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Simple Mail Transfer Protocol
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Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 10), and last updated by RFC 5321 (2008), which describes the protocol in widespread use today, also known as extended SMTP (ESMTP).
While electronic mail server software uses 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.
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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 in RFC 821 (STD 10), and last updated by RFC 5321 (2008), which describes the protocol in widespread use today, also known as extended SMTP (ESMTP).
While electronic mail server software uses 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) to access their mail box accounts on a mail server.
Description
SMTP is a relatively simple, text-based protocol, in which one or more recipients of a message are specified (and in most cases verified to exist) along with the message text and possibly other encoded objects. The message is then transferred to a remote server using a series of queries and responses between the client and server. Either an end-user's e-mail client, a.k.a. MUA (Mail User Agent), or a relaying server's MTA (Mail Transport Agent) can act as an SMTP client.
An e-mail client knows the outgoing mail SMTP server from its configuration. A relaying server typically determines which SMTP server to connect to by looking up the MX (Mail eXchange) DNS record for each recipient's domain name. Conformant MTAs (not all) fall back to a simple A record in the case of no MX (relaying servers can also be configured to use a smart host).
The SMTP client initiates a TCP connection to server's port 25 (unless overridden by configuration). It is quite easy to test an SMTP server using the netcat program (see below).
SMTP is a "push" protocol that cannot "pull" messages from a remote server on demand. To retrieve messages only on demand, which is the most common requirement on a single-user computer, a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN. It is possible to receive mail by running an SMTP server. POP3 became popular when single-user computers connected to the Internet only intermittently; SMTP is more suitable for a machine permanently connected to the Internet.
A simple aid to memory is "Send Mail To People."
History
Various forms of one-to-one electronic messaging were used in the 1960s. People communicated with one another using systems developed for a particular mainframe computer. As more computers began to be interconnected with others, especially in the US Government's ARPANET, standards were developed to allow users using different systems to be able to 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 may not 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 to send 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. The work continued throughout the 1970s, until the ARPANET converted into the modern Internet around 1980. Jon Postel then proposed a Mail Transfer Protocol in 1980 that began to remove the mail's reliance on FTP. 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 UUCP (Unix to Unix CoPy) mail, which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both use a store and forward mechanism and are examples of push technology. 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 and the Sobig worm. As of 2001 there were at least 50 programs that implemented SMTP either as clients (senders of messages) or as servers (receivers of messages).
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 Mail submission agents for Mail user agents, some of which were now relaying mail from the outside of an organization. (e.g. A company executive wishes to send 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 the SMTP protocol had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as unsolicited e-mail (spam) relaying.
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. MTAs developed after Sendmail also tended to be implemented 8-bit-clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary data via SMTP. Non-8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.
Developers
Many people edited or contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore.
Outgoing mail SMTP server
An e-mail client requires the name or the IP address of an SMTP server as part of its configuration. The server will deliver messages on behalf of the user. This setting allows for various policies and network designs. End users connected to the Internet can use the services of an e-mail provider that is not necessarily the same as their connection provider (ISP). Network topology, or the location of a client within a network or outside of a network, is no longer a limiting factor for e-mail submission or delivery. Modern SMTP servers typically use a client's credentials (authentication) rather than a client's location (IP address), to determine whether it is eligible to relay e-mail.
Server administrators choose whether clients use TCP port 25 (SMTP) or port 587 (Submission), as formalized in RFC 4409, 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. A server that relays all e-mail for all destinations for all clients connecting to port 25 is known as an open relay and is now generally considered a bad practice worthy of blacklisting.
Some Internet service providers intercept port 25, so that it is not possible for their users to send mail via a relaying SMTP server elsewhere using port 25; they are restricted to using the ISP's SMTP server only. Some independent SMTP servers support an additional port other than 25 to allow users with authenticated access to connect to them even if port 25 is blocked. The practical purpose of this is that a travelling user connecting to different ISPs otherwise has to change SMTP server settings on the mail client for each ISP; using a relaying SMTP server allows the SMTP client settings to be used unchanged worldwide.
Sample communications
After establishing a connection between the sender (the client) and the receiver (the server), the following is a valid SMTP session. In the following conversation, everything sent by the client is prefixed here with "C: " and everything sent by the server with "S: "; this prefix is not part of the conversation. On most computer systems, a connection can be established using the netcat command (or telnet if netcat is not available) on the client machine, for example:
telnet smtp.example.com 25
which opens a TCP connection from the sending machine to the MTA listening on port 25 on host smtp.example.com. By convention, SMTP servers greet clients with their fully-qualified domain name. In this example, the client computer (relay.example.org) has already determined that "smtp.example.com" is a mail exchanger for the example.com domain by doing a DNS lookup of example.coms MX records. Note that a carriage return and a line feed character (not shown) are required at the end of each line; in a manual Telnet session they are both normally generated by pressing the Enter or carriage return key once.
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 headers and 4 lines in the body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
In this example, the e-mail is sent to two mailboxes on the same SMTP server: once for each recipient listed in the "To" and "Cc" headers; if there were any in a "Bcc" list, which are not included in any headers, there would have been additional "RCPT TO" commands for those recipients as well. If the second recipient had been located elsewhere, the client would QUIT and connect to the appropriate SMTP server once the first message had been queued. Note that the information the client sends in the HELO and MAIL FROM commands can be retrieved in additional headers that the server adds to the message: Received and Return-Path respectively.
Although optional and not shown above, many clients ask the server which SMTP extensions the server supports, by using the EHLO greeting to invoke Extended SMTP (ESMTP) specified in RFC 1870. These clients fall back to HELO only if the server does not respond to EHLO.
Modern clients may use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted. Older clients and servers may try to transfer huge messages that will be rejected after wasting the network resources, including a lot of connect time to dial-up ISPs that are paid by the minute.
Users can manually determine in advance the maximum size accepted by ESMTP servers. The user telnets as above, but substitutes "EHLO host.example.org" for the HELO command line.
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.
Security and spamming
One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.
Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 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.
Other protocols for e-mail
E-mail is "handed off" (pushed) from a client (MUA) to a mail server (MSA), usually using Simple Mail Transfer Protocol. From there, the MSA delivers the mail to an MTA, usually running on the same machine. The MTA looks up the destination(s)'s MX records with a DNS lookup, and begins to relay (push) the message to the server on record via TCP port 25 and SMTP. Once the receiving MTA accepts the incoming message, it is delivered via a mail delivery agent (MDA) to a server which is designated for local mail delivery. The MDA either delivers the mail directly to storage, or forwards it over a network using either SMTP or 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). Generally speaking, mail retrieval (pull) is performed using either a type of online folders (e.g. IMAP 4, a protocol that both delivers and organizes mail) or the older single repository format (e.g. POP3, the Post Office Protocol). Webmail clients may use either method, but the retrieval protocol is often not a formal standard. Some local mail servers and MUAs are capable of either push or pull mail retrieval.
Related Requests For Comments (RFCs)
- RFC 1123 – Requirements for Internet Hosts -- Application and Support (STD 3)
- RFC 1870 – SMTP Service Extension for Message Size Declaration (?bsoletes: RFC 1653)
- RFC 2476 – Message Submission
- 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 4409 – Message Submission for Mail (obsoletes RFC 2476)
- 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)
See also
External links
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- Includes information on getting around port 25 blocking.
- - Security and Insecurity in SMTP, POP and IMAP.
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