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> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | the control
characters (ASCII codes 0 through 31 inclusive and
127)
Note that the backslash, "\", is a quote character, which is
used to indicate that the next character is to be used
literally (instead of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with comma being the fourth character of the field.
Hosts are generally known by names which are translated to
addresses in each host. Note that the name elements of domains
are the official names -- no use of nicknames or aliases is
allowed.
Sometimes a host is not known to the translation function and
communication is blocked. To bypass this barrier two numeric
forms are also allowed for host "names". One form is a decimal
integer prefixed by a pound sign, "#", which indicates the
number is the address of the host. Another form is four small
decimal integers separated by dots and enclosed by brackets,
e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
Address in four 8-bit fields.
Postel [Page 31]
August 1982 RFC 821
Simple Mail Transfer Protocol
The time stamp line and the return path line are formally
defined as follows:
<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
<stamp> ::= <from-domain> <by-domain> <opt-info> ";"
<daytime>
<from-domain> ::= "FROM" <SP> <domain> <SP>
<by-domain> ::= "BY" <SP> <domain> <SP>
<opt-info> ::= [<via>] [<with>] [<id>] [<for>]
<via> ::= "VIA" <SP> <link> <SP>
<with> ::= "WITH" <SP> <protocol> <SP>
<id> ::= "ID" <SP> <string> <SP>
<for> ::= "FOR" <SP> <path> <SP>
<link> ::= The standard names for links are registered with
the Network Information Center.
<protocol> ::= The standard names for protocols are
registered with the Network Information Center.
<daytime> ::= <SP> <date> <SP> <time>
<date> ::= <dd> <SP> <mon> <SP> <yy>
<time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
<dd> ::= the one or two decimal integer day of the month in
the range 1 to 31.
<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
<yy> ::= the two decimal integer year of the century in the
range 00 to 99.
[Page 32] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.
<zone> ::= "UT" for Universal Time (the default) or other
time zone designator (as in [2]).
-------------------------------------------------------------
Return Path Example
Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
Example 9
-------------------------------------------------------------
-------------------------------------------------------------
Time Stamp Line Example
Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
Example 10
-------------------------------------------------------------
Postel [Page 33]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2. SMTP REPLIES
Replies to SMTP commands are devised to ensure the synchronization
of requests and actions in the process of mail transfer, and to
guarantee that the sender-SMTP always knows the state of the
receiver-SMTP. Every command must generate exactly one reply.
The details of the command-reply sequence are made explicit in
Section 5.3 on Sequencing and Section 5.4 State Diagrams.
An SMTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is meant for the human user. It is intended that
the three digits contain enough encoded information that the
sender-SMTP need not examine the text and may either discard it or
pass it on to the user, as appropriate. In particular, the text
may be receiver-dependent and context dependent, so there are
likely to be varying texts for each reply code. A discussion of
the theory of reply codes is given in Appendix E. Formally, a
reply is defined to be the sequence: a three-digit code, <SP>,
one line of text, and <CRLF>, or a multiline reply (as defined in
Appendix E). Only the EXPN and HELP commands are expected to
result in multiline replies in normal circumstances, however
multiline replies are allowed for any command.
[Page 34] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.2.1. REPLY CODES BY FUNCTION GROUPS
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed
Postel [Page 35]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2.2. NUMERIC ORDER LIST OF REPLY CODES
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
354 Start mail input; end with <CRLF>.<CRLF>
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
551 User not local; please try <forward-path>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
554 Transaction failed
[Page 36] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.3. SEQUENCING OF COMMANDS AND REPLIES
The communication between the sender and receiver is intended to
be an alternating dialogue, controlled by the sender. As such,
the sender issues a command and the receiver responds with a
reply. The sender must wait for this response before sending
further commands.
One important reply is the connection greeting. Normally, a
receiver will send a 220 "Service ready" reply when the connection
is completed. The sender should wait for this greeting message
before sending any commands.
Note: all the greeting type replies have the official name of
the server host as the first word following the reply code.
For example,
220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
The table below lists alternative success and failure replies for
each command. These must be strictly adhered to; a receiver may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.
COMMAND-REPLY SEQUENCES
Each command is listed with its possible replies. The prefixes
used before the possible replies are "P" for preliminary (not
used in SMTP), "I" for intermediate, "S" for success, "F" for
failure, and "E" for error. The 421 reply (service not
available, closing transmission channel) may be given to any
command if the SMTP-receiver knows it must shut down. This
listing forms the basis for the State Diagrams in Section 4.4.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
Postel [Page 37]
August 1982 RFC 821
Simple Mail Transfer Protocol
RCPT
S: 250, 251
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 503, 421
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 503, 421
RSET
S: 250
E: 500, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251
F: 550, 551, 553
E: 500, 501, 502, 504, 421
EXPN
S: 250
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500, 421
QUIT
S: 221
E: 500
TURN
S: 250
F: 502
E: 500, 503
[Page 38] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.4. STATE DIAGRAMS
Following are state diagrams for a simple-minded SMTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of SMTP commands. The
command groupings were determined by constructing a model for each
command and then collecting together the commands with
structurally identical models.
For each command there are three possible outcomes: "success"
(S), "failure" (F), and "error" (E). In the state diagrams below
we use the symbol B for "begin", and the symbol W for "wait for
reply".
First, the diagram that represents most of the SMTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
NOOP, QUIT, TURN.
Postel [Page 39]
August 1982 RFC 821
Simple Mail Transfer Protocol
A more complex diagram models the DATA command:
+---+ DATA +---+ 1,2 +---+
| B |---------->| W |-------------------->| E |
+---+ +---+ ------------>+---+
3| |4,5 |
| | |
-------------- ----- |
| | | +---+
| ---------- -------->| S |
| | | | +---+
| | ------------
| | | |
V 1,3| |2 |
+---+ data +---+ --------------->+---+
| |---------->| W | | F |
+---+ +---+-------------------->+---+
4,5
Note that the "data" here is a series of lines sent from the
sender to the receiver with no response expected until the last
line is sent.
[Page 40] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.
1. Before sending a line of mail text the sender-SMTP checks
the first character of the line. If it is a period, one
additional period is inserted at the beginning of the line.
2. When a line of mail text is received by the receiver-SMTP
it checks the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.
The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipient's mailbox
including format effectors and other control characters. If
the transmission channel provides an 8-bit byte (octets) data
stream, the 7-bit ASCII codes are transmitted right justified
in the octets with the high order bits cleared to zero.
In some systems it may be necessary to transform the data as
it is received and stored. This may be necessary for hosts
that use a different character set than ASCII as their local
character set, or that store data in records rather than
Postel [Page 41]
August 1982 RFC 821
Simple Mail Transfer Protocol
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.
4.5.3. SIZES
There are several objects that have required minimum maximum
sizes. That is, every implementation must be able to receive
objects of at least these sizes, but must not send objects
larger than these sizes.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
user
The maximum total length of a user name is 64 characters.
domain
The maximum total length of a domain name or number is 64
characters.
path
The maximum total length of a reverse-path or
forward-path is 256 characters (including the punctuation
and element separators).
command line
The maximum total length of a command line including the
command word and the <CRLF> is 512 characters.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters.
[Page 42] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
text line
The maximum total length of a text line including the
<CRLF> is 1000 characters (but not counting the leading
dot duplicated for transparency).
recipients buffer
The maximum total number of recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
Errors due to exceeding these limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
Postel [Page 43]
August 1982 RFC 821
Simple Mail Transfer Protocol
APPENDIX A
TCP Transport service
The Transmission Control Protocol [3] is used in the ARPA
Internet, and in any network following the US DoD standards for
internetwork protocols.
Connection Establishment
The SMTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 25 (31 octal), that is L=25.
Data Transfer
The TCP connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to
zero.
[Page 44] Postel
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Simple Mail Transfer Protocol
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