accessed via the Internet. ... Look up spam, SPAM in Wiktionary, the free dictionary. ...


Since this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP. MTAs developed after Sendmail also tended to be implemented 8-bit-clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary data via SMTP. Non-8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text. Image:ASCII fullsvg There are 95 printable ASCII characters, numbered 32 to 126. ... Multipurpose Internet Mail Extensions (MIME) is an Internet Standard that extends the format of e-mail to support: text in character sets other than US-ASCII; non-text attachments; multi-part message bodies; and header information in non-ASCII character sets. ... A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. ... Sendmail is a mail transfer agent (MTA) that is a well known project of the open source, free software and Unix communities, which is distributed both as free software and proprietary software. ... 8BITMIME (RFC 1652) is an SMTP extension standardized in 1994 that facilitates the exchange of e-mail messages containing octets outside the seven-bit ASCII range. ...


Developers

Many people edited or contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore. Jon Postel (Photo by Irene Fertik, USC News Service. ... Eric Allman (born 1959) is a computer programmer. ... Ned Freed is the author or co-author of several IETF RFCs, most relating to e-mail or security. ... Keith Moore (born 12 October 1960) is the author and co-author of several IETF RFCs related to the MIME and SMTP protocols for electronic mail, among others: RFC 1870, defining a mechanism to allow SMTP clients and servers to avoid transferring messages so large that they will be rejected...


Outgoing mail SMTP server

An email client requires the name or the IP address of an SMTP server as part of its configuration. The server will deliver messages on behalf of the user. This setting allows for various policies and network designs. End users connected to the Internet can use the services of an e-mail provider that is not necessarily the same as their connection provider. Network topology, or the location of a client within a network or outside of a network, is no longer a limiting factor for email submission or delivery. Modern SMTP servers typically use a client's credentials (authentication) rather than a client's location (IP address), to determine whether it is eligible to relay email. An email client (or mail user agent [MUA]) is a computer program that is used to read and send e-mail. ... Authentication (from Greek αυθεντικός; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true. ... This article or section does not cite any references or sources. ...


Another choice is whether to use TCP port 25 (SMTP) or port 587 (Submission), as established by RFC 2476, for relaying outbound mail to a mail server. Many servers support both. Some servers still support port 465 for legacy secure SMTP, but it is preferable to use encryption on standard ports according to RFC 2487. Some servers are setup to reject all relaying on port 25, but valid users authenticating on port 587 are allowed to relay mail to any valid address. A server that relays all email for all destinations for all clients connecting to port 25 is known as an open relay and is generally considered a bad practice worthy of blacklisting. TCP is an abbreviation of all of: Transmission Control Protocol Thermal conversion process Top Cow Productions Tool Center Point of a robot A number of chemical substances: Trichlorophenol, a fungicide Trichlorophenylmethyliodosalicyl, a germicide, see TCP (antiseptic) Tricresylphosphate, a lubricant, gasoline additive, plasticizer, and flame retardant Thienylcyclohexylpiperidine, which has been sold... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... An open proxy is an Internet proxy server which is accessible by unauthorized users, specifically those from elsewhere on the internet. ... A blacklist is a list or register of people who, for one reason or another, are being denied a particular privilege, service, or mobility. ...


Sample communications

After establishing a connection between the sender (the client) and the receiver (the server), the following is a legal SMTP session. In the following conversation, everything sent by the client is prefaced with C: and everything sent by the server is prefaced with S:. On most computer systems, a connection can be established using the telnet command on the client machine, for example. This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ...

telnet www.example.com 25

which opens a TCP connection from the sending machine to the MTA listening on port 25 on host www.example.com.

 S: 220 www.example.com ESMTP Postfix C: HELO mydomain.com S: 250 Hello mydomain.com C: MAIL FROM:<sender@mydomain.com> S: 250 Ok C: RCPT TO:<friend@example.com> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: Subject: test message C: From: sender@mydomain.com C: To: friend@example.com C: C: Hello, C: This is a test. C: Goodbye. C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye 

Please note that the data the client sends in the HELO and MAIL FROM commands can be retrieved in additional headers that the server adds to the message: Received and Return-Path respectively.


Although optional and not shown above, nearly all clients ask the server which SMTP extensions the server supports, by using the EHLO greeting to invoke Extended SMTP (ESMTP). These clients use HELO only if the server does not respond to EHLO. Extended SMTP (ESMTP) is a definition of protocol extensions to the Simple Mail Transfer Protocol standard. ...


Contemporary clients will use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted. Older clients and servers will try to transfer huge messages that will be rejected after wasting the network resources, including a lot of connect time to dialup ISPs that are paid by the minute.


For the edit planning of giant files or sending with older clients, users can manually determine in advance the maximum size accepted by ESMTP servers. The user telnets as above, but substitutes "EHLO mydomain.com" for the HELO command line.

 S: 220-serverdomain.com ESMTP {postfix version and date} S: 220 NO UCE. {etc., terms of service} C: EHLO mydomain.com S: 250-serverdomain.com Hello mydomain.com [127.0.0.1] S: 250-SIZE 14680064 S: 250-PIPELINING S: 250 HELP 

This serverdomain.com declares that it will accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). Depending on the server's actual resource usage, it may be currently unable to accept a message this large. In computer technology and networking, an octet is a group of 8 bits. ...


In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction. If no number appears after the SIZE keyword, or if the current message limit must be exactly determined, the user can further interact by simulating the ESMTP header of a message with an estimated size. See External Link RFC 1870 below.


Security and spamming

Main article: Anti-spam techniques (e-mail)

One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it. To prevent e-mail spam, both end users and administrators of e-mail systems use various anti-spam techniques. ... SMTP-AUTH extends SMTP (the Internet e-mail transmission protocol) to include an authentication step through which the client effectively logs in to the mail server during the process of sending mail. ... A KMail folder full of spam emails collected over a few days. ...


Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 is one such proposal for replacement. A network effect is a characteristic that causes a good or service to have a value to a potential customer which depends on the number of other customers who own the good or are users of the service. ... Internet Mail 2000 is a new Internet mail architecture proposed by Daniel J. Bernstein (and in subsequent years separately proposed by several others), designed with the precept that the initial storage of mail messages be the responsibility of the sender, and not of the recipient as it is with the...


Spam is enabled by several factors, including vendors implementing broken MTAs (that do not adhere to standards, and therefore make it difficult for other MTAs to enforce standards), security vulnerabilities within the operating system (often exacerbated by always-on broadband connections) that allow spammers to remotely control end-user PCs and cause them to send spam, and a regrettable lack of "intelligence" in many MTAs. A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. ... Look up spam, SPAM in Wiktionary, the free dictionary. ...


There are a number of proposals for sideband protocols that will assist SMTP operation. The Anti-Spam Research Group (ASRG) of the Internet Research Task Force (IRTF) is working on a number of E-mail authentication and other proposals for providing simple source authentication that is flexible, lightweight, and scalable. Recent Internet Engineering Task Force (IETF) activities include MARID (2004) leading to two approved IETF experiments in 2005, and DomainKeys Identified Mail in 2006. The Anti-Spam Research Group or ASRG is a research group within the IRTF dedicated to research into curbing spam on an Internet-wide level. ... The Internet Research Task Force (IRTF) is a sister group to the Internet Engineering Task Force (IETF). ... Ensuring a valid identity on an e-mail has become a vital first step in stopping spam, forgery, fraud, and even more serious crimes. ... The Internet Engineering Task Force (IETF) develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standard bodies; and dealing in particular with standards of the TCP/IP and Internet protocol suite. ... A Marid (Arabic : مارد ) in common mythology is a djinn related to the element of water. ... DomainKeys Identified Mail is a method for E-mail authentication. ...


Other Protocols for Email

Email is "handed off" (pushed) from a client (MUA) to a mail server (MSA), usually using Simple Mail Transfer Protocol or IMAP. From there, the MSA delivers the mail to a MTA, usually running on the same machine. The MTA looks up the destination(s)'s MX records with a DNS lookup, and begins to relay (push) the message to the server on record via TCP port 25 and SMTP. Once the receiving MTA accepts the incoming message, it is delivered via Mail delivery agent to a server which is designed for local mail delivery. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Generally speaking, mail retrieval (pull) is performed using either a type of online folders (e.g. IMAP 4, a protocol that both delivers and organizes mail) or the older single repository format (e.g. POP3, the Post Office Protocol). Webmail clients may use either method, but the retrieval protocol is often not a formal standard. Some local mail servers and MUAs are capable of either push or pull mail retrieval. The Internet Message Access Protocol (commonly known as IMAP, and previously called Interactive Mail Access Protocol) is an application layer Internet protocol used for accessing email on a remote server from a local client. ... An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... DNS may refer to: Look up DNS in Wiktionary, the free dictionary. ... TCP is an abbreviation of all of: Transmission Control Protocol Thermal conversion process Top Cow Productions Tool Center Point of a robot A number of chemical substances: Trichlorophenol, a fungicide Trichlorophenylmethyliodosalicyl, a germicide, see TCP (antiseptic) Tricresylphosphate, a lubricant, gasoline additive, plasticizer, and flame retardant Thienylcyclohexylpiperidine, which has been sold... A Mail Delivery Agent (MDA) is software that accepts incoming e-mail messages and distributes them to recipients individual mailboxes (if the destination account is on the local machine), or forwards back to an SMTP server (if the destination is on a remote server). ... The Internet Message Access Protocol (commonly known as IMAP, and previously called Interactive Mail Access Protocol) is an application layer Internet protocol used for accessing email on a remote server from a local client. ... Post Office Protocol version 3 (POP3) is an application layer Internet standard protocol used to retrieve email from a remote server to a local client over a TCP/IP connection. ... Webmail is a class of web applications that allow users to read and write e-mail using a web browser, or in a more general sense, an e-mail account accessed through such an application. ...


References

  1. ^ RFC 469 - Network Mail Meeting Summary
  2. ^ RFC 524 - A Proposed Mail Protocol
  3. ^ RFC 772 - Mail Transfer Protocol
  4. ^ http://tldp.org/HOWTO/Usenet-News-HOWTO/x64.html
  5. ^ http://tools.ietf.org/html/draft-barber-uucp-project-conclusion-05

Related Requests For Comments (RFCs)

  • RFC 3700 Internet Official Protocol Standards (STD 1). As of 2004, this RFC Designates RFC 821 and RFC 822 as the SMTP and MAIL standards, respectively, with RFC 2821 and RFC 2822 as proposed standards. However, in practice the newer "proposed standards" are said to "obsolete" the original.
  • RFC 821 (official standard) Simple Mail Transfer Protocol
  • RFC 822 (official standard) Standard for the Format of ARPA Internet Text Messages
  • RFC 1123 Requirements for Internet Hosts -- Application and Support (STD 3)
  • RFC 1870 SMTP Service Extension for Message Size Declaration (оbsoletes: RFC 1653)
  • RFC 2505 Anti-Spam Recommendations for SMTP MTAs (BCP 30)
  • RFC 2554 SMTP Service Extension for Authentication
  • RFC 2821 The Simple Mail Transfer Protocol (obsoletes RFC 821 aka STD 10, RFC 974, and RFC 1869)
  • RFC 2822 Internet Message Format (obsoletes RFC 822 aka STD 11)
  • RFC 2920 SMTP Service Extension for Command Pipelining (STD 60)
  • RFC 3030 SMTP Service Extensions for Transmission of Large and Binary MIME Messages
  • RFC 3207 SMTP Service Extension for Secure SMTP over Transport Layer Security (obsoletes RFC 2487)
  • RFC 3461 SMTP Service Extension for Delivery Status Notifications (obsoletes RFC 1891)
  • RFC 3462 The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages (obsoletes RFC 1892)
  • RFC 3463 Enhanced Status Codes for SMTP (obsoletes RFC 1893 )
  • RFC 3464 An Extensible Message Format for Delivery Status Notifications (obsoletes RFC 1894)
  • RFC 3552 Guidelines for Writing RFC Text on Security Considerations (contains SMTP example)
  • RFC 3834 Recommendations for Automatic Responses to Electronic Mail
  • RFC 4409 Message Submission for Mail (obsoletes RFC 2476)

See also

A bounce message, or Delivery Status Notification (DSN) message or, simply, a bounce is an automated electronic mail message from the receivers mail system, the message tells the sender that the message could not be delivered. ... An E-Mail Loop is an infinite loop phenomenon created by mail servers, scripts, or mail reading programs generating automatic replies or responses, to which each automatic response generates another automatic response and so on. ... Ensuring a valid identity on an e-mail has become a vital first step in stopping spam, forgery, fraud, and even more serious crimes. ... SMTP-AUTH extends SMTP (the Internet e-mail transmission protocol) to include an authentication step through which the client effectively logs in to the mail server during the process of sending mail. ... Extended SMTP (ESMTP) is a definition of protocol extensions to the Simple Mail Transfer Protocol standard. ... QMTP is the Quick Mail Trasport Protocol that works faster than SMTP. It was designed and implemented by DJB. QMTP has fewer round trips than SMTP and is suitable for low bandwidth networks such as serial links. ... Daniel Julius Bernstein (sometimes known simply as djb; born October 29, 1971) is a professor at the University of Illinois at Chicago, a mathematician, a cryptologist, and a programmer. ... This article is about the Internet protocol. ... In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). ... The Domain Name System Security Extensions (DNSSEC) are a suite of IETF specifications for securing certain kinds of information provided by the Domain Name System (DNS) as used on Internet Protocol (IP) networks. ... There are very few or no other articles that link to this one. ...

External links

  • 2821bis draft (2005)
  • 2821 security draft (2005)
  • Internet Mail Architecture draft (2005)
  • Email Submission Access and Accountability draft (2005)
  • Essential Internet Protocols - SMTP

cr.yp.to links

Other links

  • SMTP Sequence Diagram (PDF)
  • Diagram of e-mail flow (PDF, PNG )
  • Troubleshooting SMTP in Thunderbird Includes information on getting around port 25 blocking.
  • The Case For Email Security - Security and Insecurity in SMTP, POP and IMAP.
  • RFC 2821/1123: SMTP Reply Codes
  • Levent Ozturk's Email protocol, additional proposed SMTP commands

  Results from FactBites:
 
RFC 2821 (rfc2821) - Simple Mail Transfer Protocol (18604 words)
SMTP clients that transfer all traffic, regardless of the target domain names associated with the individual messages, or that do not maintain queues for retrying message transmissions that initially cannot be completed, may otherwise conform to this specification but are not considered fully-capable.
The SMTP protocol allows a server to formally reject a transaction while still allowing the initial connection as follows: a 554 response MAY be given in the initial connection opening message instead of the 220.
SMTP clients that experience a connection close, reset, or other communications failure due to circumstances not under their control (in violation of the intent of this specification but sometimes unavoidable) SHOULD, to maintain the robustness of the mail system, treat the mail transaction as if a 451 response had been received and act accordingly.
Simple Mail Transfer Protocol - Wikipedia, the free encyclopedia (1373 words)
SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred.
To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is used, falling back to a simple A record in the case of no MX (not all MTAs (Mail Transfer Agents) support fallback).
RFC 2821 The Simple Mail Transfer Protocol (obsoletes RFC 821 aka STD 10, RFC 974, and RFC 1869)
  More results at FactBites »

 
 

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