Simple Mail Transfer Protocol

Source: Wikipedia, the free encyclopedia.
"SMTP" redirects here. For the email delivery company, see SMTP (company). For Short Message Transfer Protocol, see GSM 03.40.

Internet protocol suite
Application layer
Transport layer
Internet layer
Link layer

The Simple Mail Transfer Protocol (SMTP) is an

RFC 8314. For retrieving messages, IMAP (which replaced 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

port number
25 (for plaintext) and 587 (for encrypted communications).

History

Predecessors to SMTP

Further information: History of email

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. SMTP grew out of these standards developed during the 1970s.

Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented,

BBN adapted that year to send messages across two computers on the ARPANET.[2][3][4] A further proposal for a Mail Protocol was made in RFC 524 in June 1973,[5] which was not implemented.[6]

The use of the File Transfer Protocol (FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973.[7] 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.[8]

Original SMTP

In 1980,

RFC 788
"Simple Mail Transfer Protocol".

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

SMTP became widely used in the early 1980s. At the time, it was a complement to the

bang paths" it used as message routing headers.[11]

MTA (mail transfer agent).[13]

The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial

blocklist open relays,[16]
making original SMTP essentially impractical for general use on the Internet.

Modern SMTP

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 be identified and servers can indicate supported extensions.

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 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 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 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 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

Blue arrows depict implementation of SMTP variations

Email is submitted by a mail client (

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 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 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 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 the incoming message, it hands it to a

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), 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

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

Internet Message Format
.

Protocol overview

SMTP is a

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 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:

  1. MAIL command, to establish the return address, also called return-path,[17] reverse-path,[18] bounce address, mfrom, or envelope sender.
  2. RCPT command, to establish a recipient of the message. This command can 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 message, as 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: once to the DATA command itself, to acknowledge that it is ready to receive the text, and the second time after the end-of-data sequence, to either accept or reject the entire message.

Besides 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

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 the

port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication
.

SMTP vs mail retrieval

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

mail boxes. To permit an intermittently-connected mail server to pull messages from 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

Remote Message Queue Starting enables 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.[19]

Outgoing mail SMTP server

An

DNS
name). This server will deliver outgoing messages on behalf of the user.

Outgoing mail server access restrictions

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:

Restricting access by location

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 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 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 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.

Client authentication

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 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.

Ports

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,[20] but also to cure for the higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open.

SMTP transport example

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 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 is available).[21] 

S: 220 smtp.example.com ESMTP Postfix
C: HELO relay.example.org
S: 250 Hello relay.example.org, I am glad to meet you
C: MAIL FROM:<[email protected]>
S: 250 Ok
C: RCPT TO:<[email protected]>
S: 250 Ok
C: RCPT TO:<[email protected]>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Bob Example" <[email protected]>
C: To: "Alice Example" <[email protected]>
C: Cc: [email protected]
C: Date: Tue, 15 Jan 2008 16:02:43 -0500
C: Subject: Test message
C:
C: Hello Alice.
C: This is a test message with 5 header fields and 4 lines in the message body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
{The server closes the connection}

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 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 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 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 shortage: 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 the message that they will try to deliver.[22] 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.[23]

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 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 the last two lines may actually be omitted. This causes an error on the server when trying to send the 221 Bye reply.

SMTP Extensions

Extension discovery mechanism

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.[24]

Modern clients may use the ESMTP extension keyword SIZE to query 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.[25]

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

S: 220 smtp2.example.com ESMTP Postfix
C: EHLO bob.example.org
S: 250-smtp2.example.com Hello bob.example.org [192.0.2.201]
S: 250-SIZE 14680064
S: 250-PIPELINING
S: 250 HELP

Thus smtp2.example.com declares that it can accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes).

In the simplest case, 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 command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages.

Binary data transfer

Original SMTP supports only a single body of ASCII text, therefore any binary data needs to be encoded as text into that body of the message before transfer, and then decoded by the recipient.

uuencode and BinHex
were typically used.

The 8BITMIME command was developed to address this. It was standardized in 1994 as

e-mail messages containing octets outside the seven-bit ASCII character set by encoding them as MIME content parts, typically encoded with Base64
.

Mail delivery mechanism extensions

On-Demand Mail Relay

Main article: On-Demand Mail Relay

On-Demand Mail Relay (ODMR) is an SMTP extension standardized in

RFC 2645
that allows an intermittently-connected SMTP server to receive email queued for it when it is connected.

Internationalization extension

Main article: International email

Original SMTP supports email addresses composed of

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.[27]

Current support is limited, but there is strong interest in broad adoption of

RFC 6531 and the related RFCs in countries like China
that have a large user base where Latin (ASCII) is a foreign script.

Extensions

Like SMTP, ESMTP is a protocol used to transport Internet mail. It 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 command EHLO (Extended HELLO), rather than HELO (Hello, the original

RFC 821
standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821 compliant server returns error code 500, allowing ESMTP clients to try either HELO or QUIT.

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

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

The ESMTP format was restated in

RFC 5321
in 2008. Support for the EHLO command in servers became mandatory, and HELO designated a required fallback.

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

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

8BITMIME

At least the following servers advertise the 8BITMIME extension:

The following servers can be configured to advertise 8BITMIME, but do not perform conversion of 8-bit data to 7-bit when connecting to non-8BITMIME relays:

  • Exim and qmail do not translate eight-bit messages to seven-bit when making an attempt to relay 8-bit data to non-8BITMIME peers, as is required by the RFC.[31] This does not cause problems in practice, since virtually all modern mail relays are 8-bit clean.[32]
  • Microsoft Exchange Server 2003 advertises 8BITMIME by default, but relaying to a non-8BITMIME peer results in a bounce. This is allowed by RFC 6152 section 3.

SMTP-AUTH

Main article: SMTP Authentication

The SMTP-AUTH extension provides an access control mechanism. It consists of an

RFC 4954
.

SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as

spoofing
, in which one sender masquerades as someone else, is still 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 also allows one mail server to indicate to another that the sender has been authenticated when relaying mail. 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

Supporting servers include:

Security extensions

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

STARTTLS or "Opportunistic TLS"

Main articles: Opportunistic TLS and Email encryption

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 connection by sending the STARTTLS command. 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 client deciding to exercise this option, hence the term opportunistic TLS.

STARTTLS is effective only against passive observation attacks, since the STARTTLS negotiation happens in plain text and an active attacker can trivially remove STARTTLS commands. This type of

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 not properly support STARTTLS, defaulting to traditional plain-text mail transfer.[41] Note that STARTTLS is also defined for IMAP and POP3
in other RFCs, but these protocols serve different purposes: SMTP is used for communication between message transfer agents, while IMAP and POP3 are for end clients and message transfer agents.

In 2014 the Electronic Frontier Foundation began "STARTTLS Everywhere" project that, similarly to "HTTPS Everywhere" list, allowed relying parties to discover others supporting secure communication without prior communication. The project stopped accepting submissions on 29 April 2021, and EFF recommended switching to DANE and MTA-STS for discovering information on peers' TLS support.[42]

RFC 8314
officially declared plain text obsolete and recommend always using TLS for mail submission and access, adding ports with implicit TLS.

SMTP MTA Strict Transport Security

A newer 2018

RFC 8461 called "SMTP MTA Strict Transport Security (MTA-STS)" aims to address the problem of active 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 record and never communicate over insecure channels until record expires.[41] Note that MTA-STS records apply only to SMTP traffic between mail servers while communications between a user's client and the mail server are protected by Transport 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 third parties.

In April 2019 Google Mail announced support for MTA-STS.[43]

SMTP TLS Reporting

Protocols designed to securely deliver messages can fail due to misconfigurations or deliberate active interference, leading to undelivered messages or delivery over unencrypted or unauthenticated channels.

RFC 8460
"SMTP TLS Reporting" describes a reporting mechanism and format for sharing statistics and specific information about potential failures with recipient domains. Recipient domains can then use this information to both detect potential attacks and diagnose unintentional misconfigurations.

In April 2019 Google Mail announced support for SMTP TLS Reporting.[43]

Spoofing and spamming

Main articles: Anti-spam techniques and Email authentication

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

Occasional proposals are made to modify SMTP extensively or replace it completely. One example of this is Internet Mail 2000, but neither it, nor any other has made much headway in the face of the network effect of the huge installed base of classic SMTP.

Instead, mail servers now use a range of techniques, such as stricter enforcement of standards such as

DNSBLs and greylisting to reject or quarantine suspicious emails.[46]

Implementations

Main articles: List of mail server software and Comparison of mail servers

Related requests for comments

See also

Notes

  1. ^ The History of Electronic Mail Archived December 2, 2017, at the Wayback Machine, Tom Van Vleck: "It is not clear this protocol was ever implemented"
  2. ^ The First Network Email, Ray Tomlinson, BBN
  3. ^ Picture of "The First Email Computer" by Dan Murphy, a PDP-10
  4. ^ Dan Murphy's TENEX and TOPS-20 Papers Archived November 18, 2007, at the Wayback Machine
  5. RFC 524
    – A Proposed Mail Protocol
  6. ^ Crocker, David H. (December 1977). "Framework and Functions of the "MS" Personal Message System" (PDF). The RAND Corporation. Archived (PDF) from the original on May 13, 2022. Retrieved April 17, 2022.
  7. RFC 469
    – Network Mail Meeting Summary
  8. ^ RFC 733, 21 November 1977, Standard for the Format of ARPA Network Text Message
  9. ^ "Tldp.org". Archived from the original on August 17, 2007. Retrieved August 25, 2007.
  10. ^ "draft-barber-uucp-project-conclusion-05 – The Conclusion of the UUCP Mapping Project". Archived from the original on October 13, 2007. Retrieved August 25, 2007.
  11. RFC 1123
    .
  12. ^ Eric Allman (1983), Sendmail – An Internetwork Mail Router (PDF), BSD UNIX documentation set, Berkeley: University of California, archived (PDF) from the original on May 20, 2013, retrieved June 29, 2012
  13. S2CID 206442868, archived from the original
    (PDF) on May 12, 2011
  14. ^ Paul Hoffman (February 1, 1998). "Allowing Relaying in SMTP: A Survey". Internet Mail Consortium. Archived from the original on March 5, 2016. Retrieved May 30, 2010.
  15. ^ Paul Hoffman (August 2002). "Allowing Relaying in SMTP: A Series of Surveys". Internet Mail Consortium. Archived from the original on January 18, 2007. Retrieved May 30, 2010.
  16. ^ "In Unix, what is an open mail relay? - Knowledge Base". June 17, 2007. Archived from the original on June 17, 2007. Retrieved March 15, 2021.
  17. ^ "The MAIL, RCPT, and DATA verbs" Archived February 22, 2014, at the Wayback Machine, [D. J. Bernstein]
  18. RFC 5321
    Section-7.2
  19. ^ Systems, Message. "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities". www.prnewswire.com. Archived from the original on July 19, 2020. Retrieved July 19, 2020.
  20. ^ Cara Garretson (2005). "ISPs Pitch In to Stop Spam". PC World. Archived from the original on August 28, 2015. Retrieved January 18, 2016. Last 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.
  21. RFC 5321
    , Simple Mail Transfer Protocol, J. Klensin, The Internet Society (October 2008)
  22. RFC 1047
  23. ^ "rfc5321#section-4.5.3.2.6". Archived from the original on January 16, 2015. Retrieved June 7, 2010.
  24. doi:10.17487/RFC1869. RFC 1869
    .
  25. ^ a b "MAIL Parameters". IANA. February 14, 2020. Archived from the original on May 28, 2019. Retrieved May 28, 2019.
  26. STD
    71
  27. ^ Jiankang Yao (December 19, 2014). "Chinese email address". EAI (Mailing list). IETF. Archived from the original on October 2, 2015. Retrieved May 24, 2016.
  28. ^ "SMTP Service Extension Parameters". IANA. Archived from the original on May 28, 2019. Retrieved November 5, 2013.
  29. ^ James Server - ChangeLog Archived February 20, 2020, at the Wayback Machine. James.apache.org. Retrieved on 2013-07-17.
  30. ^ 8BITMIME service advertised in response to EHLO on gmail-smtp-in.l.google.com port 25, checked 23 November 2011
  31. ^ Qmail bugs and wishlist. Home.pages.de. Retrieved on 2013-07-17.
  32. ^ The 8BITMIME extension Archived June 7, 2011, at the Wayback Machine. Cr.yp.to. Retrieved on 2013-07-17.
  33. ^ "Postfix SMTPUTF8 support is enabled by default" Archived August 7, 2020, at the Wayback Machine, February 8, 2015, postfix.org
  34. ^ "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities" (Press release). Archived from the original on September 15, 2020. Retrieved September 17, 2020.
  35. ^ "Version 6.2 Revision History". CommuniGate.com. Archived from the original on October 29, 2020. Retrieved September 17, 2020.
  36. ^ Sam Varshavchik (September 18, 2018). "New releases of Courier packages". courier-announce (Mailing list). Archived from the original on August 17, 2021. Retrieved September 17, 2020.
  37. ^ "Halon MTA changelog". GitHub. November 9, 2021. Archived from the original on September 18, 2020. Retrieved September 17, 2020. v4.0: New SMTPUTF8 support Updated for new versions
  38. ^ "MS-OXSMTP: Simple Mail Transfer Protocol (SMTP) Extensions". July 24, 2018. Archived from the original on August 16, 2021. Retrieved September 17, 2020.
  39. ^ "EAI Readiness in TLDs" (PDF). February 12, 2019. Archived (PDF) from the original on January 24, 2021. Retrieved September 17, 2020.
  40. ^ "Communications Messaging Server Release Notes". oracle.com. October 2017. Archived from the original on November 24, 2020. Retrieved September 17, 2020.
  41. ^ a b "Introducing MTA Strict Transport Security (MTA-STS) | Hardenize Blog". www.hardenize.com. Archived from the original on April 25, 2019. Retrieved April 25, 2019.
  42. ^ "STARTTLS Everywhere". EFF. Archived from the original on August 9, 2019. Retrieved December 4, 2021.
  43. ^ a b Cimpanu, Catalin. "Gmail becomes first major email provider to support MTA-STS and TLS Reporting". ZDNet. Archived from the original on April 29, 2019. Retrieved April 25, 2019.
  44. ^ "Message Non Compliant with RFC 5322". Archived from the original on January 17, 2023. Retrieved January 20, 2021.
  45. ^ "Message could not be delivered. Please ensure the message is RFC 5322 compliant". Archived from the original on January 28, 2021. Retrieved January 20, 2021.
  46. ^ "Why are the emails sent to Microsoft Account rejected for policy reasons?". Archived from the original on February 14, 2021. Retrieved January 20, 2021.

References

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Simple_Mail_Transfer_Protocol&oldid=1149748616"