Smtp; 451 4.3.2 Please Try Again Later

Internet protocol used for relaying e-mails

"SMTP" redirects here. For the email delivery company, come across SMTP (visitor). For Short Bulletin Transfer Protocol, meet GSM 03.40.

The Unproblematic Mail Transfer Protocol (SMTP) is an internet standard advice protocol for electronic mail transmission. Mail servers and other message transfer agents employ SMTP to transport and receive mail letters. User-level e-mail clients typically apply SMTP only for sending messages to a post server for relaying, and typically submit approachable e-mail to the mail server on port 587 or 465 per RFC 8314. For retrieving letters, IMAP (which replaced the older POP3) is standard, but proprietary servers likewise often implement proprietary protocols, e.chiliad., Exchange ActiveSync.

Since SMTP's introduction in 1981, information technology has been updated, modified and extended multiple times. The protocol version in common utilise today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized e-mail addresses. SMTP servers usually use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications).

History [edit]

Predecessors to SMTP [edit]

Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems adult for specific mainframe computers. Every bit more computers were interconnected, especially in the U.S. Authorities'south ARPANET, standards were developed to allow exchange of messages betwixt dissimilar operating systems. SMTP grew out of these standards developed during the 1970s.

SMTP traces its roots to two implementations described in 1971: the Mail Box Protocol, whose implementation has been disputed,^[1] merely is discussed in RFC 196 and other RFCs, and the SNDMSG program, which, according to RFC 2235, Ray Tomlinson of BBN invented for TENEX computers to transport mail letters across the ARPANET.^{[2] [3] [four]} Fewer than 50 hosts were connected to the ARPANET at this fourth dimension.^[5]

Further implementations include FTP Mail^[vi] and Mail service Protocol, both from 1973.^[7] Development work continued throughout the 1970s, until the ARPANET transitioned into the modernistic Internet effectually 1980.

Original SMTP [edit]

In 1980, Jon Postel published RFC 772 which proposed the Mail Transfer Protocol as a replacement of the use of the File Transfer Protocol (FTP) for mail. RFC 780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP^{[ citation needed ]}, an resource allotment that has since been removed by IANA. In November 1981, Postel published RFC 788 "Simple Postal service Transfer Protocol".

The SMTP standard was developed around the aforementioned time as Usenet, a one-to-many communication network with some similarities.^{[ citation needed ]}

SMTP became widely used in the early 1980s. At the fourth dimension, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling e-mail transfers betwixt 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 used a store and frontward machinery and are examples of push technology. Though Usenet's newsgroups were still propagated with UUCP betwixt servers,^[8] UUCP every bit a mail transport has about disappeared^[9] forth with the "blindside paths" it used as bulletin routing headers.^[10]

Sendmail, released with four.1cBSD in 1982, soon afterwards RFC 788 was published in Nov 1981, was one of the first postal service transfer agents to implement SMTP.^[11] Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer amanuensis).^[12]

The original SMTP protocol supported just 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 pattern every SMTP server was an open mail relay. The Internet Mail Consortium (IMC) reported that 55% of post servers were open relays in 1998,^[xiii] simply less than 1% in 2002.^[14] Because of spam concerns most email providers blocklist open up relays,^[15] making original SMTP essentially impractical for full general use on the Internet.

Modern SMTP [edit]

In November 1995, RFC 1869 defined Extended Simple Mail Transfer Protocol (ESMTP), which established 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 exist identified and servers can bespeak supported extensions.

Message submission (RFC 2476) and SMTP-AUTH (RFC 2554) were introduced in 1998 and 1999, both describing new trends in electronic mail delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for the organization from the outside, and relaying messages from the system to the exterior. Merely equally time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to get bulletin submission agents for Mail service user agents, some of which were at present relaying mail from the exterior of an organization. (e.g. a company executive wishes to ship electronic mail while on a trip using the corporate SMTP server.) This event, a consequence of the rapid expansion and popularity of the Www, meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to forbid abuses such every bit relaying of unsolicited email (spam). Work on bulletin submission (RFC 2476) was originally started considering popular mail servers would oft rewrite post in an try to set issues in information technology, for case, calculation a domain proper 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 style to let and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, information technology was also seen as a way to provide authorization for mail being sent out from an system, also equally traceability. This separation of relay and submission apace became a foundation for modern electronic mail security practices.

As this protocol started out purely ASCII text-based, it did not deal well with binary files, or characters in many not-English languages. Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) adult after Sendmail as well tended to be implemented eight-bit-make clean, and so that the alternate "just send eight" strategy could be used to transmit capricious text data (in any eight-bit ASCII-similar character encoding) via SMTP. Mojibake was still a problem due to differing character fix mappings between vendors, although the email addresses themselves still immune only ASCII. eight-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to exist transmitted about as easily as patently text (limits on line length and permitted octet values all the same apply, and so that MIME encoding 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.

Mail processing model [edit]

Blue arrows depict implementation of SMTP variations

Email is submitted by a mail client (mail user amanuensis, MUA) to a mail server (mail submission amanuensis, MSA) using SMTP on TCP port 587. Well-nigh mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (mail service 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 a unmarried car, or split among multiple machines; mail service agent processes on i machine tin share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each auto is configured to apply the next machine equally a smart host. Each process is an MTA (an SMTP server) in its own right.

The purlieus MTA uses DNS to wait up the MX (postal service exchanger) record for the recipient's domain (the part of the email address on the correct 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 the ultimate destination, an intermediate "relay" (that is, information technology stores and forrard the message) or a "gateway" (that is, it may forwards the message using some protocol other than SMTP). Per RFC 5321 department two.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 the incoming message, it hands information technology to a postal service delivery amanuensis (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception tin be washed using one or multiple computers, simply in the diagram above the MDA is depicted as i 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 every bit Local Mail Transfer Protocol (LMTP), a derivative of SMTP designed for this purpose.

Once delivered to the local mail server, the mail service is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by stop-user applications, called e-mail clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates admission to mail service and manages stored post, or the Post Office Protocol (Popular) which typically uses the traditional mbox mail file format or a proprietary system such equally Microsoft Exchange/Outlook or Lotus Notes/Domino. Webmail clients may use either method, only the retrieval protocol is often not a formal standard.

SMTP defines message send, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header (except trace data) nor the torso of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD xi and RFC 5322 define the message (header and body), formally referred to equally the Cyberspace Message Format.

Protocol overview [edit]

SMTP is a connexion-oriented, text-based protocol in which a mail service sender communicates with a mail service receiver by issuing command strings and supplying necessary data over a reliable ordered data stream aqueduct, typically a Manual 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:

1. Postal service command, to plant the return address, also called return-path,^[xvi] reverse-path,^[17] bounciness accost, mfrom, or envelope sender.
2. RCPT command, to establish a recipient of the message. This command tin be issued multiple times, one for each recipient. These addresses are also part of the envelope.
3. Data to signal the beginning of the message text; the content of the bulletin, every bit opposed to its envelope. It consists of a message header and a message body separated by an empty line. Data is actually a group of commands, and the server replies twice: in one case to the DATA command itself, to acknowledge that it is set up to receive the text, and the second fourth dimension afterwards the end-of-data sequence, to either take or reject the entire message.

Too the intermediate reply for DATA, each server's reply tin be either positive (2xx answer codes) or negative. Negative replies tin can be permanent (5xx codes) or transient (4xx codes). A refuse is a permanent failure and the client should transport a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than commitment.

The initiating host, the SMTP customer, tin exist either an end-user's e-mail customer, functionally identified as a mail user agent (MUA), or a relay server'south post transfer amanuensis (MTA), that is an SMTP server acting as an SMTP customer, in the relevant session, in guild 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 upward the MX (Mail service eXchange) DNS resource record for each recipient'southward domain name. If no MX record is institute, a conformant relaying server (not all are) instead looks upward 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 principal departure between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication.

SMTP vs mail retrieval [edit]

SMTP is a commitment protocol only. In normal use, postal service is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, non the individual user(due south) to which it is addressed. Other protocols, such equally the Postal service Role Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by private users retrieving messages and managing post boxes. To permit an intermittently-connected mail service server to pull messages from a remote server on need, SMTP has a feature to initiate mail service queue processing on a remote server (run across Remote Message Queue Starting below). Popular and IMAP are unsuitable protocols for relaying mail by intermittently-continued machines; they are designed to operate after final delivery, when information critical to the right operation of mail relay (the "postal service envelope") has been removed.

Remote Message Queue Starting [edit]

Remote Bulletin Queue Starting enables a remote host to starting time processing of the postal service queue on a server then it may receive messages destined to it by sending a corresponding command. The original TURN command was accounted insecure and was extended in RFC 1985 with the ETRN control which operates more deeply using an authentication method based on Domain Proper noun System data.^[xviii]

Outgoing mail SMTP server [edit]

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 a DNS name). This server will evangelize outgoing letters on behalf of the user.

Approachable postal service server access restrictions [edit]

Server administrators need to impose some control on which clients can use the server. This enables them to bargain with corruption, for example spam. Two solutions have been in common utilize:

• In the past, many systems imposed usage restrictions by the location of the client, just permitting usage by clients whose IP address is one that the server administrators control. Usage from whatever other client IP accost is disallowed.
• Mod SMTP servers typically offer an alternative organisation that requires authentication of clients by credentials before allowing admission.

Restricting access by location [edit]

Under this system, an ISP'southward SMTP server will not allow access by users who are outside the Isp's network. More precisely, the server may merely allow access to users with an IP address provided by the Internet access provider, which is equivalent to requiring that they are connected to the Cyberspace using that same Isp. A mobile user may oft be on a network other than that of their normal Internet access provider, and will then find that sending email fails because the configured SMTP server pick is no longer accessible.

This system has several variations. For case, an organization'south SMTP server may only provide service to users on 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 post simply for use internally within the organisation. Withal, most of these bodies now apply customer authentication methods, as described below.

Where a user is mobile, and may use different ISPs to connect to the internet, this kind of usage brake is onerous, and altering the configured outbound electronic mail SMTP server address is impractical. It is highly desirable to exist able to use email client configuration data that does not need to change.

Client authentication [edit]

Modernistic SMTP servers typically require hallmark of clients by credentials before allowing access, rather than restricting access past location as described before. This more flexible arrangement is friendly to mobile users and allows them to have a fixed selection of configured outbound SMTP server. SMTP Hallmark, often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication machinery.

Ports [edit]

Communication betwixt post servers more often than not uses the standard TCP port 25 designated for SMTP.

Mail clients however by and large don't utilise this, instead using specific "submission" ports. Post services more often than not have email submission from clients on i of:

• 587 (Submission), as formalized in RFC 6409 (previously RFC 2476)
• 465 This port was deprecated later on RFC 2487, until the issue of RFC 8314.

Port 2525 and others may be used by some individual providers, only take never been officially supported.

Many Net service providers now block all outgoing port 25 traffic from their customers. Mainly equally an anti-spam measure,^[19] just likewise to cure for the college price they take when leaving information technology open up, perhaps by charging more from the few customers that crave it open.

SMTP transport case [edit]

A typical case of sending a message via SMTP to two mailboxes (alice and theboss) located in the aforementioned mail domain (example.com) is reproduced in the following session exchange. (In this instance, the conversation parts are prefixed with Southward: and C:, for server and client, respectively; these labels are non part of the exchange.)

After the bulletin sender (SMTP client) establishes a reliable communications channel to the bulletin receiver (SMTP server), the session is opened with a greeting past 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 is bachelor).^[20]

          South: 220 smtp.case.com ESMTP Postfix          C: HELO relay.example.org          S: 250 Hi relay.example.org, I am glad to encounter you          C: MAIL FROM:<bob@example.org>          Due south: 250 Ok          C: RCPT TO:<alice@example.com>          S: 250 Ok          C: RCPT TO:<theboss@example.com>          S: 250 Ok          C: DATA          South: 354 End data with <CR><LF>.<CR><LF>          C: From: "Bob Example" <bob@example.org> C: To: "Alice Instance" <alice@instance.com> C: Cc: theboss@case.com C: Date: Tue, 15 Jan 2008 xvi:02:43 -0500 C: Subject: Exam message C: C: Hello Alice. C: This is a examination message with 5 header fields and 4 lines in the bulletin body. C: Your friend, C: Bob C: .          South: 250 Ok: queued equally 12345          C: QUIT          S: 221 Bye          {The server closes the connectedness}        

The client notifies the receiver of the originating e-mail address of the message in a MAIL FROM control. This is also the return or bounce accost in instance the message cannot be delivered. In this instance the e-mail bulletin is sent to 2 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 a command is best-selling by the server with a result lawmaking and response message (e.g., 250 Ok).

The transmission of the body of the mail message is initiated with a Information command after which information technology is transmitted verbatim line past 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 a message torso tin can contain a line with just a menstruum as role of the text, the customer sends two periods every fourth dimension a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a unmarried one. Such escaping method is chosen dot-stuffing.

The server's positive respond to the finish-of-data, as exemplified, implies that the server has taken the responsibleness of delivering the message. A message tin exist doubled if there is a communication failure at this time, e.g. due to a ability shortage: Until the sender has received that 250 Ok reply, it must assume the bulletin was non delivered. On the other hand, after the receiver has decided to accept the message, it must assume the bulletin has been delivered to information technology. Thus, during this time span, both agents have agile copies of the message that they volition try to deliver.^[21] 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, virtually ofttimes for anti-spam purposes. The limiting timeout is specified to be 10 minutes.^[22]

The QUIT command ends the session. If the email has other recipients located elsewhere, the client would QUIT and connect to an advisable SMTP server for subsequent recipients after the current destination(s) had been queued. The information that the customer sends in the HELO and MAIL FROM commands are added (not seen in example code) equally 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 connectedness later on the bulletin is accustomed (250 Ok: queued as 12345), then the last two lines may actually exist omitted. This causes an mistake on the server when trying to send the 221 Adieu reply.

SMTP Extensions [edit]

Extension discovery mechanism [edit]

Clients larn a server's supported options by using the EHLO greeting, as exemplified beneath, instead of the original HELO. Clients fall back to HELO but if the server does not support EHLO greeting.^[23]

Mod clients may use the ESMTP extension keyword SIZE to query the server for the maximum message size that will be accustomed. Older clients and servers may try to transfer excessively sized messages that volition exist rejected subsequently consuming network resource, including connect fourth dimension to network links that is paid by the minute.^[24]

Users tin can manually determine in accelerate the maximum size accepted by ESMTP servers. The client replaces the HELO command with the EHLO command.

          S: 220 smtp2.instance.com ESMTP Postfix          C: EHLO bob.instance.org          S: 250-smtp2.instance.com Howdy bob.instance.org [192.0.2.201]          S: 250-SIZE 14680064          Due south: 250-PIPELINING          Due south: 250 Assist        

Thus smtp2.example.com declares that it can have a fixed maximum message size no larger than fourteen,680,064 octets (8-bit bytes).

In the simplest instance, an ESMTP server declares a maximum SIZE immediately after receiving an EHLO. According to RFC 1870, however, the numeric parameter to the SIZE extension in the EHLO response is optional. Clients may instead, when issuing a MAIL FROM control, include a numeric estimate of the size of the message they are transferring, so that the server can decline receipt of overly-big letters.

Binary information transfer [edit]

Original SMTP supports merely a unmarried body of ASCII text, therefore any binary data needs to be encoded as text into that body of the bulletin earlier transfer, and and then decoded past the recipient. Binary-to-text encodings, such as uuencode and BinHex were typically used.

The 8BITMIME command was adult to address this. Information technology was standardized in 1994 as RFC 1652^[25] Information technology facilitates the transparent exchange of e-mail letters containing octets exterior the 7-bit ASCII character set past encoding them as MIME content parts, typically encoded with Base64.

Mail delivery mechanism extensions [edit]

On-Demand Mail Relay [edit]

On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows an intermittently-connected SMTP server to receive e-mail queued for information technology when it is connected.

Internationalization extension [edit]

Original SMTP supports email addresses composed of ASCII characters only, which is inconvenient for users whose native script is not Latin based, or who apply diacritic not in the ASCII character set. This limitation was alleviated via extensions enabling UTF-8 in address names. RFC 5336 introduced experimental^[24] UTF8SMTP command and later was superseded by RFC 6531 that introduced SMTPUTF8 command. These extensions provide support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as Greek and Chinese.^[26]

Current support is limited, simply there is strong interest in broad adoption of RFC 6531 and the related RFCs in countries similar Mainland china that have a large user base of operations where Latin (ASCII) is a strange script.

Extensions [edit]

Like SMTP, ESMTP is a protocol used to transport Internet mail. Information technology is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol.

The main identification feature for ESMTP clients is to open a transmission with the control EHLO (Extended Hullo), rather than HELO (How-do-you-do, the original RFC 821 standard). A server will answer with success (code 250), failure (code 550) or error (lawmaking 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the lawmaking 250 OK in a multi-line respond with its domain and a list of keywords to point supported extensions. A RFC 821 compliant server returns error code 500, allowing ESMTP clients to try either HELO or QUIT.

Each service extension is divers in an canonical format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA). The outset definitions were the RFC 821 optional services: SEND, SOML (Send or Mail), SAML (Send and Post), EXPN, Assist, and Turn. The format of additional SMTP verbs was set up and for new parameters in MAIL and RCPT.

Some relatively mutual keywords (not all of them corresponding to commands) used today are:

• 8BITMIME – eight bit data manual, RFC 6152
• ATRN – Authenticated Plough for On-Need Mail Relay, RFC 2645
• AUTH – Authenticated SMTP, RFC 4954
• CHUNKING – Chunking, RFC 3030
• DSN – Delivery status notification, RFC 3461 (Come across Variable envelope render path)
• ETRN – Extended version of remote bulletin queue starting control TURN, RFC 1985
• Aid – Supply helpful information, RFC 821
• PIPELINING – Command pipelining, RFC 2920
• SIZE – Message size annunciation, RFC 1870
• STARTTLS – Ship Layer Security, RFC 3207 (2002)
• SMTPUTF8 – Permit UTF-8 encoding in mailbox names and header fields, RFC 6531
• UTF8SMTP – Allow UTF-8 encoding in mailbox names and header fields, RFC 5336 (deprecated^[27])

The ESMTP format was restated in RFC 2821 (superseding RFC 821) and updated to the latest definition in RFC 5321 in 2008. Support for the EHLO control in servers became mandatory, and HELO designated a required fallback.

Non-standard, unregistered, service extensions can exist used by bilateral agreement, these services are indicated past an EHLO message keyword starting with "X", and with any additional parameters or verbs similarly marked.

SMTP commands are case-insensitive. They are presented hither in capitalized form for emphasis merely. An SMTP server that requires a specific capitalization method is a violation of the standard.^{[ citation needed ]}

8BITMIME [edit]

At least the following servers advertise the 8BITMIME extension:

• Apache James (since two.3.0a1)^[28]
• Citadel (since vii.30)
• Courier Mail Server
• Gmail^[29]
• IceWarp
• IIS SMTP Service
• Kerio Connect
• Lotus Domino
• Microsoft Substitution Server (as of Exchange Server 2000)
• Novell GroupWise
• OpenSMTPD
• Oracle Communications Messaging Server
• Postfix
• Sendmail (since 6.57)

The following servers tin exist configured to annunciate 8BITMIME, only practise not perform conversion of 8-bit data to 7-bit when connecting to not-8BITMIME relays:

• Exim and qmail do not translate eight-fleck messages to vii-bit when making an endeavor to relay 8-flake data to non-8BITMIME peers, as is required by the RFC.^[30] This does not cause bug in practice, since virtually all modernistic mail relays are viii-bit clean.^[31]
• Microsoft Commutation Server 2003 advertises 8BITMIME by default, but relaying to a non-8BITMIME peer results in a bounciness. This is allowed past RFC 6152 section three.

SMTP-AUTH [edit]

The SMTP-AUTH extension provides an access control mechanism. It consists of an hallmark step through which the client effectively logs into the mail server during the procedure of sending mail. Servers that back up SMTP-AUTH can normally be configured to require clients to use this extension, ensuring the true identity of the sender is known. The SMTP-AUTH extension is divers in RFC 4954.

SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as spammers. It does not necessarily guarantee the authenticity of either the SMTP envelope sender or the RFC 2822 "From:" header. For example, spoofing, in which one sender masquerades every bit someone else, is however possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for.

The SMTP-AUTH extension as well allows one mail server to indicate to another that the sender has been authenticated when relaying mail service. In general this requires the recipient server to trust the sending server, meaning that this aspect of SMTP-AUTH is rarely used on the Internet.^{[ citation needed ]}

SMTPUTF8 [edit]

Supporting servers include:

• Postfix (version 3.0 and later)^[32]
• Momentum (versions iv.1^[33] and iii.half dozen.5, and later on)
• Sendmail (under development)
• Exim (experimental as of the iv.86 release)
• CommuniGate Pro every bit of version 6.2.2^[34]
• Courier-MTA equally of version one.0^[35]
• Halon as of version 4.0^[36]
• Microsoft Commutation Server as of protocol revision xiv.0^[37]
• Haraka and other servers.^[38]
• Oracle Communications Messaging Server as of release 8.0.2.^[39]

Security extensions [edit]

Mail delivery can occur both over patently text and encrypted connections, nevertheless the communicating parties might not know in advance of other party's ability to apply secure channel.

STARTTLS or "Opportunistic TLS" [edit]

The STARTTLS extensions enables supporting SMTP servers to notify connecting clients that it supports TLS encrypted communication and offers the opportunity for clients to upgrade their connexion by sending the STARTTLS control. Servers supporting the extension do not inherently gain any security benefits from its implementation on its own, as upgrading to a TLS encrypted session is dependent on the connecting customer deciding to exercise this option, hence the term opportunistic TLS.

STARTTLS is constructive merely against passive observation attacks, since the STARTTLS negotiation happens in obviously text and an active attacker can trivially remove STARTTLS commands. This blazon of human being-in-the-middle attack is sometimes referred to as STRIPTLS, where the encryption negotiation information sent from one end never reaches the other. In this scenario both parties take the invalid or unexpected responses as indication that the other does non properly support STARTTLS, defaulting to traditional plain-text mail transfer.^[40] Annotation that STARTTLS is also defined for IMAP and POP3 in other RFCs, but these protocols serve different purposes: SMTP is used for advice between message transfer agents, while IMAP and POP3 are for end clients and bulletin transfer agents.

In 2014 the Electronic Borderland Foundation began "STARTTLS Everywhere" project that, similarly to "HTTPS Everywhere" list, immune relying parties to detect others supporting secure communication without prior advice. The projection stopped accepting submissions on 29 April 2021, and EFF recommended switching to DANE and MTA-STS for discovering information on peers' TLS support.^[41]

RFC 8314 officially alleged plain text obsolete and recommend always using TLS, adding ports with implicit TLS.

SMTP MTA Strict Transport Security [edit]

A newer 2018 RFC 8461 called "SMTP MTA Strict Transport Security (MTA-STS)" aims to accost the problem of agile adversary by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific DNS TXT records. The relying party would regularly check existence of such record, and cache it for the amount of time specified in the tape and never communicate over insecure channels until record expires.^[40] Note that MTA-STS records apply merely to SMTP traffic between mail servers while communications between a user's client and the mail server are protected by Send Layer Security with SMTP/MSA, IMAP, POP3, or HTTPS in combination with an organizational or technical policy. Essentially, MTA-STS is a means to extend such a policy to 3rd parties.

In April 2019 Google Mail service announced support for MTA-STS.^[42]

SMTP TLS Reporting [edit]

A number of protocols allows secure commitment of letters, but they can neglect due to misconfigurations or deliberate agile interference, leading to undelivered messages or commitment over unencrypted or unauthenticated channels. RFC 8460 "SMTP TLS Reporting" describes a reporting mechanism and format for sharing statistics and specific information most potential failures with recipient domains. Recipient domains can so employ this data to both detect potential attacks and diagnose unintentional misconfigurations.

In April 2019 Google Mail appear support for SMTP TLS Reporting.^[42]

Spoofing and spamming [edit]

The original design of SMTP had no facility to cosign senders, or check that servers were authorized to send on their behalf, with the consequence that email spoofing is possible, and commonly used in email spam and phishing.

Occasional proposals are fabricated to change SMTP extensively or replace information technology completely. One case of this is Internet Postal service 2000, but neither it, nor whatsoever other has made much headway in the face up of the network effect of the huge installed base of archetype SMTP.

Instead, mail servers at present use a range of techniques, such every bit stricter enforcement of standards such as RFC 5322,^{[43] [44]} DomainKeys Identified Postal service, Sender Policy Framework and DMARC, DNSBLs and greylisting to reject or quarantine suspicious emails.^[45]

Implementations [edit]

[edit]

• RFC 1123 – Requirements for Internet Hosts—Awarding and Support (STD iii)
• RFC 1870 – SMTP Service Extension for Message Size Proclamation (оbsoletes: RFC 1653)
• RFC 2505 – Anti-Spam Recommendations for SMTP MTAs (BCP 30)
• RFC 2821 – Simple Mail service Transfer Protocol
• RFC 2920 – SMTP Service Extension for Command Pipelining (STD 60)
• RFC 3030 – SMTP Service Extensions for Transmission of Large and Binary MIME Letters
• RFC 3207 – SMTP Service Extension for Secure SMTP over Send Layer Security (obsoletes RFC 2487)
• RFC 3461 – SMTP Service Extension for Delivery Status Notifications (obsoletes RFC 1891)
• RFC 3463 – Enhanced Status Codes for SMTP (obsoletes RFC 1893, updated past RFC 5248)
• RFC 3464 – An Extensible Message Format for Commitment Status Notifications (obsoletes RFC 1894)
• RFC 3798 – Message Disposition Notification (updates RFC 3461)
• RFC 3834 – Recommendations for Automated Responses to Electronic Mail service
• RFC 3974 – SMTP Operational Experience in Mixed IPv4/v6 Environments
• RFC 4952 – Overview and Framework for Internationalized Electronic mail (updated by RFC 5336)
• RFC 4954 – SMTP Service Extension for Authentication (obsoletes RFC 2554, updates RFC 3463, updated past RFC 5248)
• RFC 5068 – Email Submission Operations: Access and Accountability Requirements (BCP 134)
• RFC 5248 – A Registry for SMTP Enhanced Mail System Status Codes (BCP 138) (updates RFC 3463)
• RFC 5321 – The Simple Mail service Transfer Protocol (obsoletes RFC 821 aka STD x, RFC 974, RFC 1869, RFC 2821, updates RFC 1123)
• RFC 5322 – Internet Message Format (obsoletes RFC 822 aka STD 11, and RFC 2822)
• RFC 5504 – Downgrading Mechanism for Email Address Internationalization
• RFC 6409 – Message Submission for Post (STD 72) (obsoletes RFC 4409, RFC 2476)
• RFC 6522 – The Multipart/Report Content Type for the Reporting of Mail Arrangement Administrative Messages (obsoletes RFC 3462, and in plough RFC 1892)
• RFC 6531 – SMTP Extension for Internationalized Electronic mail Addresses (updates RFC 2821, RFC 2822, RFC 4952, and RFC 5336)
• RFC 8314 – Cleartext Considered Obsolete: Use of Send Layer Security (TLS) for Email Submission and Admission

See besides [edit]

• Bounce accost
• CRAM-MD5 (a SASL machinery for ESMTPA) RFC 2195
• Email
  • Email encryption
• DKIM
• Ident
• List of mail server software
• List of SMTP server return codes
• POP earlier SMTP / SMTP after POP
• Internet Message Access Protocol Binary Content Extension RFC 3516
• Sender Policy Framework (SPF)
• Simple Authentication and Security Layer (SASL) RFC 4422
• SMTP Authentication
• Variable envelope return path
• Comparison of email clients for information about SMTP back up

Notes [edit]

1. ^ The History of Electronic mail, Tom Van Vleck: "It is non clear this protocol was ever implemented"
2. ^ The Commencement Network Electronic mail, Ray Tomlinson, BBN
3. ^ Motion picture of "The First E-mail Computer" by Dan Murphy, a PDP-ten
4. ^ Dan Murphy's TENEX and TOPS-twenty Papers Archived Nov 18, 2007, at the Wayback Auto
5. ^ RFC 2235
6. ^ RFC 469 – Network Post Coming together Summary
7. ^ RFC 524 – A Proposed Mail Protocol
8. ^ "Tldp.org".
9. ^ "typhoon-hairdresser-uucp-project-conclusion-05 – The Conclusion of the UUCP Mapping Project".
10. ^ The commodity virtually sender rewriting contains technical background info nearly the early SMTP history and source routing before RFC 1123.
11. ^ Eric Allman (1983), Sendmail – An Internetwork Mail Router (PDF), BSD UNIX documentation gear up, Berkeley: University of California, retrieved June 29, 2012
12. ^ Craig Partridge (2008), The Technical Development of Internet Email (PDF), IEEE Annals of the History of Computing, vol. 30, IEEE Computer Society, pp. three–29, doi:10.1109/MAHC.2008.32, S2CID 206442868, archived from the original (PDF) on May 12, 2011
13. ^ Paul Hoffman (February ane, 1998). "Assuasive Relaying in SMTP: A Survey". Net Postal service Consortium. Retrieved May 30, 2010.
14. ^ Paul Hoffman (Baronial 2002). "Allowing Relaying in SMTP: A Serial of Surveys". Internet Mail Consortium. Archived from the original on January xviii, 2007. Retrieved May thirty, 2010.
15. ^ "In Unix, what is an open mail relay? - Knowledge Base". June 17, 2007. Archived from the original on June 17, 2007. Retrieved March xv, 2021.
16. ^ "The Postal service, RCPT, and Information verbs", [D. J. Bernstein]
17. ^ RFC 5321 Department-7.2
18. ^ Systems, Message. "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities". www.prnewswire.com . Retrieved July 19, 2020.
19. ^ Cara Garretson (2005). "ISPs Pitch In to Stop Spam". PC World . Retrieved January 18, 2016. Concluding month, the Anti-Spam Technical Alliance, formed last year by Yahoo, America Online, EarthLink, and Microsoft, issued a list of antispam recommendations that includes filtering Port 25.
20. ^ RFC 5321, Simple Mail Transfer Protocol, J. Klensin, The Cyberspace Social club (October 2008)
21. ^ RFC 1047
22. ^ "rfc5321#section-four.v.3.ii.6".
23. ^ John Klensin; Ned Freed; Marshall T. Rose; Einar A. Stefferud; Dave Crocker (November 1995). SMTP Service Extensions. IETF. doi:x.17487/RFC1869. RFC 1869.
24. ^ ^{a b} "Mail service Parameters". IANA. February 14, 2020.
25. ^ Which was obsoleted in 2011 by RFC 6152 corresponding to the and then new STD 71
26. ^ Jiankang Yao (December 19, 2014). "Chinese email address". EAI (Mailing listing). IETF. Retrieved May 24, 2016.
27. ^ "SMTP Service Extension Parameters". IANA. Retrieved November 5, 2013.
28. ^ James Server - ChangeLog. James.apache.org. Retrieved on 2013-07-17.
29. ^ 8BITMIME service advertised in response to EHLO on gmail-smtp-in.l.google.com port 25, checked 23 November 2011
30. ^ Qmail bugs and wishlist. Domicile.pages.de. Retrieved on 2013-07-17.
31. ^ The 8BITMIME extension. Cr.yp.to. Retrieved on 2013-07-17.
32. ^ "Postfix SMTPUTF8 support is enabled by default", February viii, 2015, postfix.org
33. ^ "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities" (Press release).
34. ^ "Version vi.ii Revision History". CommuniGate.com.
35. ^ Sam Varshavchik (September 18, 2018). "New releases of Courier packages". courier-announce (Mailing list).
36. ^ "Halon MTA changelog". GitHub. November 9, 2021. v4.0: New SMTPUTF8 back up Updated for new versions
37. ^ "MS-OXSMTP: Elementary Mail Transfer Protocol (SMTP) Extensions". July 24, 2018.
38. ^ "EAI Readiness in TLDs" (PDF). Feb 12, 2019.
39. ^ "Communications Messaging Server Release Notes". oracle.com. October 2017.
40. ^ ^{a b} "Introducing MTA Strict Transport Security (MTA-STS) | Hardenize Blog". www.hardenize.com . Retrieved Apr 25, 2019.
41. ^ "STARTTLS Everywhere". EFF. Retrieved December iv, 2021.
42. ^ ^{a b} Cimpanu, Catalin. "Gmail becomes first major email provider to support MTA-STS and TLS Reporting". ZDNet . Retrieved April 25, 2019.
43. ^ "Message Non Compliant with RFC 5322".
44. ^ "Message could not exist delivered. Please ensure the message is RFC 5322 compliant".
45. ^ "Why are the emails sent to Microsoft Business relationship rejected for policy reasons?".

References [edit]

• Hughes, Fifty (1998). Internet E-post: Protocols, Standards and Implementation. Artech House Publishers. ISBN978-0-89006-939-4.
• Chase, C (2003). sendmail Cookbook. O'Reilly Media. ISBN978-0-596-00471-2.
• Johnson, Chiliad (2000). Net E-mail Protocols: A Developer'south Guide. Addison-Wesley Professional. ISBN978-0-201-43288-6.
• Loshin, P (1999). Essential Email Standards: RFCs and Protocols Made Practical. John Wiley & Sons. ISBN978-0-471-34597-viii.
• Rhoton, J (1999). Programmer's Guide to Cyberspace Mail: SMTP, Pop, IMAP, and LDAP. Elsevier. ISBN978-ane-55558-212-8.
• Woods, D (1999). Programming Internet Mail . O'Reilly. ISBN978-ane-56592-479-6.

External links [edit]

• RFC 1869 SMTP Service Extensions
• RFC 5321 Uncomplicated Mail Transfer Protocol
• RFC 4954 SMTP Service Extension for Authentication (obsoletes RFC 2554)
• RFC 3848 SMTP and LMTP Transmission Types Registration (with ESMTPA)
• RFC 6409 Message Submission for Mail (obsoletes RFC 4409, which obsoletes RFC 2476)

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Source: https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol

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