Simple Mail Transfer Protocol
Encyclopedia
Simple Mail Transfer Protocol (SMTP) is an Internet standard
for electronic mail
(e-mail) transmission across Internet Protocol
(IP) networks. SMTP was first defined by RFC 821 (1982, eventually declared STD
10), and last updated by RFC 5321 (2008) which includes the extended SMTP
(ESMTP) additions, and is the protocol in widespread use today. SMTP is specified for outgoing mail transport and uses TCP
port
25. The protocol for new submissions is effectively the same as SMTP, but it uses port 587 instead. SMTP connections secured by SSL are known by the shorthand SMTPS
, though SMTPS is not a protocol in its own right.
While electronic mail servers and other mail transfer agent
s use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol
(POP) or the Internet Message Access Protocol
(IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes
/Domino
) to access their mail box accounts on a mail server.
were used in the 1960s. People communicated with one another using systems developed for specific mainframe
computers. As more computers were interconnected, especially in the US Government's ARPANET
, standards were developed to allow users of different systems to e-mail
one another. SMTP grew out of these standards developed during the 1970s.
SMTP can trace its roots to two implementations described in 1971, the Mail Box Protocol, which has been disputed to actually have been implemented, but is discussed in RFC 196 and other RFCs, and the SNDMSG program, which, according to RFC 2235, Ray Tomlinson
of BBN
"invents" for TENEX computers the sending of mail across the ARPANET. Fewer than 50 hosts were connected to the ARPANET at this time.
Further implementations include FTP Mail and Mail Protocol, both from 1973. Development 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. SMTP was published as RFC 821 in August 1982, also by Postel.
The SMTP standard was developed around the same time as Usenet
, a one-to-many communication network with some similarities.
SMTP became widely used in the early 1980s. At the time, it was a complement to Unix to Unix Copy Program
(UUCP) mail, which was better suited for handling 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
. Though Usenet's newsgroups are still propagated with UUCP between servers, UUCP mail has virtually disappeared along with the "bang paths" it used as message routing headers.
The article about sender rewriting
contains technical background info about the early SMTP history and source routing before RFC 1123.
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
, Sun Java System Messaging Server
.
Message submission (RFC 2476) and SMTP-AUTH (RFC 2554) were introduced in 1998 and 1999, both describing new trends in e-mail 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 message submission agents for Mail user agents, some of which were now relaying mail from the outside of an organization. (e.g. a company executive wishes to send e-mail 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 SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited e-mail (spam
).
As this protocol started out 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. 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. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.
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
.
) using SMTP on TCP
port 587. Most mailbox providers still allow submission on traditional port 25. From there, the MSA delivers the mail to its mail transfer agent (MTA, mail transfer agent
). Often, these two agents are just different 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 various appliances; in the former case, involved processes can share files; in the latter case, SMTP is used to transfer the message internally, with each host configured to use the next appliance as a smart host
. Each process is an MTA in its own right; that is, an SMTP server.
The boundary MTA has to locate the target host. It uses the Domain name system
(DNS) to look up the mail exchanger record (MX record) for the recipient's domain (the part of the address on the right of @). The returned MX record contains the name of the target host. The MTA next connects to the exchange server as an SMTP client. (The article on MX record
discusses many factors in determining which server the sending MTA connects to.)
Once the MX target accepts the incoming message, it hands it to a mail delivery agent
(MDA) for local mail delivery. An MDA is able to save messages in the relevant mailbox
format. Again, mail reception can be done using many computers or just one —the picture displays two nearby boxes in either case. An MDA may deliver messages directly to storage, or forward them over a network using SMTP, or any other means, including the 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 Internet Message Access Protocol
(IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol
(POP) which typically uses the traditional mbox
mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes
/Domino
. Webmail clients may use either method, but the retrieval protocol is often not a formal standard.
SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header or the body of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (header and body), formally referred to as the Internet Message Format.
(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 (see example below.) They are:
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 by an SMTP server; in this case the SMTP client should send a bounce message. A drop is a positive response followed by message discard rather than delivery.
The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent
(MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.
A MUA knows the outgoing mail SMTP server from its configuration. An SMTP server acting as client, i.e. relaying, typically determines which SMTP server to connect to by looking up the MX
(Mail eXchange) DNS
resource record for each recipient's domain name
. Conformant MTAs (not all) fall back to a simple A record in case no MX record can be found. Relaying servers can also be configured to use a smart host
.
An SMTP server acting as client initiates a TCP
connection to the server on the "well-known port" designated for SMTP: port
25. MUAs should use port 587 to connect to an MSA. The main difference between an MTA and an MSA is that SMTP Authentication
is mandatory for the latter only.
(POP) and the Internet Message Access Protocol
(IMAP) are specifically designed for retrieving messages and managing mail boxes
. However, SMTP has a feature to initiate mail queue processing on a remote server so that the requesting system may receive any messages destined for it (see Remote Message Queue Starting below). POP and IMAP are preferred protocols when a user's personal computer is only intermittently powered up, or Internet connectivity is only transient and hosts cannot receive message during off-line periods.
method based on Domain Name System information.
needs to know the IP address of an SMTP server and this has to be given as part of its configuration (usually given as a DNS
name). The server will deliver outgoing messages on behalf of the user.
ming. Two solutions have been in common use:
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.
By restricting access to certain IP addresses, server administrators can readily recognise the IP address of any abuser. As it will be a meaningful address to them, the administrators can deal with the rogue machine or user.
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.
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.
ing.
port 25 (SMTP) or port 587 (Submission), as formalized in RFC 6409, for relaying outbound mail to a mail server. The specifications and many servers support both. Although some servers support port 465 for legacy secure SMTP in violation of the specifications, it is preferable to use standard ports and standard ESMTP commands according to RFC 3207 if a secure session needs to be used between the client and the server.
Some servers are set up to reject all relaying on port 25, but valid users authenticating on port 587 are allowed to relay mail to any valid address.
Some Internet service provider
s intercept port 25, redirecting traffic to their own SMTP server regardless of the destination address. This means that it is not possible for their users to access an SMTP server outside the ISP's network using port 25.
Some SMTP servers support authenticated access on an additional port other than 25 to allow users to connect to them even if port 25 is blocked.
For illustration purposes here (not part of protocol), the protocol exchanges are prefixed for the server (S:) and the client (C:).
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
The client notifies the receiver of the originating email address of the message in a
The transmission of the body of the mail message is initiated with a), a single full stop
(period), followed by another new-line. 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 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. The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.
The
Modern clients may use the ESMTP extension keyword
Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the
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 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 the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction.
, SASL) for subsequent message transfers.
Microsoft products implement the proprietary Secure Password Authentication
(SPA) protocol through the use of the SMTP-AUTH extension.
However, the impracticality of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.
Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effect
s of the huge installed base of SMTP. Internet Mail 2000
was one such proposal for replacement.
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 lack of "intelligence" in many MTAs.
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.
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
for electronic mail
E-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
(e-mail) transmission across Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) networks. SMTP was first defined by RFC 821 (1982, eventually declared STD
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
10), and last updated by RFC 5321 (2008) which includes the extended SMTP
Extended SMTP
Extended SMTP , sometimes referred to as Enhanced SMTP, is a definition of protocol extensions to the Simple Mail Transfer Protocol standard...
(ESMTP) additions, and is the protocol in widespread use today. SMTP is specified for outgoing mail transport and uses TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
port
Computer port (software)
In computer programming, port has a wide range of meanings.A software port is a virtual/logical data connection that can be used by programs to exchange data directly, instead of going through a file or other temporary storage location...
25. The protocol for new submissions is effectively the same as SMTP, but it uses port 587 instead. SMTP connections secured by SSL are known by the shorthand SMTPS
SMTPS
SMTPS refers to a method for securing SMTP with transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality.SMTPS is not a proprietary protocol and not an extension of SMTP...
, though SMTPS is not a protocol in its own right.
While electronic mail servers and other mail transfer agent
Mail transfer agent
Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
s use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol
Post Office Protocol
In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP) or the Internet Message Access Protocol
Internet Message Access Protocol
Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...
(IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes
Lotus Notes
Lotus Notes is the client of a collaborative platform originally created by Lotus Development Corp. in 1989. In 1995 Lotus was acquired by IBM and became known as the Lotus Development division of IBM and is now part of the IBM Software Group...
/Domino
IBM Lotus Domino
Lotus Domino is an IBM server product that provides enterprise-grade e-mail, collaboration capabilities, and a custom application platform. Domino began life as Lotus Notes Server, the server component of Lotus Development Corporation's client-server messaging technology. It can be used as an...
) to access their mail box accounts on a mail server.
History
Various forms of one-to-one electronic messagingElectronic messaging
Electronic messaging may refer to:One to One communication*Instant message *Personal message *Text message *SMTP *Email...
were used in the 1960s. People communicated with one another using systems developed for specific mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
computers. As more computers were interconnected, especially in the US Government's ARPANET
ARPANET
The Advanced Research Projects Agency Network , was the world's first operational packet switching network and the core network of a set that came to compose the global Internet...
, standards were developed to allow users of different systems to e-mail
E-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
one another. SMTP grew out of these standards developed during the 1970s.
SMTP can trace its roots to two implementations described in 1971, the Mail Box Protocol, which has been disputed to actually have been implemented, but is discussed in RFC 196 and other RFCs, and the SNDMSG program, which, according to RFC 2235, Ray Tomlinson
Ray Tomlinson
Raymond Samuel Tomlinson is a programmer who implemented an email system in 1971 on the ARPANET. Email had been previously sent on other networks such as AUTODIN and PLATO. It was the first system able to send mail between users on different hosts connected to the ARPAnet...
of BBN
BBN Technologies
BBN Technologies is a high-technology company which provides research and development services. BBN is based next to Fresh Pond in Cambridge, Massachusetts, USA...
"invents" for TENEX computers the sending of mail across the ARPANET. Fewer than 50 hosts were connected to the ARPANET at this time.
Further implementations include FTP Mail and Mail Protocol, both from 1973. Development work continued throughout the 1970s, until the ARPANET converted into the modern Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
around 1980. Jon Postel
Jon Postel
Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards...
then proposed a Mail Transfer Protocol
Mail Transfer Protocol
The Mail Transfer Protocol is an obsolete network protocol used to reliably transfer mail. Request for Comments 780, which defined this protocol, allocated Transmission Control Protocol and User Datagram Protocol port 57 for MTP...
in 1980 that began to remove the mail's reliance on FTP. SMTP was published as RFC 821 in August 1982, also by Postel.
The SMTP standard was developed around the same time as Usenet
Usenet
Usenet is a worldwide distributed Internet discussion system. It developed from the general purpose UUCP architecture of the same name.Duke University graduate students Tom Truscott and Jim Ellis conceived the idea in 1979 and it was established in 1980...
, a one-to-many communication network with some similarities.
SMTP became widely used in the early 1980s. At the time, it was a complement to Unix to Unix Copy Program
UUCP
UUCP is an abbreviation for Unix-to-Unix Copy. The term generally refers to a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers. Specifically, a command named uucp is one of the programs in the suite; it...
(UUCP) mail, which was better suited for handling 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
Store and forward
Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in a networking context, verifies the integrity of...
mechanism and are examples of push technology
Push technology
Push technology, or server push, describes a style of Internet-based communication where the request for a given transaction is initiated by the publisher or central server...
. Though Usenet's newsgroups are still propagated with UUCP between servers, UUCP mail has virtually disappeared along with the "bang paths" it used as message routing headers.
The article about sender rewriting
Sender Rewriting Scheme
Sender Rewriting Scheme is a technique to re-mail an email message so that eventual Delivery Status Notifications can reach the original message sender...
contains technical background info about the early SMTP history and source routing before RFC 1123.
Sendmail
Sendmail
Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and -delivery methods, including the Simple Mail Transfer Protocol used for email transport over the Internet....
was one of the first (if not the first) mail transfer agents to implement SMTP. Some other popular SMTP server programs include Postfix
Postfix (software)
In computing, Postfix is a free and open-source mail transfer agent that routes and delivers electronic mail. It is intended as a fast, easier-to-administer, and secure alternative to the widely-used Sendmail MTA....
, qmail
Qmail
qmail is a mail transfer agent that runs on Unix. It was written, starting December 1995, by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program...
, Novell GroupWise
Novell GroupWise
GroupWise is a messaging and collaborative software platform from Novell that supports email, calendaring, personal information management, instant messaging, and document management. The platform consists of the client software, which is available for Windows, Mac OS X, and Linux, and the server...
, Exim
Exim
Exim is a mail transfer agent used on Unix-like operating systems. Exim is free software distributed under the terms of the GNU General Public License, and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail....
, Novell NetMail, Microsoft Exchange Server
Microsoft Exchange Server
Microsoft Exchange Server is the server side of a client–server, collaborative application product developed by Microsoft. It is part of the Microsoft Servers line of server products and is used by enterprises using Microsoft infrastructure products...
, Sun Java System Messaging Server
Sun Java System Messaging Server
Oracle Communications Messaging Exchange Server is Oracle’s highly scalable and robust messaging server software. The software was obtained by Oracle as part of the company's acquisition of Sun in 2010....
.
Message submission (RFC 2476) and SMTP-AUTH (RFC 2554) were introduced in 1998 and 1999, both describing new trends in e-mail 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 message submission agents for Mail user agents, some of which were now relaying mail from the outside of an organization. (e.g. a company executive wishes to send e-mail while on a trip using the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
, meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited e-mail (spam
E-mail spam
Email spam, also known as junk email or unsolicited bulk email , is a subset of spam that involves nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. One subset of UBE is UCE...
).
As this protocol started out purely ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
text-based, it did not deal well with binary files. Standards such as Multipurpose Internet Mail Extensions (MIME
MIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after Sendmail
Sendmail
Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and -delivery methods, including the Simple Mail Transfer Protocol used for email transport over the Internet....
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. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.
Many people contributed to the core SMTP specifications, among them Jon Postel
Jon Postel
Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards...
, Eric Allman
Eric Allman
Eric Paul Allman is an American computer programmer who developed sendmail and its precursor delivermail in the late 1970s and early 1980s at UC Berkeley.-Education and training:...
, Dave Crocker, Ned Freed
Ned Freed
Ned Freed has contributed as an IETF participant and RFC writer to a significant number of internet protocol standards.-Life:Edwin Earl "Ned" Freed was born in 1959 in Oklahoma City, Oklahoma. He graduated from Groton School in 1978...
, Randall Gellens, John Klensin
John Klensin
Dr. John C. Klensin is a computer science professional who is active in Internet-related issues.His career includes 30 years as a Principal Research Scientist at MIT, a stint as INFOODS Project Coordinator for the United Nations University, Distinguished Engineering Fellow at MCI WorldCom, and...
, and Keith Moore
Keith Moore
Keith Moore is the author and co-author of severalIETFRFCs related to the MIMEand SMTP protocols for electronic mail, among others:...
.
Mail processing model
Email is submitted by a mail client (MUA, mail user agent) to a mail server (MSA, mail submission agentMail submission agent
A mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent and cooperates with a mail transfer agent for delivery of the mail...
) using SMTP on TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
port 587. Most mailbox providers still allow submission on traditional port 25. From there, the MSA delivers the mail to its mail transfer agent (MTA, mail transfer agent
Mail transfer agent
Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
). Often, these two agents are just different 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 various appliances; in the former case, involved processes can share files; in the latter case, SMTP is used to transfer the message internally, with each host configured to use the next appliance as a smart host
Smart host
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. Often this smart host requires authentication from the sender to verify that the sender has privileges to have mail forwarded...
. Each process is an MTA in its own right; that is, an SMTP server.
The boundary MTA has to locate the target host. It uses the Domain name system
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS) to look up the mail exchanger record (MX record) for the recipient's domain (the part of the address on the right of @). The returned MX record contains the name of the target host. The MTA next connects to the exchange server as an SMTP client. (The article on MX record
MX record
A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
discusses many factors in determining which server the sending MTA connects to.)
Once the MX target accepts the incoming message, it hands it to a mail delivery agent
Mail delivery agent
A mail delivery agent or message delivery agent is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox...
(MDA) for local mail delivery. An MDA is able to save messages in the relevant mailbox
Email mailbox
An email box is the equivalent of a letter box for electronic mail. It is the destination where electronic mail messages are delivered.-Definitions:...
format. Again, mail reception can be done using many computers or just one —the picture displays two nearby boxes in either case. An MDA may deliver messages directly to storage, or forward them over a network using SMTP, or any other means, including the 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 Internet Message Access Protocol
Internet Message Access Protocol
Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...
(IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol
Post Office Protocol
In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP) which typically uses the traditional mbox
Mbox
mbox is a generic term for a family of related file formats used for holding collections of electronic mail messages. All messages in an mbox mailbox are concatenated and stored as plain text in a single file...
mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes
Lotus Notes
Lotus Notes is the client of a collaborative platform originally created by Lotus Development Corp. in 1989. In 1995 Lotus was acquired by IBM and became known as the Lotus Development division of IBM and is now part of the IBM Software Group...
/Domino
IBM Lotus Domino
Lotus Domino is an IBM server product that provides enterprise-grade e-mail, collaboration capabilities, and a custom application platform. Domino began life as Lotus Notes Server, the server component of Lotus Development Corporation's client-server messaging technology. It can be used as an...
. Webmail clients may use either method, but the retrieval protocol is often not a formal standard.
SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header or the body of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (header and body), formally referred to as the Internet Message Format.
Protocol overview
SMTP is a connection-oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control ProtocolTransmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) connection. An SMTP session consists of commands originated by an SMTP client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
(the initiating agent
Software agent
In computer science, a software agent is a piece of software that acts for a user or other program in a relationship of agency, which derives from the Latin agere : an agreement to act on one's behalf...
, sender, or transmitter) and corresponding responses from the SMTP server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
(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 (see example below.) They are:
- MAIL command, to establish the return address, a.k.a. Return-Path, 5321.From, mfrom, or envelope sender. This is the address for bounce messageBounce messageIn the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt , a Delivery Status Notification message, a Non-Delivery Notification or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another...
s. - RCPT command, to establish a recipient of this message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope.
- DATA to send the message text. This is 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 proper, 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 by an SMTP server; in this case the SMTP client should send a bounce message. A drop is a positive response followed by message discard rather than delivery.
The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent
Mail transfer agent
Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
(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. An SMTP server acting as client, i.e. relaying, typically determines which SMTP server to connect to by looking up the MX
MX record
A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
(Mail eXchange) DNS
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
resource record for each recipient's domain name
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
. Conformant MTAs (not all) fall back to a simple A record in case no MX record can be found. Relaying servers can also be configured to use a smart host
Smart host
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. Often this smart host requires authentication from the sender to verify that the sender has privileges to have mail forwarded...
.
An SMTP server acting as client initiates a TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
connection to the server on the "well-known port" designated for SMTP: port
TCP and UDP port
In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication...
25. MUAs should use port 587 to connect to an MSA. The main difference between an MTA and an MSA is that SMTP Authentication
SMTP Authentication
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol whereby an SMTP client may log in, using an authentication mechanism chosen among those supported by the SMTP server...
is mandatory for the latter only.
SMTP vs mail retrieval
SMTP is a delivery protocol only. It cannot pull messages from a remote server on demand. Other protocols, such as the Post Office ProtocolPost Office Protocol
In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP) and the Internet Message Access Protocol
Internet Message Access Protocol
Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...
(IMAP) are specifically designed for retrieving messages and managing mail boxes
Email mailbox
An email box is the equivalent of a letter box for electronic mail. It is the destination where electronic mail messages are delivered.-Definitions:...
. However, SMTP has a feature to initiate mail queue processing on a remote server so that the requesting system may receive any messages destined for it (see Remote Message Queue Starting below). POP and IMAP are preferred protocols when a user's personal computer is only intermittently powered up, or Internet connectivity is only transient and hosts cannot receive message during off-line periods.
Remote Message Queue Starting
Remote Message Queue Starting is a feature of SMTP that permits a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending the TURN command. This feature however was deemed insecure and was extended in RFC 1985 with the ETRN command which operates more securely using an authenticationAuthentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
method based on Domain Name System information.
On-Demand Mail Relay
On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows e-mail to be relayed to an authenticated recipient.Internationalization
RFC 5336 describes internationalization features for SMTP, the UTF8SMTP extension, which provides support for multi-byte and non-ASCII characters in email addresses, such as Pelé@live.com (simple diacritic), δοκιμή@παράδειγμα.δοκιμή, and 测试@测试.测试.Outgoing mail SMTP server
An e-mail clientE-mail client
An email client, email reader, or more formally mail user agent , is a computer program used to manage a user's email.The term can refer to any system capable of accessing the user's email mailbox, regardless of it being a mail user agent, a relaying server, or a human typing on a terminal...
needs to know the IP address of an SMTP server and this has to be given as part of its configuration (usually given as a DNS
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
name). The 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 spamSpam
Spam may refer to:* Spam , a canned pork meat product* Spam , unsolicited or undesired electronic messages* E-mail spam, unsolicited or undesired email messages* "Spam" , a comedy sketch...
ming. Two solutions have been in common use:
- In the past, many systems imposed usage restrictions by the location of the client, only permitting usage by clients whose IP address is one that the server administrators control. Usage from any other client IP address is disallowed.
- Modern SMTP servers typically offer an alternative system that requires authenticationAuthenticationAuthentication is the act of confirming the truth of an attribute of a datum or entity...
of clients by credentials before allowing access.
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.
By restricting access to certain IP addresses, server administrators can readily recognise the IP address of any abuser. As it will be a meaningful address to them, the administrators can deal with the rogue machine or user.
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 authenticationAuthentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
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.
Open relay
A server that is accessible on the wider Internet and does not enforce these kinds of access restrictions is known as an open relay. This is now generally considered a bad practice worthy of blacklistBlacklist (computing)
In computing, a blacklist or block list is a basic access control mechanism that allows everyone access, except for the members of the black list . The opposite is a whitelist, which means allow nobody, except members of the white list...
ing.
Ports
Server administrators choose whether clients use TCPTransmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
port 25 (SMTP) or port 587 (Submission), as formalized in RFC 6409, for relaying outbound mail to a mail server. The specifications and many servers support both. Although some servers support port 465 for legacy secure SMTP in violation of the specifications, it is preferable to use standard ports and standard ESMTP commands according to RFC 3207 if a secure session needs to be used between the client and the server.
Some servers are set up to reject all relaying on port 25, but valid users authenticating on port 587 are allowed to relay mail to any valid address.
Some Internet service provider
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...
s intercept port 25, redirecting traffic to their own SMTP server regardless of the destination address. This means that it is not possible for their users to access an SMTP server outside the ISP's network using port 25.
Some SMTP servers support authenticated access on an additional port other than 25 to allow users to connect to them even if port 25 is blocked.
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.For illustration purposes here (not part of protocol), the protocol exchanges are prefixed for the server (S:) and the client (C:).
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).
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:
S: 250 Ok
C: RCPT TO:
S: 250 Ok
C: RCPT TO:
S: 250 Ok
C: DATA
S: 354 End data with .
C: From: "Bob Example"
C: To: "Alice Example"
C: Cc: theboss@example.com
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. In this example, the email message is sent to two mailboxes on the same SMTP server: one each 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 (Full stop
A full stop is the punctuation mark commonly placed at the end of sentences. In American English, the term used for this punctuation is period. In the 21st century, it is often also called a dot by young people...
(period), followed by another new-line. 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 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. The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.
The
QUIT command ends the session. If the second recipient were located elsewhere, the client would QUIT and connect to the appropriate SMTP server after the first message 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.Optional extensions
Although optional and not shown in this example, many clients ask the server for the SMTP extensions that the server supports, by using theEHLO greeting of the extended SMTP specification (RFC 1870). Clients fall back to HELO only if the server does not respond to EHLO.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.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 will accept a fixed maximum message size no larger than 14,680,064 octets
Octet (computing)
An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:...
(8-bit bytes). Depending on the server's actual resource usage, it may be currently unable to accept a message this large. In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction.
Security and spamming
The original SMTP specification did not include a facility for authentication of senders. Subsequently, the SMTP-AUTH extension was defined by RFC 2554. The SMTP extension (ESMTP) provides a mechanism for email clients to specify a security mechanism to a mail server, authenticate the exchange, and negotiate a security profile (Simple Authentication and Security LayerSimple Authentication and Security Layer
Simple Authentication and Security Layer is a framework for authentication and data security in Internet protocols. It decouples authentication mechanisms from application protocols, in theory allowing any authentication mechanism supported by SASL to be used in any application protocol that uses...
, SASL) for subsequent message transfers.
Microsoft products implement the proprietary Secure Password Authentication
Secure Password Authentication
Secure Password Authentication is a proprietary Microsoft protocol used to authenticate Microsoft email clients with an electronic mail server when using the Simple Mail Transfer Protocol , Post Office Protocol , or Internet Message Access Protocol . The protocol was based on the Integrated...
(SPA) protocol through the use of the SMTP-AUTH extension.
However, the impracticality of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.
Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effect
Network effect
In economics and business, a network effect is the effect that one user of a good or service has on the value of that product to other people. When network effect is present, the value of a product or service is dependent on the number of others using it.The classic example is the telephone...
s of the huge installed base of SMTP. Internet Mail 2000
Internet Mail 2000
Internet Mail 2000 is an Internet mail architecture proposed by Daniel J. Bernstein , 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 SMTP-based Internet mail architecture.Whereas the SMTP-based...
was one such proposal for replacement.
Spam is enabled by several factors, including vendors implementing broken MTAs
Mail transfer agent
Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
(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 lack of "intelligence" in many MTAs.
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
Internet Research Task Force
The Internet Research Task Force focuses on longer term research issues related to the Internet while the parallel organization, the Internet Engineering Task Force , focuses on the shorter term issues of engineering and standards making...
(IRTF) is working on a number of E-mail authentication
E-mail authentication
Email authentication is the effort to equip messages of the email transport system with enough verifiable information, so that recipients can recognize the nature of each incoming message automatically...
and other proposals for providing simple source authentication that is flexible, lightweight, and scalable. Recent Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF) activities include MARID
MARID
MARID was an IETF working group in the applications area tasked to propose standards for E-mail authentication in 2004.The name is an acronym of MTA Authorization Records In DNS.- Background :Lightweight MTA Authentication Protocol...
(2004) leading to two approved IETF experiments in 2005, and DomainKeys Identified Mail
DomainKeys Identified Mail
DomainKeys Identified Mail is a method for associating a domain name to an email message, thereby allowing a person, role, or organization to claim some responsibility for the message. The association is set up by means of a digital signature which can be validated by recipients...
in 2006.
Related Requests For Comments
- 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 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 3834 – Recommendations for Automatic Responses to Electronic Mail
- RFC 4952 – Overview and Framework for Internationalized E-mail
- RFC 4954 – SMTP Service Extension for Authentication (obsoletes RFC 2554)
- RFC 5068 – E-mail Submission Operations: Access and Accountability Requirements (BCP 134)
- RFC 5321 – The Simple Mail Transfer Protocol (obsoletes RFC 821 aka STD 10, RFC 974, RFC 1869, RFC 2821)
- RFC 5322 – Internet Message Format (obsoletes RFC 822 aka STD 11, and RFC 2822)
- RFC 5336 - SMTP Extension for Internationalized Email Addresses (updates RFC 2821, RFC 2822, and RFC 4952)
- RFC 5504 - Downgrading Mechanism for Email Address Internationalization
- RFC 6409 – Message Submission for Mail (obsoletes RFC 4409, RFC 2476)
See also
- Bounce messageBounce messageIn the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt , a Delivery Status Notification message, a Non-Delivery Notification or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another...
s (SMTP non-delivery reports), bounce addressBounce addressA bounce address is an e-mail address to which bounce messages are delivered. There are many variants of the name, none of them used universally, including return path, reverse path, envelope from, envelope sender, MAIL FROM, 5321-FROM, return address, From_, Errors-to, etc... - Comparison of mail serversComparison of mail serversThis is a comparison of mail servers: mail transfer agents, mail delivery agents, and other computer software which provide e-mail services.- Feature comparison :- Authentication :- Antispam Features :...
- E-mail authenticationE-mail authenticationEmail authentication is the effort to equip messages of the email transport system with enough verifiable information, so that recipients can recognize the nature of each incoming message automatically...
- E-mail encryptionE-mail encryptionEmail encryption refers to encryption, and often authentication, of email messages, which can be done in order to protect the content from being read by unintended recipients....
- Extended SMTPExtended SMTPExtended SMTP , sometimes referred to as Enhanced SMTP, is a definition of protocol extensions to the Simple Mail Transfer Protocol standard...
(ESMTP) - IdentIdentThe Ident Protocol, specified in RFC 1413, is an Internet protocol that helps identify the user of a particular TCP connection. One popular daemon program for providing the ident service is identd.-How ident works:...
- List of mail servers
- POP before SMTPPOP before SMTPPOP before SMTP or SMTP after POP is a method of authorization used by mail server software which helps allow users the option to send e-mail from any location, as long as they can demonstrably also fetch their mail from the same place....
/ SMTP after POP - Sender Policy FrameworkSender Policy FrameworkSender Policy Framework is an email validation system designed to prevent email spam by detecting email spoofing, a common vulnerability, by verifying sender IP addresses. SPF allows administrators to specify which hosts are allowed to send mail from a given domain by creating a specific SPF...
(SPF) - SMTP-AUTH (ESMTPA)
- SMTPSSMTPSSMTPS refers to a method for securing SMTP with transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality.SMTPS is not a proprietary protocol and not an extension of SMTP...
- Variable envelope return pathVariable envelope return pathVariable envelope return path is a technique used by some electronic mailing list software to enable automatic detection and removal of undeliverable e-mail addresses...
External links
- Essential Internet Protocols - SMTP
- SMTP Sequence Diagram (PDF)
- Diagram of e-mail flow (PDF, PNG )
- The Case For E-mail Security - Security and Insecurity in SMTP, POP and IMAP.
- Picture of the first computers to send and receive a network email, 2 PDP-10s
- Email Address Internationalization IETF Working Group
- SMTP TLS Transport real-time test - Live version of above example, mostly for TLS (i.e. secure) email but useful for non-TLS too
