SMTP [rfc]

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SMTP [rfc]

Postby weazy » Sat May 31, 2003 4:00 pm

RFC 821





SIMPLE MAIL TRANSFER PROTOCOL



Jonathan B. Postel

August 1982



Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291


RFC 821 August 1982
Simple Mail Transfer Protocol



TABLE OF CONTENTS

1. INTRODUCTION .................................................. 1

2. THE SMTP MODEL ................................................ 2

3. THE SMTP PROCEDURE ............................................ 4

3.1. Mail ..................................................... 4
3.2. Forwarding ............................................... 7
3.3. Verifying and Expanding .................................. 8
3.4. Sending and Mailing ..................................... 11
3.5. Opening and Closing ..................................... 13
3.6. Relaying ................................................ 14
3.7. Domains ................................................. 17
3.8. Changing Roles .......................................... 18

4. THE SMTP SPECIFICATIONS ...................................... 19

4.1. SMTP Commands ........................................... 19
4.1.1. Command Semantics ..................................... 19
4.1.2. Command Syntax ........................................ 27
4.2. SMTP Replies ............................................ 34
4.2.1. Reply Codes by Function Group ......................... 35
4.2.2. Reply Codes in Numeric Order .......................... 36
4.3. Sequencing of Commands and Replies ...................... 37
4.4. State Diagrams .......................................... 39
4.5. Details ................................................. 41
4.5.1. Minimum Implementation ................................ 41
4.5.2. Transparency .......................................... 41
4.5.3. Sizes ................................................. 42

APPENDIX A: TCP ................................................. 44
APPENDIX B: NCP ................................................. 45
APPENDIX C: NITS ................................................ 46
APPENDIX D: X.25 ................................................ 47
APPENDIX E: Theory of Reply Codes ............................... 48
APPENDIX F: Scenarios ........................................... 51

GLOSSARY ......................................................... 64

REFERENCES ....................................................... 67




Network Working Group J. Postel
Request for Comments: DRAFT ISI
Replaces: RFC 788, 780, 772 August 1982

SIMPLE MAIL TRANSFER PROTOCOL


1. INTRODUCTION

The objective of 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. Appendices A,
B, C, and D describe the use of SMTP with various transport services.
A Glossary provides the definitions of terms as used in this
document.

An important feature of SMTP is its capability to relay mail across
transport service environments. A transport service provides an
interprocess communication environment (IPCE). An IPCE may cover one
network, several networks, or a subset of a network. It is important
to realize that transport systems (or IPCEs) are not one-to-one with
networks. A process can communicate directly with another process
through any mutually known IPCE. Mail is an application or use of
interprocess communication. Mail can be communicated between
processes in different IPCEs by relaying through a process connected
to two (or more) IPCEs. More specifically, mail can be relayed
between hosts on different transport systems by a host on both
transport systems.
























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August 1982 RFC 821
Simple Mail Transfer Protocol



2. THE SMTP MODEL

The SMTP design is based on the following model of communication: as
the result of a user mail request, the sender-SMTP establishes a
two-way transmission channel to a receiver-SMTP. The receiver-SMTP
may be either the ultimate destination or an intermediate. SMTP
commands are generated by the sender-SMTP and sent to the
receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
sender-SMTP in response to the commands.

Once the transmission channel is established, the SMTP-sender sends a
MAIL command indicating the sender of the mail. If the SMTP-receiver
can accept mail it responds with an OK reply. The SMTP-sender then
sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an
OK reply; if not, it responds with a reply rejecting that recipient
(but not the whole mail transaction). The SMTP-sender and
SMTP-receiver may negotiate several recipients. When the recipients
have been negotiated the SMTP-sender sends the mail data, terminating
with a special sequence. If the SMTP-receiver successfully processes
the mail data it responds with an OK reply. The dialog is purposely
lock-step, one-at-a-time.

-------------------------------------------------------------


+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+


Sender-SMTP Receiver-SMTP

Model for SMTP Use

Figure 1

-------------------------------------------------------------

The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the



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RFC 821 August 1982
Simple Mail Transfer Protocol



two host are connected to the same transport service, or via one or
more relay SMTP-servers when the source and destination hosts are not
connected to the same transport service.

To be able to provide the relay capability the SMTP-server must be
supplied with the name of the ultimate destination host as well as
the destination mailbox name.

The argument to the MAIL command is a reverse-path, which specifies
who the mail is from. The argument to the RCPT command is a
forward-path, which specifies who the mail is to. The forward-path
is a source route, while the reverse-path is a return route (which
may be used to return a message to the sender when an error occurs
with a relayed message).

When the same message is sent to multiple recipients the SMTP
encourages the transmission of only one copy of the data for all the
recipients at the same destination host.

The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears in Section 4 on specifications.

Commands and replies are not case sensitive. That is, a command or
reply word may be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive, and SMTP implementations
must take case to preserve the case of user names as they appear in
mailbox arguments. Host names are not case sensitive.

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.

When specifying the general form of a command or reply, an argument
(or special symbol) will be denoted by a meta-linguistic variable (or
constant), for example, "<string>" or "<reverse-path>". Here the
angle brackets indicate these are meta-linguistic variables.
However, some arguments use the angle brackets literally. For
example, an actual reverse-path is enclosed in angle brackets, i.e.,
"<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
angle brackets are actually transmitted in the command or reply).





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August 1982 RFC 821
Simple Mail Transfer Protocol



3. THE SMTP PROCEDURES

This section presents the procedures used in SMTP in several parts.
First comes the basic mail procedure defined as a mail transaction.
Following this are descriptions of forwarding mail, verifying mailbox
names and expanding mailing lists, sending to terminals instead of or
in combination with mailboxes, and the opening and closing exchanges.
At the end of this section are comments on relaying, a note on mail
domains, and a discussion of changing roles. Throughout this section
are examples of partial command and reply sequences, several complete
scenarios are presented in Appendix F.

3.1. MAIL

There are three steps to SMTP mail transactions. The transaction
is started with a MAIL command which gives the sender
identification. A series of one or more RCPT commands follows
giving the receiver information. Then a DATA command gives the
mail data. And finally, the end of mail data indicator confirms
the transaction.

The first step in the procedure is the MAIL command. The
<reverse-path> contains the source mailbox.

MAIL <SP> FROM:<reverse-path> <CRLF>

This command tells the SMTP-receiver that a new mail
transaction is starting and to reset all its state tables and
buffers, including any recipients or mail data. It gives the
reverse-path which can be used to report errors. If accepted,
the receiver-SMTP returns a 250 OK reply.

The <reverse-path> can contain more than just a mailbox. The
<reverse-path> is a reverse source routing list of hosts and
source mailbox. The first host in the <reverse-path> should be
the host sending this command.

The second step in the procedure is the RCPT command.

RCPT <SP> TO:<forward-path> <CRLF>

This command gives a forward-path identifying one recipient.
If accepted, the receiver-SMTP returns a 250 OK reply, and
stores the forward-path. If the recipient is unknown the
receiver-SMTP returns a 550 Failure reply. This second step of
the procedure can be repeated any number of times.



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Simple Mail Transfer Protocol



The <forward-path> can contain more than just a mailbox. The
<forward-path> is a source routing list of hosts and the
destination mailbox. The first host in the <forward-path>
should be the host receiving this command.

The third step in the procedure is the DATA command.

DATA <CRLF>

If accepted, the receiver-SMTP returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.

Since the mail data is sent on the transmission channel the end
of the mail data must be indicated so that the command and
reply dialog can be resumed. SMTP indicates the end of the
mail data by sending a line containing only a period. A
transparency procedure is used to prevent this from interfering
with the user's text (see Section 4.5.2).

Please note that the mail data includes the memo header
items such as Date, Subject, To, Cc, From [2].

The end of mail data indicator also confirms the mail
transaction and tells the receiver-SMTP to now process the
stored recipients and mail data. If accepted, the
receiver-SMTP returns a 250 OK reply. The DATA command should
fail only if the mail transaction was incomplete (for example,
no recipients), or if resources are not available.

The above procedure is an example of a mail transaction. These
commands must be used only in the order discussed above.
Example 1 (below) illustrates the use of these commands in a mail
transaction.














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Simple Mail Transfer Protocol



-------------------------------------------------------------

Example of the SMTP Procedure

This SMTP example shows mail sent by Smith at host Alpha.ARPA,
to Jones, Green, and Brown at host Beta.ARPA. Here we assume
that host Alpha contacts host Beta directly.

S: MAIL FROM:<Smith@Alpha.ARPA>
R: 250 OK

S: RCPT TO:<Jones@Beta.ARPA>
R: 250 OK

S: RCPT TO:<Green@Beta.ARPA>
R: 550 No such user here

S: RCPT TO:<Brown@Beta.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 OK

The mail has now been accepted for Jones and Brown. Green did
not have a mailbox at host Beta.

Example 1

-------------------------------------------------------------
















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Simple Mail Transfer Protocol



3.2. FORWARDING

There are some cases where the destination information in the
<forward-path> is incorrect, but the receiver-SMTP knows the
correct destination. In such cases, one of the following replies
should be used to allow the sender to contact the correct
destination.

251 User not local; will forward to <forward-path>

This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use in the future. Note that either the
host or user or both may be different. The receiver takes
responsibility for delivering the message.

551 User not local; please try <forward-path>

This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use. Note that either the host or user or
both may be different. The receiver refuses to accept mail
for this user, and the sender must either redirect the mail
according to the information provided or return an error
response to the originating user.

Example 2 illustrates the use of these responses.

-------------------------------------------------------------

Example of Forwarding

Either

S: RCPT TO:<Postel@USC-ISI.ARPA>
R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>

Or

S: RCPT TO:<Paul@USC-ISIB.ARPA>
R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>

Example 2

-------------------------------------------------------------




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3.3. VERIFYING AND EXPANDING

SMTP provides as additional features, commands to verify a user
name or expand a mailing list. This is done with the VRFY and
EXPN commands, which have character string arguments. For the
VRFY command, the string is a user name, and the response may
include the full name of the user and must include the mailbox of
the user. For the EXPN command, the string identifies a mailing
list, and the multiline response may include the full name of the
users and must give the mailboxes on the mailing list.

"User name" is a fuzzy term and used purposely. If a host
implements the VRFY or EXPN commands then at least local mailboxes
must be recognized as "user names". If a host chooses to
recognize other strings as "user names" that is allowed.

In some hosts the distinction between a mailing list and an alias
for a single mailbox is a bit fuzzy, since a common data structure
may hold both types of entries, and it is possible to have mailing
lists of one mailbox. If a request is made to verify a mailing
list a positive response can be given if on receipt of a message
so addressed it will be delivered to everyone on the list,
otherwise an error should be reported (e.g., "550 That is a
mailing list, not a user"). If a request is made to expand a user
name a positive response can be formed by returning a list
containing one name, or an error can be reported (e.g., "550 That
is a user name, not a mailing list").

In the case of a multiline reply (normal for EXPN) exactly one
mailbox is to be specified on each line of the reply. In the case
of an ambiguous request, for example, "VRFY Smith", where there
are two Smith's the response must be "553 User ambiguous".

The case of verifying a user name is straightforward as shown in
example 3.














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Simple Mail Transfer Protocol



-------------------------------------------------------------

Example of Verifying a User Name

Either

S: VRFY Smith
R: 250 Fred Smith <Smith@USC-ISIF.ARPA>

Or

S: VRFY Smith
R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>

Or

S: VRFY Jones
R: 550 String does not match anything.

Or

S: VRFY Jones
R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>

Or

S: VRFY Gourzenkyinplatz
R: 553 User ambiguous.

Example 3

-------------------------------------------------------------

















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The case of expanding a mailbox list requires a multiline reply as
shown in example 4.

-------------------------------------------------------------

Example of Expanding a Mailing List

Either

S: EXPN Example-People
R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>

Or

S: EXPN Executive-Washroom-List
R: 550 Access Denied to You.

Example 4

-------------------------------------------------------------

The character string arguments of the VRFY and EXPN commands
cannot be further restricted due to the variety of implementations
of the user name and mailbox list concepts. On some systems it
may be appropriate for the argument of the EXPN command to be a
file name for a file containing a mailing list, but again there is
a variety of file naming conventions in the Internet.

The VRFY and EXPN commands are not included in the minimum
implementation (Section 4.5.1), and are not required to work
across relays when they are implemented.













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Simple Mail Transfer Protocol



3.4. SENDING AND MAILING

The main purpose of SMTP is to deliver messages to user's
mailboxes. A very similar service provided by some hosts is to
deliver messages to user's terminals (provided the user is active
on the host). The delivery to the user's mailbox is called
"mailing", the delivery to the user's terminal is called
"sending". Because in many hosts the implementation of sending is
nearly identical to the implementation of mailing these two
functions are combined in SMTP. However the sending commands are
not included in the required minimum implementation
(Section 4.5.1). Users should have the ability to control the
writing of messages on their terminals. Most hosts permit the
users to accept or refuse such messages.

The following three command are defined to support the sending
options. These are used in the mail transaction instead of the
MAIL command and inform the receiver-SMTP of the special semantics
of this transaction:

SEND <SP> FROM:<reverse-path> <CRLF>

The SEND command requires that the mail data be delivered to
the user's terminal. If the user is not active (or not
accepting terminal messages) on the host a 450 reply may
returned to a RCPT command. The mail transaction is
successful if the message is delivered the terminal.

SOML <SP> FROM:<reverse-path> <CRLF>

The Send Or MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. If the user is
not active (or not accepting terminal messages) then the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered either
to the terminal or the mailbox.

SAML <SP> FROM:<reverse-path> <CRLF>

The Send And MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. In any case the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered the
mailbox.



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August 1982 RFC 821
Simple Mail Transfer Protocol



The same reply codes that are used for the MAIL commands are used
for these commands.















































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Simple Mail Transfer Protocol



3.5. OPENING AND CLOSING

At the time the transmission channel is opened there is an
exchange to ensure that the hosts are communicating with the hosts
they think they are.

The following two commands are used in transmission channel
opening and closing:

HELO <SP> <domain> <CRLF>

QUIT <CRLF>

In the HELO command the host sending the command identifies
itself; the command may be interpreted as saying "Hello, I am
<domain>".

-------------------------------------------------------------

Example of Connection Opening

R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA

Example 5

-------------------------------------------------------------

-------------------------------------------------------------

Example of Connection Closing

S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel

Example 6

-------------------------------------------------------------










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August 1982 RFC 821
Simple Mail Transfer Protocol



3.6. RELAYING

The forward-path may be a source route of the form
"@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. This
form is used to emphasize the distinction between an address and a
route. The mailbox is an absolute address, and the route is
information about how to get there. The two concepts should not
be confused.

Conceptually the elements of the forward-path are moved to the
reverse-path as the message is relayed from one server-SMTP to
another. The reverse-path is a reverse source route, (i.e., a
source route from the current location of the message to the
originator of the message). When a server-SMTP deletes its
identifier from the forward-path and inserts it into the
reverse-path, it must use the name it is known by in the
environment it is sending into, not the environment the mail came
from, in case the server-SMTP is known by different names in
different environments.

If when the message arrives at an SMTP the first element of the
forward-path is not the identifier of that SMTP the element is not
deleted from the forward-path and is used to determine the next
SMTP to send the message to. In any case, the SMTP adds its own
identifier to the reverse-path.

Using source routing the receiver-SMTP receives mail to be relayed
to another server-SMTP The receiver-SMTP may accept or reject the
task of relaying the mail in the same way it accepts or rejects
mail for a local user. The receiver-SMTP transforms the command
arguments by moving its own identifier from the forward-path to
the beginning of the reverse-path. The receiver-SMTP then becomes
a sender-SMTP, establishes a transmission channel to the next SMTP
in the forward-path, and sends it the mail.

The first host in the reverse-path should be the host sending the
SMTP commands, and the first host in the forward-path should be
the host receiving the SMTP commands.

Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That
is, there is no need for these paths and especially this syntax to
appear in the "To:" , "From:", "CC:", etc. fields of the message
header.

If a server-SMTP has accepted the task of relaying the mail and



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RFC 821 August 1982
Simple Mail Transfer Protocol



later finds that the forward-path is incorrect or that the mail
cannot be delivered for whatever reason, then it must construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the
reverse-path).

This notification message must be from the server-SMTP at this
host. Of course, server-SMTPs should not send notification
messages about problems with notification messages. One way to
prevent loops in error reporting is to specify a null reverse-path
in the MAIL command of a notification message. When such a
message is relayed it is permissible to leave the reverse-path
null. A MAIL command with a null reverse-path appears as follows:

MAIL FROM:<>

An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
to HOSTZ. What we see in the example is the transaction between
HOSTY and HOSTX, which is the first step in the return of the
notification message.



























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-------------------------------------------------------------

Example Undeliverable Mail Notification Message

S: MAIL FROM:<>
R: 250 ok
S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
R: 250 ok
S: DATA
R: 354 send the mail data, end with .
S: Date: 23 Oct 81 11:22:33
S: From: SMTP@HOSTY.ARPA
S: To: JOE@HOSTW.ARPA
S: Subject: Mail System Problem
S:
S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
S: HOSTZ.ARPA said this:
S: "550 No Such User"
S: .
R: 250 ok

Example 7

-------------------------------------------------------------

























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Simple Mail Transfer Protocol



3.7. DOMAINS

Domains are a recently introduced concept in the ARPA Internet
mail system. The use of domains changes the address space from a
flat global space of simple character string host names to a
hierarchically structured rooted tree of global addresses. The
host name is replaced by a domain and host designator which is a
sequence of domain element strings separated by periods with the
understanding that the domain elements are ordered from the most
specific to the most general.

For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
"PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.

Whenever domain names are used in SMTP only the official names are
used, the use of nicknames or aliases is not allowed.

































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Simple Mail Transfer Protocol



3.8. CHANGING ROLES

The TURN command may be used to reverse the roles of the two
programs communicating over the transmission channel.

If program-A is currently the sender-SMTP and it sends the TURN
command and receives an ok reply (250) then program-A becomes the
receiver-SMTP.

If program-B is currently the receiver-SMTP and it receives the
TURN command and sends an ok reply (250) then program-B becomes
the sender-SMTP.

To refuse to change roles the receiver sends the 502 reply.

Please note that this command is optional. It would not normally
be used in situations where the transmission channel is TCP.
However, when the cost of establishing the transmission channel is
high, this command may be quite useful. For example, this command
may be useful in supporting be mail exchange using the public
switched telephone system as a transmission channel, especially if
some hosts poll other hosts for mail exchanges.



























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Simple Mail Transfer Protocol



4. THE SMTP SPECIFICATIONS

4.1. SMTP COMMANDS

4.1.1. COMMAND SEMANTICS

The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character
strings terminated by <CRLF>. The command codes themselves are
alphabetic characters terminated by <SP> if parameters follow
and <CRLF> otherwise. The syntax of mailboxes must conform to
receiver site conventions. The SMTP commands are discussed
below. The SMTP replies are discussed in the Section 4.2.

A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the argument of the MAIL command, the
forward-path is the argument of the RCPT command, and the mail
data is the argument of the DATA command. These arguments or
data objects must be transmitted and held pending the
confirmation communicated by the end of mail data indication
which finalizes the transaction. The model for this is that
distinct buffers are provided to hold the types of data
objects, that is, there is a reverse-path buffer, a
forward-path buffer, and a mail data buffer. Specific commands
cause information to be appended to a specific buffer, or cause
one or more buffers to be cleared.

HELLO (HELO)

This command is used to identify the sender-SMTP to the
receiver-SMTP. The argument field contains the host name of
the sender-SMTP.

The receiver-SMTP identifies itself to the sender-SMTP in
the connection greeting reply, and in the response to this
command.

This command and an OK reply to it confirm that both the
sender-SMTP and the receiver-SMTP are in the initial state,
that is, there is no transaction in progress and all state
tables and buffers are cleared.







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MAIL (MAIL)

This command is used to initiate a mail transaction in which
the mail data is delivered to one or more mailboxes. The
argument field contains a reverse-path.

The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different). In some types of error
reporting messages (for example, undeliverable mail
notifications) the reverse-path may be null (see Example 7).

This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.

RECIPIENT (RCPT)

This command is used to identify an individual recipient of
the mail data; multiple recipients are specified by multiple
use of this command.

The forward-path consists of an optional list of hosts and a
required destination mailbox. When the list of hosts is
present, it is a source route and indicates that the mail
must be relayed to the next host on the list. If the
receiver-SMTP does not implement the relay function it may
user the same reply it would for an unknown local user
(550).

When mail is relayed, the relay host must remove itself from
the beginning forward-path and put itself at the beginning
of the reverse-path. When mail reaches its ultimate
destination (the forward-path contains only a destination
mailbox), the receiver-SMTP inserts it into the destination
mailbox in accordance with its host mail conventions.





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Simple Mail Transfer Protocol



For example, mail received at relay host A with arguments

FROM:<USERX@HOSTY.ARPA>
TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>

will be relayed on to host B with arguments

FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.

This command causes its forward-path argument to be appended
to the forward-path buffer.

DATA (DATA)

The receiver treats the lines following the command as mail
data from the sender. This command causes the mail data
from this command to be appended to the mail data buffer.
The mail data may contain any of the 128 ASCII character
codes.

The mail data is terminated by a line containing only a
period, that is the character sequence "<CRLF>.<CRLF>" (see
Section 4.5.2 on Transparency). This is the end of mail
data indication.

The end of mail data indication requires that the receiver
must now process the stored mail transaction information.
This processing consumes the information in the reverse-path
buffer, the forward-path buffer, and the mail data buffer,
and on the completion of this command these buffers are
cleared. If the processing is successful the receiver must
send an OK reply. If the processing fails completely the
receiver must send a failure reply.

When the receiver-SMTP accepts a message either for relaying
or for final delivery it inserts at the beginning of the
mail data a time stamp line. The time stamp line indicates
the identity of the host that sent the message, and the
identity of the host that received the message (and is
inserting this time stamp), and the date and time the
message was received. Relayed messages will have multiple
time stamp lines.

When the receiver-SMTP makes the "final delivery" of a
message it inserts at the beginning of the mail data a



Postel [Page 21]



August 1982 RFC 821
Simple Mail Transfer Protocol



return path line. The return path line preserves the
information in the <reverse-path> from the MAIL command.
Here, final delivery means the message leaves the SMTP
world. Normally, this would mean it has been delivered to
the destination user, but in some cases it may be further
processed and transmitted by another mail system.

It is possible for the mailbox in the return path be
different from the actual sender's mailbox, for example,
if error responses are to be delivered a special error
handling mailbox rather than the message senders.

The preceding two paragraphs imply that the final mail data
will begin with a return path line, followed by one or more
time stamp lines. These lines will be followed by the mail
data header and body [2]. See Example 8.

Special mention is needed of the response and further action
required when the processing following the end of mail data
indication is partially successful. This could arise if
after accepting several recipients and the mail data, the
receiver-SMTP finds that the mail data can be successfully
delivered to some of the recipients, but it cannot be to
others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA
command must be an OK reply. But, the receiver-SMTP must
compose and send an "undeliverable mail" notification
message to the originator of the message. Either a single
notification which lists all of the recipients that failed
to get the message, or separate notification messages must
be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the
MAIL command (even if they result from processing a SEND,
SOML, or SAML command).















[Page 22] Postel



RFC 821 August 1982
Simple Mail Transfer Protocol



-------------------------------------------------------------

Example of Return Path and Received Time Stamps

Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
Date: 27 Oct 81 15:01:01 PST
From: JOE@ABC.ARPA
Subject: Improved Mailing System Installed
To: SAM@JKL.ARPA

This is to inform you that ...

Example 8

-------------------------------------------------------------

SEND (SEND)

This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals. The
argument field contains a reverse-path. This command is
successful if the message is delivered to a terminal.

The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).

This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.

SEND OR MAIL (SOML)

This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals or



Postel [Page 23]



August 1982 RFC 821
Simple Mail Transfer Protocol



mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), otherwise to the
recipient's mailbox. The argument field contains a
reverse-path. This command is successful if the message is
delivered to a terminal or the mailbox.

The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).

This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.

SEND AND MAIL (SAML)

This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals and
mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), and for all
recipients to the recipient's mailbox. The argument field
contains a reverse-path. This command is successful if the
message is delivered to the mailbox.

The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).

This command clears the reverse-path buffer, the



[Page 24] Postel



RFC 821 August 1982
Simple Mail Transfer Protocol



forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.

RESET (RSET)

This command specifies that the current mail transaction is
to be aborted. Any stored sender, recipients, and mail data
must be discarded, and all buffers and state tables cleared.
The receiver must send an OK reply.

VERIFY (VRFY)

This command asks the receiver to confirm that the argument
identifies a user. If it is a user name, the full name of
the user (if known) and the fully specified mailbox are
returned.

This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.

EXPAND (EXPN)

This command asks the receiver to confirm that the argument
identifies a mailing list, and if so, to return the
membership of that list. The full name of the users (if
known) and the fully specified mailboxes are returned in a
multiline reply.

This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.

HELP (HELP)

This command causes the receiver to send helpful information
to the sender of the HELP command. The command may take an
argument (e.g., any command name) and return more specific
information as a response.

This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.








Postel [Page 25]



August 1982 RFC 821
Simple Mail Transfer Protocol



NOOP (NOOP)

This command does not affect any parameters or previously
entered commands. It specifies no action other than that
the receiver send an OK reply.

This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.

QUIT (QUIT)

This command specifies that the receiver must send an OK
reply, and then close the transmission channel.

The receiver should not close the transmission channel until
it receives and replies to a QUIT command (even if there was
an error). The sender should not close the transmission
channel until it send a QUIT command and receives the reply
(even if there was an error response to a previous command).
If the connection is closed prematurely the receiver should
act as if a RSET command had been received (canceling any
pending transaction, but not undoing any previously
completed transaction), the sender should act as if the
command or transaction in progress had received a temporary
error (4xx).

TURN (TURN)

This command specifies that the receiver must either (1)
send an OK reply and then take on the role of the
sender-SMTP, or (2) send a refusal reply and retain the role
of the receiver-SMTP.

If program-A is currently the sender-SMTP and it sends the
TURN command and receives an OK reply (250) then program-A
becomes the receiver-SMTP. Program-A is then in the initial
state as if the transmission channel just opened, and it
then sends the 220 service ready greeting.

If program-B is currently the receiver-SMTP and it receives
the TURN command and sends an OK reply (250) then program-B
becomes the sender-SMTP. Program-B is then in the initial
state as if the transmission channel just opened, and it
then expects to receive the 220 service ready greeting.

To refuse to change roles the receiver sends the 502 reply.



[Page 26] Postel



RFC 821 August 1982
Simple Mail Transfer Protocol



There are restrictions on the order in which these command may
be used.

The first command in a session must be the HELO command.
The HELO command may be used later in a session as well. If
the HELO command argument is not acceptable a 501 failure
reply must be returned and the receiver-SMTP must stay in
the same state.

The NOOP, HELP, EXPN, and VRFY commands can be used at any
time during a session.

The MAIL, SEND, SOML, or SAML commands begin a mail
transaction. Once started a mail transaction consists of
one of the transaction beginning commands, one or more RCPT
commands, and a DATA command, in that order. A mail
transaction may be aborted by the RSET command. There may
be zero or more transactions in a session.

If the transaction beginning command argument is not
acceptable a 501 failure reply must be returned and the
receiver-SMTP must stay in the same state. If the commands
in a transaction are out of order a 503 failure reply must
be returned and the receiver-SMTP must stay in the same
state.

The last command in a session must be the QUIT command. The
QUIT command can not be used at any other time in a session.

4.1.2. COMMAND SYNTAX

The commands consist of a command code followed by an argument
field. Command codes are four alphabetic characters. Upper
and lower case alphabetic characters are to be treated
identically. Thus, any of the following may represent the mail
command:

MAIL Mail mail MaIl mAIl

This also applies to any symbols representing parameter values,
such as "TO" or "to" for the forward-path. Command codes and
the argument fields are separated by one or more spaces.
However, within the reverse-path and forward-path arguments
case is important. In particular, in some hosts the user
"smith" is different from the user "Smith".




Postel [Page 27]



August 1982 RFC 821
Simple Mail Transfer Protocol



The argument field consists of a variable length character
string ending with the character sequence <CRLF>. The receiver
is to take no action until this sequence is received.

Square brackets denote an optional argument field. If the
option is not taken, the appropriate default is implied.











































[Page 28] Postel



RFC 821 August 1982
Simple Mail Transfer Protocol



The following are the SMTP commands:

HELO <SP> <domain> <CRLF>

MAIL <SP> FROM:<reverse-path> <CRLF>

RCPT <SP> TO:<forward-path> <CRLF>

DATA <CRLF>

RSET <CRLF>

SEND <SP> FROM:<reverse-path> <CRLF>

SOML <SP> FROM:<reverse-path> <CRLF>

SAML <SP> FROM:<reverse-path> <CRLF>

VRFY <SP> <string> <CRLF>

EXPN <SP> <string> <CRLF>

HELP [<SP> <string>] <CRLF>

NOOP <CRLF>

QUIT <CRLF>

TURN <CRLF>




















Postel [Page 29]



August 1982 RFC 821
Simple Mail Transfer Protocol



The syntax of the above argument fields (using BNF notation
where applicable) is given below. The "..." notation indicates
that a field may be repeated one or more times.

<reverse-path> ::= <path>

<forward-path> ::= <path>

<path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"

<a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>

<at-domain> ::= "@" <domain>

<domain> ::= <element> | <element> "." <domain>

<element> ::= <name> | "#" <number> | "[" <dotnum> "]"

<mailbox> ::= <local-part> "@" <domain>

<local-part> ::= <dot-string> | <quoted-string>

<name> ::= <a> <ldh-str> <let-dig>

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig> ::= <a> | <d>

<let-dig-hyp> ::= <a> | <d> | "-"

<dot-string> ::= <string> | <string> "." <dot-string>

<string> ::= <char> | <char> <string>

<quoted-string> ::= """ <qtext> """

<qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>

<char> ::= <c> | "\" <x>

<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>

<number> ::= <d> | <d> <number>

<CRLF> ::= <CR> <LF>




[Page 30] Postel



RFC 821 August 1982
Simple Mail Transfer Protocol



<CR> ::= the carriage return character (ASCII code 13)

<LF> ::= the line feed character (ASCII code 10)

<SP> ::= the space character (ASCII code 32)

<snum> ::= one, two, or three digits representing a decimal
integer value in the range 0 through 255

<a> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case

<c> ::= any one of the 128 ASCII characters, but not any
<special> or <SP>

<d> ::= any one of the ten digits 0 through 9

<q> ::= any one of the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)

<x> ::= any one of the 128 ASCII characters (no exceptions)

<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" |
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