particular non-standard command; this reply is useful only
to the human user]
220 <host> Service ready
221 <host> Service closing transmission channel
421 <host> 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
November 1981 RFC788
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 <host> Service ready
221 <host> 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 <host> 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
RFC788 November 1981
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 "Awaiting input" 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 <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
S: 250
F: 552, 451, 452
E: 500, 501, 421
November 1981 RFC788
Simple Mail Transfer Protocol
RCPT
S: 250, 251
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 421
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 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
F: 550
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
RFC788 November 1981
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.
November 1981 RFC788
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.
RFC788 November 1981
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
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the 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 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 recipients mailbox
including format effectors and other control characters. The
7-bit ASCII codes are transmitted right justified in 8-bit
bytes (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
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.
November 1981 RFC788
Simple Mail Transfer Protocol
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.
host
The maximum total length of a host name or number is 40
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.
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).
RFC788 November 1981
Simple Mail Transfer Protocol
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.
November 1981 RFC788
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 a 8-bit byte with the high-order bit cleared to
zero.
RFC788 November 1981
Simple Mail Transfer Protocol
APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [4] (implemented by the Network
Control Program) may be used in the ARPANET.
Connection Establishment
The SMTP transmission channel is established via NCP between
the the sender process socket U and receiver process socket L.
The Initial Connection Protocol [5] is followed resulting in a
pair of simplex connections. This pair of connections is used
as the transmission channel. This protocol is assigned the
contact socket 25 (31 octal), that is L=25.
Data Transfer
The NCP data connections are established in 8-bit byte mode.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
November 1981 RFC788
Simple Mail Transfer Protocol
APPENDIX C
NITS
The Network Independent Transport Service [6] may be used.
Connection Establishment
The SMTP transmission channel is established via NITS between
the sender process and receiver process. The sender process
executes the CONNECT primitive, and the waiting receiver
process executes the ACCEPT primitive.
Data Transfer
The NITS connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
RFC788 November 1981
Simple Mail Transfer Protocol
APPENDIX D
X.25 Transport service
It may be possible to use the X.25 service [7] as provided by the
Public Data Networks directly, but there are indications that it
is too error prone to qualify as a reliable channel. It is
suggested that a reliable end-to-end protocol such as TCP be used
on top of X.25 connections.
November 1981 RFC788
Simple Mail Transfer Protocol
APPENDIX E
Theory of Reply Codes
The three digits of the reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated sender-SMTP will be able to
determine its next action (proceed as planned, redo, retrench,
etc.) by simply examining this first digit. A sender-SMTP that
wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit, reserving the third digit for the finest gradation of
information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action
is being held in abeyance, pending confirmation of the
information in this reply. The sender-SMTP should send
another command specifying whether to continue or abort
the action.
[Note: SMTP does not have any commands that allow this
type of reply, and so does not have the continue or
abort commands.]
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The sender-SMTP should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not occur. However, the error condition is temporary and
the action may be requested again. The sender should
RFC788 November 1981
Simple Mail Transfer Protocol
return to the beginning of the command sequence (if any).
It is difficult to assign a meaning to "transient" when
two different sites (receiver- and sender- SMTPs) must
agree on the interpretation. Each reply in this category
might have a different time value, but the sender-SMTP is
encouraged to try again. A rule of thumb to determine if
a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be repeated without
any change in command form or in properties of the sender
or receiver. (E.g., the command is repeated identically
and the receiver does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not occur. The sender-SMTP is discouraged from repeating
the exact request (in the same sequence). Even some
"permanent" error conditions can be corrected, so the
human user may want to direct the sender-SMTP to
reinitiate the command sequence by direct action at some
point in the future (e.g., after the spelling has been
changed, or the user has altered the account status).
The second digit encodes responses in specific categories:
x0z Syntax -- These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, and unimplemented or superfluous
commands.
x1z Information -- These are replies to requests for
information, such as status or help.
x2z Connections -- These are replies referring to the
transmission channel.
x3z Unspecified as yet.
x4z Unspecified as yet.
x5z Mail system -- These replies indicate the status of
the receiver mail system vis-a-vis the requested
transfer or other mail system action.
The third digit gives a finer gradation of meaning in each
category specified by the second digit. The list of replies
November 1981 RFC788
Simple Mail Transfer Protocol
illustrates this. Each reply text is recommended rather than
mandatory, and may even change according to the command with
which it is associated. On the other hand, the reply codes
must strictly follow the specifications in this section.
Receiver implementations should not invent new codes for
slightly different situations from the ones described here, but
rather adapt codes already defined.
For example, a command such as NOOP whose successful execution
does not offer the sender-SMTP any new information will return
a 250 reply. The response is 502 when the command requests an
unimplemented non-site-specific action. A refinement of that
is the 504 reply for a command that is implemented, but that
requests an unimplemented parameter.
The reply text may be longer than a single line; in these cases
the complete text must be marked so the sender-SMTP knows when it
can stop reading the reply. This requires a special format to
indicate a multiple line reply.
The format for multi-line replies requires that every line,
except the last, begin with the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
The sender-SMTP then simply needs to search for the reply code
followed by <SP> at the beginning of a line, and ignore all
preceding lines.
RFC788 November 1981
Simple Mail Transfer Protocol
APPENDIX F
Scenarios
This section presents complete scenarios of several types of SMTP
sessions.
A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that
host USC-ISIF contacts host BBN-UNIX directly. The mail is
accepted for Jones and Brown. Green does not have a mailbox at
host BBN-UNIX.
-------------------------------------------------------------
R: 220 BBN-UNIX Simple Mail Transfer Service Ready
S: HELO USC-ISIF
R: 250 BBN-UNIX
S: MAIL FROM:<Smith@USC-ISIF>
R: 250 OK
S: RCPT TO:<Jones@BBN-UNIX>
R: 250 OK
S: RCPT TO:<Green@BBN-UNIX>
R: 550 No such user here
S: RCPT TO:<Brown@BBN-UNIX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BBN-UNIX Service closing transmission channel
Scenario 1
-------------------------------------------------------------
November 1981 RFC788
Simple Mail Transfer Protocol
Aborted SMTP Transaction Scenario
-------------------------------------------------------------
R: 220 MIT-Multics Simple Mail Transfer Service Ready
S: HELO ISI-VAXA
R: 250 MIT-Multics
S: MAIL FROM:<Smith@ISI-VAXA>
R: 250 OK
S: RCPT TO:<Jones@MIT-Multics>
R: 250 OK
S: RCPT TO:<Green@MIT-Multics>
R: 550 No such user here
S: RSET
R: 250 OK
S: QUIT
R: 221 MIT-Multics Service closing transmission channel
Scenario 2
-------------------------------------------------------------
RFC788 November 1981
Simple Mail Transfer Protocol
Relayed Mail Scenario
-------------------------------------------------------------
Step 1 -- Source Host to Relay Host
R: 220 USC-ISIE Simple Mail Transfer Service Ready
S: HELO MIT-AI
R: 250 USC-ISIE
S: MAIL FROM:<JQP@MIT-AI>
R: 250 OK
S: RCPT TO:<@ISIE,Jones@BBN-VAX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2-Nov-81 22:33:44
S: From: John Q. Public <JQP at MIT-AI>
S: Subject: The Next Meeting of the Board
S: To: Jones at BBN-Vax
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
November 1981 RFC788
Simple Mail Transfer Protocol
Step 2 -- Relay Host to Destination Host
R: 220 BBN-VAX Simple Mail Transfer Service Ready
S: HELO USC-ISIE
R: 250 BBN-VAX
S: MAIL FROM:<@ISIE,JQP@MIT-AI>
R: 250 OK
S: RCPT TO:<Jones@BBN-VAX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Mail-From: NCP host MIT-AI received by USC-ISIE at
2-Nov-81 22:40:10
S: Date: 2-Nov-81 22:33:44
S: From: John Q. Public <JQP at MIT-AI>
S: Subject: The Next Meeting of the Board
S: To: Jones at BBN-Vax
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
Scenario 3
-------------------------------------------------------------
RFC788 November 1981
Simple Mail Transfer Protocol
Verifying and Sending Scenario
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SEND FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 4
-------------------------------------------------------------
November 1981 RFC788
Simple Mail Transfer Protocol
Sending and Mailing Scenarios
First the user's name is verified, then an attempt is made to
send to the user's terminal. When that fails, the messages is
mailed to the user's mailbox.
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SEND FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 450 User not active now
S: RSET
R: 250 OK
S: MAIL FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 5
-------------------------------------------------------------
RFC788 November 1981
Simple Mail Transfer Protocol
Doing the preceding scenario more efficiently.
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SOML FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 User not active now, so will do mail.
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 6
-------------------------------------------------------------
November 1981 RFC788
Simple Mail Transfer Protocol
Mailing List Scenario
First each of two mailing lists are expanded in separate sessions
with different hosts. Then the message is sent to everyone that
appeared on either list (but no duplicates) via a relay host.
-------------------------------------------------------------
Step 1 -- Expanding the First List
R: 220 MIT-AI Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 MIT-AI
S: EXPN Example-People
R: 250-<ABC@MIT-MC>
R: 250-Fred Fonebone <Fonebone@ISIQ>
R: 250-Xenon Y. Zither <XYZ@MIT-AI>
R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
R: 250-<joe@foo-unix>
R: 250 <xyz@bar-unix>
S: QUIT
R: 221 MIT-AI Service closing transmission channel
RFC788 November 1981
Simple Mail Transfer Protocol
Step 2 -- Expanding the Second List
R: 220 MIT-MC Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 MIT-MC
S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC>
R: 250-<XYZ@MIT-AI>
R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
R: 250-<fred@BBN-UNIX>
R: 250 <xyz@bar-unix>
S: QUIT
R: 221 MIT-MC Service closing transmission channel
November 1981 RFC788
Simple Mail Transfer Protocol
Step 3 -- Mailing to All via a Relay Host
R: 220 USC-ISIE Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 USC-ISIE
S: MAIL FROM:<Account.Person@SU-SCORE>
R: 250 OK
S: RCPT TO:<@ISIE,ABC@MIT-MC>
R: 250 OK
S: RCPT TO:<@ISIE,Fonebone@ISIQ>
R: 250 OK
S: RCPT TO:<@ISIE,XYZ@MIT-AI>
R: 250 OK
S: RCPT TO:<@ISIE,@ISIF,Q-Smith@ISI-VAXA>
R: 250 OK
S: RCPT TO:<@ISIE,joe@FOO-UNIX>
R: 250 OK
S: RCPT TO:<@ISIE,xyz@BAR-UNIX>
R: 250 OK
S: RCPT TO:<@ISIE,fred@BBN-UNIX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
Scenario 7
-------------------------------------------------------------
RFC788 November 1981
Simple Mail Transfer Protocol
Forwarding Scenarios
-------------------------------------------------------------
R: 220 USC-ISIF Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIF
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF>
R: 251 User not local; will forward to <Jones@USC-ISIA>
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIF Service closing transmission channel
Scenario 8
-------------------------------------------------------------
November 1981 RFC788
Simple Mail Transfer Protocol
-------------------------------------------------------------
Step 1 -- Trying the Mailbox at the First Host
R: 220 USC-ISIF Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIF
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF>
R: 251 User not local; will forward to <Jones@USC-ISIA>
S: RSET
R: 250 OK
S: QUIT
R: 221 USC-ISIF Service closing transmission channel
Step 2 -- Delivering the Mail at the Second Host
R: 220 USC-ISIA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIA
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<Jones@USC-ISIA>
R: OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIA Service closing transmission channel
Scenario 9
-------------------------------------------------------------
RFC788 November 1981
Simple Mail Transfer Protocol
Too Many Recipients Scenario
-------------------------------------------------------------
R: 220 BERKELEY Simple Mail Transfer Service Ready
S: HELO USC-ISIF
R: 250 BERKELEY
S: MAIL FROM:<Postel@USC-ISIF>
R: 250 OK
S: RCPT TO:<fabry@BERKELEY>
R: 250 OK
S: RCPT TO:<eric@BERKELEY>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<Postel@USC-ISIF>
R: 250 OK
S: RCPT TO:<eric@BERKELEY>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY Service closing transmission channel
Scenario 10
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November 1981 RFC788
Simple Mail Transfer Protocol
GLOSSARY
ASCII
American Standard Code for Information Interchange [1].
command
A request for a mail service action sent by the sender-SMTP to the
receiver-SMTP.
end of mail data indication
A special sequence of characters that indicates the end of the
mail data. In particular, the five characters carriage return,
line feed, period, carriage return, line feed, in that order.
host
A computer in the internetwork environment on which mailboxes or
SMTP processes reside.
line
A line of text ending with a <CRLF>.
mail data
A sequence of ASCII characters of arbitrary length, which conforms
to the standard set in the Standard for the Format of ARPA Network
Text Messages (RFC733 [2]).
mailbox
A character string (address) which identifies a user to whom mail
is to be sent. Mailbox normally consists of the host and user
specifications. The standard mailbox naming convention is defined
to be "user@host". Additionally, the "container" in which mail is
stored.
receiver-SMTP process
A process which transfers mail in cooperation with a sender-SMTP
process. It waits for a connection to be established via the
transport service. It receives SMTP commands from the
sender-SMTP, sends replies, and performs the specified operations.
RFC788 November 1981
Simple Mail Transfer Protocol
reply
A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
SMTP command. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes are
for use by programs and the text is usually intended for human
users.
sender-SMTP process
A process which transfers mail in cooperation with a receiver-SMTP
process. A local language may be used in the user interface
command/reply dialogue. The sender-SMTP initiates the transport
service connection. It initiates SMTP commands, receives replies,
and governs the transfer of mail.
session
The set of exchanges that occur while the transmission channel is
open.
transaction
The set of exchanges required for one message to be transmitted
for one or more recipients.
transmission channel
A full-duplex communication path between a sender-SMTP and a
receiver-SMTP for the exchange of commands, replies, and mail
text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
November 1981 RFC788
Simple Mail Transfer Protocol
word
A sequence of printing characters.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
RFC788 November 1981
Simple Mail Transfer Protocol
REFERENCES
[1] ASCII
ASCII, "USA Code for Information Interchange", United States of
America Standards Institute, X3.4, 1968. Also in: Feinler, E.
and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
the Defense Communications Agency by SRI International, Menlo
Park, California, Revised January 1978.
[2] RFC733
Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
the Format of ARPA Network Text Messages," RFC733, NIC 41952,
November 1977. Also in: Feinler, E. and J. Postel, eds.,
"ARPANET Protocol Handbook", NIC 7104, for the Defense
Communications Agency by SRI International, Menlo Park,
California, Revised January 1978.
[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, USC/Information Sciences
Institute, September 1981.
[4] NCP
McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[5] Initial Connection Protocol
Postel, J., "Official Initial Connection Protocol", NIC 7101,
11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[6] NITS
PSS/SG3, "A Network Independent Transport Service", Study Group 3,
The Post Office PSS Users Group, February 1980. Available from
the DCPU, National Physical Laboratory, Teddington, UK.
November 1981 RFC788
Simple Mail Transfer Protocol
[7] X.25
CCITT, "Recommendation X.25 - Interface Between Data Terminal
Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
Terminals Operating in the Packet Mode on Public Data Networks,"
CCITT Orange Book, Vol. VIII.2, International Telephone and
Telegraph Consultative Committee, Geneva, 1976.