Section 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 identifier(s) ("names") of the
   domain(s) to which mail messages are to be transferred, is a local
   matter, and is not addressed by this document.  In some cases, the
   designated domain(s), or those 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 (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
   protocols, or when the SMTP client is inside an isolated transport
   service environment, the domain 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 domains 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.  In many situations and configurations, the less-
   capable clients discussed above SHOULD be using the message
   submission protocol (RFC 4409 [18]) rather than SMTP.
   The means by which an SMTP client, once it has determined a target
   domain, 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, once the server has issued a success response at the end of the
   mail data, a formal handoff of responsibility for the message occurs:
   the protocol requires that a server MUST accept responsibility for
   either delivering the message or properly reporting the failure to do
   so (see Sections 6.1, 6.2, and 7.8, below).
   Once the transmission channel is established and initial handshaking
   is 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 lines in the header section 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 upon extension
   requests such as command pipelining (RFC 2920 [19]).
   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.  Historically, this transmission normally
   occurred 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.  A very
   common case in the Internet today involves submission of the original
   message to an intermediate, "message submission" server, which is
   similar to a relay but has some additional properties; such servers
   are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
   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 Appendix C and
   Appendix 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:
   o  The SMTP command EHLO, superseding the earlier HELO,
   o  a registry of SMTP service extensions,
   o  additional parameters to the SMTP MAIL and RCPT commands, and
   o  optional replacements for commands defined in this protocol, such
      as for DATA in non-ASCII transmissions (RFC 3030 [20]).
   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:
   o  the textual name of the SMTP service extension;
   o  the EHLO keyword value associated with the extension;
   o  the syntax and possible values of parameters associated with the
      EHLO keyword value;
   o  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);
   o  any new parameters the extension associates with the MAIL or RCPT
      verbs;
   o  a description of how support for the extension affects the
      behavior of a server and client SMTP; and
   o  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.
2.2.3.  Special Issues with Extensions
   Extensions that change fairly basic properties of SMTP operation are
   permitted.  The text in other sections of this document must be
   understood in that context.  In particular, extensions can change the
   minimum limits specified in Section 4.5.3, can change the ASCII
   character set requirement as mentioned above, or can introduce some
   optional modes of message handling.
   In particular, if an extension implies that the delivery path
   normally supports special features of that extension, and an
   intermediate SMTP system finds a next hop that does not support the
   required extension, it MAY choose, based on the specific extension
   and circumstances, to requeue the message and try later and/or try an
   alternate MX host.  If this strategy is employed, the timeout to fall
   back to an unextended format (if one is available) SHOULD be less
   than the normal timeout for bouncing as undeliverable (e.g., if
   normal timeout is three days, the requeue timeout before attempting
   to transmit the mail without the extension might be one day).
2.3.  SMTP Terminology
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 reverse-path (originator) address specification
   command (MAIL) could be used to specify alternate delivery modes,
   such as immediate display; those variations have now been deprecated
   (see Appendix F and Appendix F.6).
   The SMTP content is sent in the SMTP DATA protocol unit and has two
   parts: the header section and the body.  If the content conforms to
   other contemporary standards, the header section consists of a
   collection of header fields, each consisting of a header name, a
   colon, and data, structured as in the message format specification
   (RFC 5322 [4]); the body, if structured, is defined according to MIME
   (RFC 2045 [21]).  The content is textual in nature, expressed using
   the US-ASCII repertoire [6].  Although SMTP extensions (such as
   "8BITMIME", RFC 1652 [22]) may relax this restriction for the content
   body, the content header fields are always encoded using the US-ASCII
   repertoire.  Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
   define 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 the next section); they SHOULD NOT be identified by numerical
   addresses, i.e., by address literals as described in Section 4.1.2.
2.3.5.  Domain Names
   A domain name (or often just a "domain") consists of one or more
   components, separated by dots if more than one appears.  In the case
   of a top-level domain used by itself in an email address, a single
   string is used without any dots.  This makes the requirement,
   described in more detail below, that only fully-qualified domain
   names appear in SMTP transactions on the public Internet,
   particularly important where top-level domains are involved.  These
   components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
   for SMTP purposes to consist of a sequence of letters, digits, and
   hyphens drawn from the ASCII character set [6].  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 RFC 1035 [2]
   and Section 5 of this specification.
   The domain name, as described in this document and in RFC 1035 [2],
   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.
   Only resolvable, fully-qualified domain names (FQDNs) are permitted
   when domain names are used in SMTP.  In other words, names that can
   be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
   in Section 5) are permitted, as are CNAME RRs whose targets can be
   resolved, in turn, to MX or address RRs.  Local nicknames or
   unqualified names MUST NOT be used.  There are two exceptions to the
   rule requiring FQDNs:
   o  The domain name given in the EHLO command MUST be either a primary
      host name (a domain name that resolves to an address RR) or, if
      the host has no name, an address literal, as described in
      Section 4.1.3 and discussed further in the EHLO discussion of
      Section 4.1.4.
   o  The reserved mailbox name "postmaster" may be used in a RCPT
      command without domain qualification (see Section 4.1.1.3) and
      MUST be accepted if so used.
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 that
   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.  Commands and Replies
   SMTP commands and, unless altered by a service extension, message
   data, are transmitted from the sender to the receiver via the
   transmission channel in "lines".
   An SMTP reply is an acknowledgment (positive or negative) sent in
   "lines" 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.  RFC 3463 [25], specifies further
   structuring of the reply strings, including the use of supplemental
   and more specific completion codes (see also RFC 5248 [26]).
2.3.8.  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).
   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.
2.3.9.  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 the message header section and
   the possibly structured message body.  The MIME specification (RFC
   2045 [21]) provides the standard mechanisms for structured message
   bodies.
2.3.10.  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 that
   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 RFC 2979 [27]).
2.3.11.  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.
2.4.  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 are required following
   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.  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.  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.  Mailbox domains follow normal DNS rules and are
   hence not case sensitive.
   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 clause 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 [6].  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 7-bit
   transport only.  An originating SMTP client that has not successfully
   negotiated an appropriate extension with a particular server (see the
   next paragraph) 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
   the ultimate delivery of a severely garbled message to the recipient.
   Delivery SMTP systems MAY reject such messages, or return them as
   undeliverable, rather than deliver them.  In the absence of a server-
   offered extension explicitly permitting it, a sending SMTP system is
   not 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.
   8-bit message content transmission MAY be requested of the server by
   a client using extended SMTP facilities, notably the "8BITMIME"
   extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
   servers.  However, it MUST NOT be construed as authorization to
   transmit unrestricted 8-bit material, nor does 8BITMIME authorize
   transmission of any envelope material in other than ASCII. 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 in RFC 5234 [7].  Metalanguage terms used in running
   text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
   The reader is cautioned that the grammar expressed in the
   metalanguage is not comprehensive.  There are many instances in which
   provisions in the text constrain or otherwise modify the syntax or
   semantics implied by the grammar.