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RFC821 - Simple Mail Transfer Protocol

王朝other·作者佚名  2008-05-31
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SIMPLE MAIL TRANSFER PROTOCOL

Jonathan B. Postel

August 1982

Information Sciences Institute

University of Southern California

4676 Admiralty Way

Marina del Rey, California 90291

(213) 822-1511

RFC821 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: RFC788, 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.

August 1982 RFC821

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

RFC821 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).

August 1982 RFC821

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.

RFC821 August 1982

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.

August 1982 RFC821

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

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

RFC821 August 1982

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

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

August 1982 RFC821

Simple Mail Transfer Protocol

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.

RFC821 August 1982

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

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

August 1982 RFC821

Simple Mail Transfer Protocol

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.

RFC821 August 1982

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.

August 1982 RFC821

Simple Mail Transfer Protocol

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

for these commands.

RFC821 August 1982

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

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

August 1982 RFC821

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

RFC821 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.

August 1982 RFC821

Simple Mail Transfer Protocol

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

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

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

RFC821 August 1982

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.

August 1982 RFC821

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.

RFC821 August 1982

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.

August 1982 RFC821

Simple Mail Transfer Protocol

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.

RFC821 August 1982

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

August 1982 RFC821

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).

RFC821 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

August 1982 RFC821

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

RFC821 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.

August 1982 RFC821

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.

RFC821 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".

August 1982 RFC821

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.

RFC821 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>

August 1982 RFC821

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>

RFC821 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.

August 1982 RFC821

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.

RFC821 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

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

August 1982 RFC821

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.

RFC821 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

August 1982 RFC821

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

RFC821 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

August 1982 RFC821

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

RFC821 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.

August 1982 RFC821

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.

RFC821 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

August 1982 RFC821

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.

RFC821 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.

August 1982 RFC821

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.

RFC821 August 1982

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 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 an 8-bit byte with the high-order bit cleared to

zero.

August 1982 RFC821

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 an 8-bit byte with the high-order bit cleared to

zero.

RFC821 August 1982

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, however, it is suggested that a

reliable end-to-end protocol such as TCP be used on top of X.25

connections.

August 1982 RFC821

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

RFC821 August 1982

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

August 1982 RFC821

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 multiline 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

In many cases 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. In a few cases, there is important

data for the sender in the reply "text". The sender will know

these cases from the current context.

RFC821 August 1982

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.ARPA Simple Mail Transfer Service Ready

S: HELO USC-ISIF.ARPA

R: 250 BBN-UNIX.ARPA

S: MAIL FROM:<Smith@USC-ISIF.ARPA>

R: 250 OK

S: RCPT TO:<Jones@BBN-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<Green@BBN-UNIX.ARPA>

R: 550 No such user here

S: RCPT TO:<

Brown@BBN-UNIX.ARPA>

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.ARPA Service closing transmission channel

Scenario 1

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

August 1982 RFC821

Simple Mail Transfer Protocol

Aborted SMTP Transaction Scenario

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

R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready

S: HELO ISI-VAXA.ARPA

R: 250 MIT-Multics.ARPA

S: MAIL FROM:<Smith@ISI-VAXA.ARPA>

R: 250 OK

S: RCPT TO:<Jones@MIT-Multics.ARPA>

R: 250 OK

S: RCPT TO:<Green@MIT-Multics.ARPA>

R: 550 No such user here

S: RSET

R: 250 OK

S: QUIT

R: 221 MIT-Multics.ARPA Service closing transmission channel

Scenario 2

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

RFC821 August 1982

Simple Mail Transfer Protocol

Relayed Mail Scenario

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

Step 1 -- Source Host to Relay Host

R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready

S: HELO MIT-AI.ARPA

R: 250 USC-ISIE.ARPA

S: MAIL FROM:<JQP@MIT-AI.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>

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@MIT-AI.ARPA>

S: Subject: The Next Meeting of the Board

S: To: Jones@BBN-Vax.ARPA

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.ARPA Service closing transmission channel

August 1982 RFC821

Simple Mail Transfer Protocol

Step 2 -- Relay Host to Destination Host

R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready

S: HELO USC-ISIE.ARPA

R: 250 BBN-VAX.ARPA

S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>

R: 250 OK

S: RCPT TO:<Jones@BBN-VAX.ARPA>

R: 250 OK

S: DATA

R: 354 Start mail input; end with <CRLF>.<CRLF>

S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;

2 Nov 81 22:40:10 UT

S: Date: 2 Nov 81 22:33:44

S: From: John Q. Public <JQP@MIT-AI.ARPA>

S: Subject: The Next Meeting of the Board

S: To: Jones@BBN-Vax.ARPA

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.ARPA Service closing transmission channel

Scenario 3

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

RFC821 August 1982

Simple Mail Transfer Protocol

Verifying and Sending Scenario

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

R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready

S: HELO MIT-MC.ARPA

R: 250 SU-SCORE.ARPA

S: VRFY Crispin

R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

S: SEND FROM:<EAK@MIT-MC.ARPA>

R: 250 OK

S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>

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.ARPA Service closing transmission channel

Scenario 4

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

August 1982 RFC821

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.ARPA Simple Mail Transfer Service Ready

S: HELO MIT-MC.ARPA

R: 250 SU-SCORE.ARPA

S: VRFY Crispin

R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

S: SEND FROM:<EAK@MIT-MC.ARPA>

R: 250 OK

S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>

R: 450 User not active now

S: RSET

R: 250 OK

S: MAIL FROM:<EAK@MIT-MC.ARPA>

R: 250 OK

S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>

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.ARPA Service closing transmission channel

Scenario 5

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

RFC821 August 1982

Simple Mail Transfer Protocol

Doing the preceding scenario more efficiently.

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

R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready

S: HELO MIT-MC.ARPA

R: 250 SU-SCORE.ARPA

S: VRFY Crispin

R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

S: SOML FROM:<EAK@MIT-MC.ARPA>

R: 250 OK

S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>

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.ARPA Service closing transmission channel

Scenario 6

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

August 1982 RFC821

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.ARPA Simple Mail Transfer Service Ready

S: HELO SU-SCORE.ARPA

R: 250 MIT-AI.ARPA

S: EXPN Example-People

R: 250-<ABC@MIT-MC.ARPA>

R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>

R: 250-Xenon Y. Zither <XYZ@MIT-AI.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>

S: QUIT

R: 221 MIT-AI.ARPA Service closing transmission channel

RFC821 August 1982

Simple Mail Transfer Protocol

Step 2 -- Expanding the Second List

R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready

S: HELO SU-SCORE.ARPA

R: 250 MIT-MC.ARPA

S: EXPN Interested-Parties

R: 250-Al Calico <ABC@MIT-MC.ARPA>

R: 250-<XYZ@MIT-AI.ARPA>

R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>

R: 250-<fred@BBN-UNIX.ARPA>

R: 250 <xyz@bar-unix.ARPA>

S: QUIT

R: 221 MIT-MC.ARPA Service closing transmission channel

August 1982 RFC821

Simple Mail Transfer Protocol

Step 3 -- Mailing to All via a Relay Host

R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready

S: HELO SU-SCORE.ARPA

R: 250 USC-ISIE.ARPA

S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>

R: 250 OK

S: RCPT

TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>

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.ARPA Service closing transmission channel

Scenario 7

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

RFC821 August 1982

Simple Mail Transfer Protocol

Forwarding Scenarios

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

R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready

S: HELO LBL-UNIX.ARPA

R: 250 USC-ISIF.ARPA

S: MAIL FROM:<mo@LBL-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<fred@USC-ISIF.ARPA>

R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>

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.ARPA Service closing transmission channel

Scenario 8

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

August 1982 RFC821

Simple Mail Transfer Protocol

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

Step 1 -- Trying the Mailbox at the First Host

R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready

S: HELO LBL-UNIX.ARPA

R: 250 USC-ISIF.ARPA

S: MAIL FROM:<mo@LBL-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<fred@USC-ISIF.ARPA>

R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>

S: RSET

R: 250 OK

S: QUIT

R: 221 USC-ISIF.ARPA Service closing transmission channel

Step 2 -- Delivering the Mail at the Second Host

R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready

S: HELO LBL-UNIX.ARPA

R: 250 USC-ISI.ARPA

S: MAIL FROM:<mo@LBL-UNIX.ARPA>

R: 250 OK

S: RCPT TO:<Jones@USC-ISI.ARPA>

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-ISI.ARPA Service closing transmission channel

Scenario 9

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

RFC821 August 1982

Simple Mail Transfer Protocol

Too Many Recipients Scenario

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

R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready

S: HELO USC-ISIF.ARPA

R: 250 BERKELEY.ARPA

S: MAIL FROM:<Postel@USC-ISIF.ARPA>

R: 250 OK

S: RCPT TO:<fabry@BERKELEY.ARPA>

R: 250 OK

S: RCPT TO:<eric@BERKELEY.ARPA>

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.ARPA>

R: 250 OK

S: RCPT TO:<eric@BERKELEY.ARPA>

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.ARPA Service closing transmission channel

Scenario 10

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

Note that a real implementation must handle many recipients as

specified in Section 4.5.3.

August 1982 RFC821

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.

domain

The hierarchially structured global character string address of a

host computer in the mail system.

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 a sequence of ASCII characters 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

Internet Text Messages (RFC822 [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@domain". Additionally, the "container" in which mail

is stored.

RFC821 August 1982

Simple Mail Transfer Protocol

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.

reply

A reply is an acknowledgment (positive or negative) sent from

receiver to sender via the transmission channel in response to a

command. The general form of a reply is a 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.

August 1982 RFC821

Simple Mail Transfer Protocol

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.

word

A sequence of printing characters.

<CRLF>

The characters carriage return and line feed (in that order).

<SP>

The space character.

RFC821 August 1982

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] RFC822

Crocker, D., "Standard for the Format of ARPA Internet Text

Messages," RFC822, Department of Electrical Engineering,

University of Delaware, August 1982.

[3] TCP

Postel, J., ed., "Transmission Control Protocol - DARPA Internet

Program Protocol Specification", RFC793, USC/Information Sciences

Institute, NTIS AD Number A111091, September 1981. Also in:

Feinler, E. and J. Postel, eds., "Internet Protocol Transition

Workbook", SRI International, Menlo Park, California, March 1982.

[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.

August 1982 RFC821

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.

 
 
 
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