RFC 2821
Simple Mail Transfer Protocol
Network Working Group J. Klensin, Editor Request for Comments: 2821 AT&T Laboratories Obsoletes: 821, 974, 1869 April 2001 Updates: 1123 Category: Standards Track Simple Mail Transfer Protocol Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved.Abstract
This document is a self-contained specification of the basic protocol for the Internet electronic mail transport. It consolidates, updates and clarifies, but doesn't add new or change existing functionality of the following: - the original SMTP (Simple Mail Transfer Protocol) specification of RFC 821 [30], - domain name system requirements and implications for mail transport from RFC 1035 [22] and RFC 974 [27], - the clarifications and applicability statements in RFC 1123 [2], and - material drawn from the SMTP Extension mechanisms [19]. It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces the mail transport materials of RFC 1123). However, RFC 821 specifies some features that were not in significant use in the Internet by the mid-1990s and (in appendices) some additional transport models. Those sections are omitted here in the interest of clarity and brevity; readers needing them should refer to RFC 821.
It also includes some additional material from RFC 1123 that required amplification. This material has been identified in multiple ways, mostly by tracking flaming on various lists and newsgroups and problems of unusual readings or interpretations that have appeared as the SMTP extensions have been deployed. Where this specification moves beyond consolidation and actually differs from earlier documents, it supersedes them technically as well as textually. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a 'mail submission' protocol, as recommended for POP [3, 26] and IMAP [6]. Additional submission issues are discussed in RFC 2476 [15]. Section 2.3 provides definitions of terms specific to this document. Except when the historical terminology is necessary for clarity, this document uses the current 'client' and 'server' terminology to identify the sending and receiving SMTP processes, respectively. A companion document [32] discusses message headers, message bodies and formats and structures for them, and their relationship.Table of Contents
1. Introduction .................................................. 4 2. The SMTP Model ................................................ 5 2.1 Basic Structure .............................................. 5 2.2 The Extension Model .......................................... 7 2.2.1 Background ................................................. 7 2.2.2 Definition and Registration of Extensions .................. 8 2.3 Terminology .................................................. 9 2.3.1 Mail Objects ............................................... 10 2.3.2 Senders and Receivers ...................................... 10 2.3.3 Mail Agents and Message Stores ............................. 10 2.3.4 Host ....................................................... 11 2.3.5 Domain ..................................................... 11 2.3.6 Buffer and State Table ..................................... 11 2.3.7 Lines ...................................................... 12 2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12 2.3.9 Message Content and Mail Data .............................. 13 2.3.10 Mailbox and Address ....................................... 13 2.3.11 Reply ..................................................... 13 2.4 General Syntax Principles and Transaction Model .............. 13 3. The SMTP Procedures: An Overview .............................. 15 3.1 Session Initiation ........................................... 15 3.2 Client Initiation ............................................ 16 3.3 Mail Transactions ............................................ 16 3.4 Forwarding for Address Correction or Updating ................ 19
3.5 Commands for Debugging Addresses ............................. 20 3.5.1 Overview ................................................... 20 3.5.2 VRFY Normal Response ....................................... 22 3.5.3 Meaning of VRFY or EXPN Success Response ................... 22 3.5.4 Semantics and Applications of EXPN ......................... 23 3.6 Domains ...................................................... 23 3.7 Relaying ..................................................... 24 3.8 Mail Gatewaying .............................................. 25 3.8.1 Header Fields in Gatewaying ................................ 26 3.8.2 Received Lines in Gatewaying ............................... 26 3.8.3 Addresses in Gatewaying .................................... 26 3.8.4 Other Header Fields in Gatewaying .......................... 27 3.8.5 Envelopes in Gatewaying .................................... 27 3.9 Terminating Sessions and Connections ......................... 27 3.10 Mailing Lists and Aliases ................................... 28 3.10.1 Alias ..................................................... 28 3.10.2 List ...................................................... 28 4. The SMTP Specifications ....................................... 29 4.1 SMTP Commands ................................................ 29 4.1.1 Command Semantics and Syntax ............................... 29 4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO) ................... 29 4.1.1.2 MAIL (MAIL) .............................................. 31 4.1.1.3 RECIPIENT (RCPT) ......................................... 31 4.1.1.4 DATA (DATA) .............................................. 33 4.1.1.5 RESET (RSET) ............................................. 34 4.1.1.6 VERIFY (VRFY) ............................................ 35 4.1.1.7 EXPAND (EXPN) ............................................ 35 4.1.1.8 HELP (HELP) .............................................. 35 4.1.1.9 NOOP (NOOP) .............................................. 35 4.1.1.10 QUIT (QUIT) ............................................. 36 4.1.2 Command Argument Syntax .................................... 36 4.1.3 Address Literals ........................................... 38 4.1.4 Order of Commands .......................................... 39 4.1.5 Private-use Commands ....................................... 40 4.2 SMTP Replies ................................................ 40 4.2.1 Reply Code Severities and Theory ........................... 42 4.2.2 Reply Codes by Function Groups ............................. 44 4.2.3 Reply Codes in Numeric Order .............................. 45 4.2.4 Reply Code 502 ............................................. 46 4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46 4.3 Sequencing of Commands and Replies ........................... 47 4.3.1 Sequencing Overview ........................................ 47 4.3.2 Command-Reply Sequences .................................... 48 4.4 Trace Information ............................................ 49 4.5 Additional Implementation Issues ............................. 53 4.5.1 Minimum Implementation ..................................... 53 4.5.2 Transparency ............................................... 53 4.5.3 Sizes and Timeouts ......................................... 54
4.5.3.1 Size limits and minimums ................................. 54 4.5.3.2 Timeouts ................................................. 56 4.5.4 Retry Strategies ........................................... 57 4.5.4.1 Sending Strategy ......................................... 58 4.5.4.2 Receiving Strategy ....................................... 59 4.5.5 Messages with a null reverse-path .......................... 59 5. Address Resolution and Mail Handling .......................... 60 6. Problem Detection and Handling ................................ 62 6.1 Reliable Delivery and Replies by Email ....................... 62 6.2 Loop Detection ............................................... 63 6.3 Compensating for Irregularities .............................. 63 7. Security Considerations ....................................... 64 7.1 Mail Security and Spoofing ................................... 64 7.2 "Blind" Copies ............................................... 65 7.3 VRFY, EXPN, and Security ..................................... 65 7.4 Information Disclosure in Announcements ...................... 66 7.5 Information Disclosure in Trace Fields ....................... 66 7.6 Information Disclosure in Message Forwarding ................. 67 7.7 Scope of Operation of SMTP Servers ........................... 67 8. IANA Considerations ........................................... 67 9. References .................................................... 68 10. Editor's Address ............................................. 70 11. Acknowledgments .............................................. 70 Appendices ....................................................... 71 A. TCP Transport Service ......................................... 71 B. Generating SMTP Commands from RFC 822 Headers ................. 71 C. Source Routes ................................................. 72 D. Scenarios ..................................................... 73 E. Other Gateway Issues .......................................... 76 F. Deprecated Features of RFC 821 ................................ 76 Full Copyright Statement ......................................... 791. Introduction
The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently. SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. While this document specifically discusses transport over TCP, other transports are possible. Appendices to RFC 821 describe some of them. An important feature of SMTP is its capability to transport mail across networks, usually referred to as "SMTP mail relaying" (see section 3.8). A network consists of the mutually-TCP-accessible hosts on the public Internet, the mutually-TCP-accessible hosts on a firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN environment utilizing a non-TCP transport-level protocol. Using
SMTP, a process can transfer mail to another process on the same network or to some other network via a relay or gateway process accessible to both networks. In this way, a mail message may pass through a number of intermediate relay or gateway hosts on its path from sender to ultimate recipient. The Mail eXchanger mechanisms of the domain name system [22, 27] (and section 5 of this document) are used to identify the appropriate next-hop destination for a message being transported.2. The SMTP Model
2.1 Basic Structure
The SMTP design can be pictured as: +----------+ +----------+ +------+ | | | | | User |<-->| | SMTP | | +------+ | Client- |Commands/Replies| Server- | +------+ | SMTP |<-------------->| SMTP | +------+ | File |<-->| | and Mail | |<-->| File | |System| | | | | |System| +------+ +----------+ +----------+ +------+ SMTP client SMTP server When an SMTP client has a message to transmit, it establishes a two- way transmission channel to an SMTP server. The responsibility of an SMTP client is to transfer mail messages to one or more SMTP servers, or report its failure to do so. The means by which a mail message is presented to an SMTP client, and how that client determines the domain name(s) to which mail messages are to be transferred is a local matter, and is not addressed by this document. In some cases, the domain name(s) transferred to, or determined by, an SMTP client will identify the final destination(s) of the mail message. In other cases, common with SMTP clients associated with implementations of the POP [3, 26] or IMAP [6] protocols, or when the SMTP client is inside an isolated transport service environment, the domain name determined will identify an intermediate destination through which all mail messages are to be relayed. SMTP clients that transfer all traffic, regardless of the target domain names associated with the individual messages, or that do not maintain queues for retrying message transmissions that initially cannot be completed, may otherwise conform to this specification but are not considered fully-capable. Fully-capable SMTP implementations, including the relays used by these less capable
ones, and their destinations, are expected to support all of the queuing, retrying, and alternate address functions discussed in this specification. The means by which an SMTP client, once it has determined a target domain name, determines the identity of an SMTP server to which a copy of a message is to be transferred, and then performs that transfer, is covered by this document. To effect a mail transfer to an SMTP server, an SMTP client establishes a two-way transmission channel to that SMTP server. An SMTP client determines the address of an appropriate host running an SMTP server by resolving a destination domain name to either an intermediate Mail eXchanger host or a final target host. An SMTP server may be either the ultimate destination or an intermediate "relay" (that is, it may assume the role of an SMTP client after receiving the message) or "gateway" (that is, it may transport the message further using some protocol other than SMTP). SMTP commands are generated by the SMTP client and sent to the SMTP server. SMTP replies are sent from the SMTP server to the SMTP client in response to the commands. In other words, message transfer can occur in a single connection between the original SMTP-sender and the final SMTP-recipient, or can occur in a series of hops through intermediary systems. In either case, a formal handoff of responsibility for the message occurs: the protocol requires that a server accept responsibility for either delivering a message or properly reporting the failure to do so. Once the transmission channel is established and initial handshaking completed, the SMTP client normally initiates a mail transaction. Such a transaction consists of a series of commands to specify the originator and destination of the mail and transmission of the message content (including any headers or other structure) itself. When the same message is sent to multiple recipients, this protocol encourages the transmission of only one copy of the data for all recipients at the same destination (or intermediate relay) host. The server responds to each command with a reply; replies may indicate that the command was accepted, that additional commands are expected, or that a temporary or permanent error condition exists. Commands specifying the sender or recipients may include server- permitted SMTP service extension requests as discussed in section 2.2. The dialog is purposely lock-step, one-at-a-time, although this can be modified by mutually-agreed extension requests such as command pipelining [13].
Once a given mail message has been transmitted, the client may either request that the connection be shut down or may initiate other mail transactions. In addition, an SMTP client may use a connection to an SMTP server for ancillary services such as verification of email addresses or retrieval of mailing list subscriber addresses. As suggested above, this protocol provides mechanisms for the transmission of mail. This transmission normally occurs directly from the sending user's host to the receiving user's host when the two hosts are connected to the same transport service. When they are not connected to the same transport service, transmission occurs via one or more relay SMTP servers. An intermediate host that acts as either an SMTP relay or as a gateway into some other transmission environment is usually selected through the use of the domain name service (DNS) Mail eXchanger mechanism. Usually, intermediate hosts are determined via the DNS MX record, not by explicit "source" routing (see section 5 and appendices C and F.2).2.2 The Extension Model
2.2.1 Background
In an effort that started in 1990, approximately a decade after RFC 821 was completed, the protocol was modified with a "service extensions" model that permits the client and server to agree to utilize shared functionality beyond the original SMTP requirements. The SMTP extension mechanism defines a means whereby an extended SMTP client and server may recognize each other, and the server can inform the client as to the service extensions that it supports. Contemporary SMTP implementations MUST support the basic extension mechanisms. For instance, servers MUST support the EHLO command even if they do not implement any specific extensions and clients SHOULD preferentially utilize EHLO rather than HELO. (However, for compatibility with older conforming implementations, SMTP clients and servers MUST support the original HELO mechanisms as a fallback.) Unless the different characteristics of HELO must be identified for interoperability purposes, this document discusses only EHLO. SMTP is widely deployed and high-quality implementations have proven to be very robust. However, the Internet community now considers some services to be important that were not anticipated when the protocol was first designed. If support for those services is to be added, it must be done in a way that permits older implementations to continue working acceptably. The extension framework consists of:
- The SMTP command EHLO, superseding the earlier HELO,
- a registry of SMTP service extensions,
- additional parameters to the SMTP MAIL and RCPT commands, and
- optional replacements for commands defined in this protocol, such
as for DATA in non-ASCII transmissions [33].
SMTP's strength comes primarily from its simplicity. Experience with
many protocols has shown that protocols with few options tend towards
ubiquity, whereas protocols with many options tend towards obscurity.
Each and every extension, regardless of its benefits, must be
carefully scrutinized with respect to its implementation, deployment,
and interoperability costs. In many cases, the cost of extending the
SMTP service will likely outweigh the benefit.
2.2.2 Definition and Registration of Extensions
The IANA maintains a registry of SMTP service extensions. A
corresponding EHLO keyword value is associated with each extension.
Each service extension registered with the IANA must be defined in a
formal standards-track or IESG-approved experimental protocol
document. The definition must include:
- the textual name of the SMTP service extension;
- the EHLO keyword value associated with the extension;
- the syntax and possible values of parameters associated with the
EHLO keyword value;
- any additional SMTP verbs associated with the extension
(additional verbs will usually be, but are not required to be, the
same as the EHLO keyword value);
- any new parameters the extension associates with the MAIL or RCPT
verbs;
- a description of how support for the extension affects the
behavior of a server and client SMTP; and,
- the increment by which the extension is increasing the maximum
length of the commands MAIL and/or RCPT, over that specified in
this standard.
In addition, any EHLO keyword value starting with an upper or lower case "X" refers to a local SMTP service extension used exclusively through bilateral agreement. Keywords beginning with "X" MUST NOT be used in a registered service extension. Conversely, keyword values presented in the EHLO response that do not begin with "X" MUST correspond to a standard, standards-track, or IESG-approved experimental SMTP service extension registered with IANA. A conforming server MUST NOT offer non-"X"-prefixed keyword values that are not described in a registered extension. Additional verbs and parameter names are bound by the same rules as EHLO keywords; specifically, verbs beginning with "X" are local extensions that may not be registered or standardized. Conversely, verbs not beginning with "X" must always be registered.2.3 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described below. 1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification. 2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the definition is an absolute prohibition of the specification. 3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. 4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. 5. MAY This word, or the adjective "OPTIONAL", mean that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which
does include a particular option MUST be prepared to interoperate
with another implementation which does not include the option
(except, of course, for the feature the option provides.)
2.3.1 Mail Objects
SMTP transports a mail object. A mail object contains an envelope
and content.
The SMTP envelope is sent as a series of SMTP protocol units
(described in section 3). It consists of an originator address (to
which error reports should be directed); one or more recipient
addresses; and optional protocol extension material. Historically,
variations on the recipient address specification command (RCPT TO)
could be used to specify alternate delivery modes, such as immediate
display; those variations have now been deprecated (see appendix F,
section F.6).
The SMTP content is sent in the SMTP DATA protocol unit and has two
parts: the headers and the body. If the content conforms to other
contemporary standards, the headers form a collection of field/value
pairs structured as in the message format specification [32]; the
body, if structured, is defined according to MIME [12]. The content
is textual in nature, expressed using the US-ASCII repertoire [1].
Although SMTP extensions (such as "8BITMIME" [20]) may relax this
restriction for the content body, the content headers are always
encoded using the US-ASCII repertoire. A MIME extension [23] defines
an algorithm for representing header values outside the US-ASCII
repertoire, while still encoding them using the US-ASCII repertoire.
2.3.2 Senders and Receivers
In RFC 821, the two hosts participating in an SMTP transaction were
described as the "SMTP-sender" and "SMTP-receiver". This document
has been changed to reflect current industry terminology and hence
refers to them as the "SMTP client" (or sometimes just "the client")
and "SMTP server" (or just "the server"), respectively. Since a
given host may act both as server and client in a relay situation,
"receiver" and "sender" terminology is still used where needed for
clarity.
2.3.3 Mail Agents and Message Stores
Additional mail system terminology became common after RFC 821 was
published and, where convenient, is used in this specification. In
particular, SMTP servers and clients provide a mail transport service
and therefore act as "Mail Transfer Agents" (MTAs). "Mail User
Agents" (MUAs or UAs) are normally thought of as the sources and
targets of mail. At the source, an MUA might collect mail to be
transmitted from a user and hand it off to an MTA; the final
("delivery") MTA would be thought of as handing the mail off to an
MUA (or at least transferring responsibility to it, e.g., by
depositing the message in a "message store"). However, while these
terms are used with at least the appearance of great precision in
other environments, the implied boundaries between MUAs and MTAs
often do not accurately match common, and conforming, practices with
Internet mail. Hence, the reader should be cautious about inferring
the strong relationships and responsibilities that might be implied
if these terms were used elsewhere.
2.3.4 Host
For the purposes of this specification, a host is a computer system
attached to the Internet (or, in some cases, to a private TCP/IP
network) and supporting the SMTP protocol. Hosts are known by names
(see "domain"); identifying them by numerical address is discouraged.
2.3.5 Domain
A domain (or domain name) consists of one or more dot-separated
components. These components ("labels" in DNS terminology [22]) are
restricted for SMTP purposes to consist of a sequence of letters,
digits, and hyphens drawn from the ASCII character set [1]. Domain
names are used as names of hosts and of other entities in the domain
name hierarchy. For example, a domain may refer to an alias (label
of a CNAME RR) or the label of Mail eXchanger records to be used to
deliver mail instead of representing a host name. See [22] and
section 5 of this specification.
The domain name, as described in this document and in [22], is the
entire, fully-qualified name (often referred to as an "FQDN"). A
domain name that is not in FQDN form is no more than a local alias.
Local aliases MUST NOT appear in any SMTP transaction.
2.3.6 Buffer and State Table
SMTP sessions are stateful, with both parties carefully maintaining a
common view of the current state. In this document we model this
state by a virtual "buffer" and a "state table" on the server which
may be used by the client to, for example, "clear the buffer" or
"reset the state table," causing the information in the buffer to be
discarded and the state to be returned to some previous state.
2.3.7 Lines
SMTP commands and, unless altered by a service extension, message data, are transmitted in "lines". Lines consist of zero or more data characters terminated by the sequence ASCII character "CR" (hex value 0D) followed immediately by ASCII character "LF" (hex value 0A). This termination sequence is denoted as <CRLF> in this document. Conforming implementations MUST NOT recognize or generate any other character or character sequence as a line terminator. Limits MAY be imposed on line lengths by servers (see section 4.5.3). In addition, the appearance of "bare" "CR" or "LF" characters in text (i.e., either without the other) has a long history of causing problems in mail implementations and applications that use the mail system as a tool. SMTP client implementations MUST NOT transmit these characters except when they are intended as line terminators and then MUST, as indicated above, transmit them only as a <CRLF> sequence.2.3.8 Originator, Delivery, Relay, and Gateway Systems
This specification makes a distinction among four types of SMTP systems, based on the role those systems play in transmitting electronic mail. An "originating" system (sometimes called an SMTP originator) introduces mail into the Internet or, more generally, into a transport service environment. A "delivery" SMTP system is one that receives mail from a transport service environment and passes it to a mail user agent or deposits it in a message store which a mail user agent is expected to subsequently access. A "relay" SMTP system (usually referred to just as a "relay") receives mail from an SMTP client and transmits it, without modification to the message data other than adding trace information, to another SMTP server for further relaying or for delivery. A "gateway" SMTP system (usually referred to just as a "gateway") receives mail from a client system in one transport environment and transmits it to a server system in another transport environment. Differences in protocols or message semantics between the transport environments on either side of a gateway may require that the gateway system perform transformations to the message that are not permitted to SMTP relay systems. For the purposes of this specification, firewalls that rewrite addresses should be considered as gateways, even if SMTP is used on both sides of them (see [11]).
2.3.9 Message Content and Mail Data
The terms "message content" and "mail data" are used interchangeably in this document to describe the material transmitted after the DATA command is accepted and before the end of data indication is transmitted. Message content includes message headers and the possibly-structured message body. The MIME specification [12] provides the standard mechanisms for structured message bodies.2.3.10 Mailbox and Address
As used in this specification, an "address" is a character string that identifies a user to whom mail will be sent or a location into which mail will be deposited. The term "mailbox" refers to that depository. The two terms are typically used interchangeably unless the distinction between the location in which mail is placed (the mailbox) and a reference to it (the address) is important. An address normally consists of user and domain specifications. The standard mailbox naming convention is defined to be "local- part@domain": contemporary usage permits a much broader set of applications than simple "user names". Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.2.3.11 Reply
An SMTP reply is an acknowledgment (positive or negative) sent from receiver to sender via the transmission channel in response to a command. The general form of a reply is a numeric completion code (indicating failure or success) usually followed by a text string. The codes are for use by programs and the text is usually intended for human users. Recent work [34] has specified further structuring of the reply strings, including the use of supplemental and more specific completion codes.2.4 General Syntax Principles and Transaction Model
SMTP commands and replies have a rigid syntax. All commands begin with a command verb. All Replies begin with a three digit numeric code. In some commands and replies, arguments MUST follow the verb or reply code. Some commands do not accept arguments (after the verb), and some reply codes are followed, sometimes optionally, by free form text. In both cases, where text appears, it is separated from the verb or reply code by a space character. Complete definitions of commands and replies appear in section 4.
Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command and extension name keywords) are not case sensitive, with the sole exception in this specification of a mailbox local-part (SMTP Extensions may explicitly specify case-sensitive elements). That is, a command verb, an argument value other than a mailbox local-part, and free form text MAY be encoded in upper case, lower case, or any mixture of upper and lower case with no impact on its meaning. This is NOT true of a mailbox local-part. The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged. A few SMTP servers, in violation of this specification (and RFC 821) require that command verbs be encoded by clients in upper case. Implementations MAY wish to employ this encoding to accommodate those servers. The argument field consists of a variable length character string ending with the end of the line, i.e., with the character sequence <CRLF>. The receiver will take no action until this sequence is received. The syntax for each command is shown with the discussion of that command. Common elements and parameters are shown in section 4.1.2. Commands and replies are composed of characters from the ASCII character set [1]. When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero. More specifically, the unextended SMTP service provides seven bit transport only. An originating SMTP client which has not successfully negotiated an appropriate extension with a particular server MUST NOT transmit messages with information in the high-order bit of octets. If such messages are transmitted in violation of this rule, receiving SMTP servers MAY clear the high-order bit or reject the message as invalid. In general, a relay SMTP SHOULD assume that the message content it has received is valid and, assuming that the envelope permits doing so, relay it without inspecting that content. Of course, if the content is mislabeled and the data path cannot accept the actual content, this may result in ultimate delivery of a severely garbled message to the recipient. Delivery SMTP systems MAY reject ("bounce") such messages rather than deliver them. No sending SMTP system is permitted to send envelope commands in any character
set other than US-ASCII; receiving systems SHOULD reject such commands, normally using "500 syntax error - invalid character" replies. Eight-bit message content transmission MAY be requested of the server by a client using extended SMTP facilities, notably the "8BITMIME" extension [20]. 8BITMIME SHOULD be supported by SMTP servers. However, it MUST not be construed as authorization to transmit unrestricted eight bit material. 8BITMIME MUST NOT be requested by senders for material with the high bit on that is not in MIME format with an appropriate content-transfer encoding; servers MAY reject such messages. The metalinguistic notation used in this document corresponds to the "Augmented BNF" used in other Internet mail system documents. The reader who is not familiar with that syntax should consult the ABNF specification [8]. Metalanguage terms used in running text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
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