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RFC 821
SIMPLE MAIL TRANSFER PROTOCOL
Jonathan B. Postel
August 1982
Information Sciences
Institute
University of Southern
California
4676 Admiralty Way
Marina del Rey, California
90291
(213) 822-1511
RFC 821 August 1982
Simple Mail Transfer Protocol
TABLE OF CONTENTS
1. INTRODUCTION
.................................................. 1
2. THE SMTP MODEL
................................................ 2
3. THE SMTP PROCEDURE
............................................ 4
3.1. Mail
..................................................... 4
3.2. Forwarding
............................................... 7
3.3. Verifying and Expanding
.................................. 8
3.4. Sending and Mailing
..................................... 11
3.5. Opening and Closing
..................................... 13
3.6. Relaying
................................................ 14
3.7. Domains
................................................. 17
3.8. Changing Roles
.......................................... 18
4. THE SMTP SPECIFICATIONS
...................................... 19
4.1. SMTP Commands
........................................... 19
4.1.1. Command Semantics
..................................... 19
4.1.2. Command Syntax
........................................ 27
4.2. SMTP Replies
............................................ 34
4.2.1. Reply Codes by
Function Group ......................... 35
4.2.2. Reply Codes in Numeric
Order .......................... 36
4.3. Sequencing of Commands
and Replies ...................... 37
4.4. State Diagrams
.......................................... 39
4.5. Details
................................................. 41
4.5.1. Minimum Implementation
................................ 41
4.5.2. Transparency
.......................................... 41
4.5.3. Sizes
................................................. 42
APPENDIX A: TCP
................................................. 44
APPENDIX B: NCP
................................................. 45
APPENDIX C: NITS
................................................ 46
APPENDIX D: X.25
................................................ 47
APPENDIX E: Theory of Reply
Codes ............................... 48
APPENDIX F: Scenarios
........................................... 51
GLOSSARY
......................................................... 64
REFERENCES
....................................................... 67
Network Working Group J.
Postel
Request for Comments: DRAFT
ISI
Replaces: RFC 788, 780, 772
August 1982
SIMPLE MAIL TRANSFER PROTOCOL
1. INTRODUCTION
The objective of Simple Mail
Transfer Protocol (SMTP) is to transfer
mail reliably and efficiently.
SMTP is independent of the
particular transmission subsystem and
requires only a reliable
ordered data stream channel. Appendices A,
B, C, and D describe the use
of SMTP with various transport services.
A Glossary provides the
definitions of terms as used in this
document.
An important feature of SMTP
is its capability to relay mail across
transport service
environments. A transport service provides an
interprocess communication
environment (IPCE). An IPCE may cover one
network, several networks, or
a subset of a network. It is important
to realize that transport
systems (or IPCEs) are not one-to-one with
networks. A process can
communicate directly with another process
through any mutually known
IPCE. Mail is an application or use of
interprocess communication.
Mail can be communicated between
processes in different IPCEs
by relaying through a process connected
to two (or more) IPCEs. More
specifically, mail can be relayed
between hosts on different
transport systems by a host on both
transport systems.
Postel [Page 1]
August 1982 RFC 821
Simple Mail Transfer Protocol
2. THE SMTP MODEL
The SMTP design is based on
the following model of communication: as
the result of a user mail
request, the sender-SMTP establishes a
two-way transmission channel
to a receiver-SMTP. The receiver-SMTP
may be either the ultimate
destination or an intermediate. SMTP
commands are generated by the
sender-SMTP and sent to the
receiver-SMTP. SMTP replies
are sent from the receiver-SMTP to the
sender-SMTP in response to
the commands.
Once the transmission channel
is established, the SMTP-sender sends a
MAIL command indicating the
sender of the mail. If the SMTP-receiver
can accept mail it responds
with an OK reply. The SMTP-sender then
sends a RCPT command
identifying a recipient of the mail. If the
SMTP-receiver can accept mail
for that recipient it responds with an
OK reply; if not, it responds
with a reply rejecting that recipient
(but not the whole mail
transaction). The SMTP-sender and
SMTP-receiver may negotiate
several recipients. When the recipients
have been negotiated the
SMTP-sender sends the mail data, terminating
with a special sequence. If
the SMTP-receiver successfully processes
the mail data it responds
with an OK reply. The dialog is purposely
lock-step, one-at-a-time.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP |
|
+------+ | Sender- |Commands/Replies|
Receiver-|
+------+ | SMTP |<-------------->|
SMTP | +------+
| File |<-->| | and
Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+
+------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
-------------------------------------------------------------
The SMTP provides mechanisms
for the transmission of mail; directly
from the sending user's host
to the receiving user's host when the
[Page 2] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
two host are connected to the
same transport service, or via one or
more relay SMTP-servers when
the source and destination hosts are not
connected to the same
transport service.
To be able to provide the
relay capability the SMTP-server must be
supplied with the name of the
ultimate destination host as well as
the destination mailbox name.
The argument to the MAIL
command is a reverse-path, which specifies
who the mail is from. The
argument to the RCPT command is a
forward-path, which specifies
who the mail is to. The forward-path
is a source route, while the
reverse-path is a return route (which
may be used to return a
message to the sender when an error occurs
with a relayed message).
When the same message is sent
to multiple recipients the SMTP
encourages the transmission
of only one copy of the data for all the
recipients at the same
destination host.
The mail commands and replies
have a rigid syntax. Replies also have
a numeric code. In the
following, examples appear which use actual
commands and replies. The
complete lists of commands and replies
appears in Section 4 on
specifications.
Commands and replies are not
case sensitive. That is, a command or
reply word may be upper case,
lower case, or any mixture of upper and
lower case. Note that this is
not true of mailbox user names. For
some hosts the user name is
case sensitive, and SMTP implementations
must take case to preserve
the case of user names as they appear in
mailbox arguments. Host names
are not case sensitive.
Commands and replies are
composed of characters from the ASCII
character set [1]. When the
transport service provides an 8-bit byte
(octet) transmission channel,
each 7-bit character is transmitted
right justified in an octet
with the high order bit cleared to zero.
When specifying the general
form of a command or reply, an argument
(or special symbol) will be
denoted by a meta-linguistic variable (or
constant), for example,
"<string>" or
"<reverse-path>". Here the
angle brackets indicate these
are meta-linguistic variables.
However, some arguments use
the angle brackets literally. For
example, an actual
reverse-path is enclosed in angle brackets, i.e.,
"<John.Smith@USC-ISI.ARPA>"
is an instance of <reverse-path> (the
angle brackets are actually
transmitted in the command or reply).
Postel [Page 3]
August 1982 RFC 821
Simple Mail Transfer Protocol
3. THE SMTP PROCEDURES
This section presents the
procedures used in SMTP in several parts.
First comes the basic mail
procedure defined as a mail transaction.
Following this are
descriptions of forwarding mail, verifying mailbox
names and expanding mailing
lists, sending to terminals instead of or
in combination with mailboxes,
and the opening and closing exchanges.
At the end of this section
are comments on relaying, a note on mail
domains, and a discussion of
changing roles. Throughout this section
are examples of partial
command and reply sequences, several complete
scenarios are presented in
Appendix F.
3.1. MAIL
There are three steps to SMTP
mail transactions. The transaction
is started with a MAIL
command which gives the sender
identification. A series of
one or more RCPT commands follows
giving the receiver
information. Then a DATA command gives the
mail data. And finally, the
end of mail data indicator confirms
the transaction.
The first step in the
procedure is the MAIL command. The
<reverse-path> contains
the source mailbox.
MAIL <SP> FROM:<reverse-path>
<CRLF>
This command tells the
SMTP-receiver that a new mail
transaction is starting and
to reset all its state tables and
buffers, including any
recipients or mail data. It gives the
reverse-path which can be
used to report errors. If accepted,
the receiver-SMTP returns a
250 OK reply.
The <reverse-path> can
contain more than just a mailbox. The
<reverse-path> is a
reverse source routing list of hosts and
source mailbox. The first
host in the <reverse-path> should be
the host sending this command.
The second step in the
procedure is the RCPT command.
RCPT <SP>
TO:<forward-path> <CRLF>
This command gives a
forward-path identifying one recipient.
If accepted, the
receiver-SMTP returns a 250 OK reply, and
stores the forward-path. If
the recipient is unknown the
receiver-SMTP returns a 550
Failure reply. This second step of
the procedure can be repeated
any number of times.
[Page 4] Postel
RFC 821 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.
Postel [Page 5]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 6] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 7]
August 1982 RFC 821
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.
[Page 8] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 9]
August 1982 RFC 821
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.
[Page 10] Postel
RFC 821 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.
Postel [Page 11]
August 1982 RFC 821
Simple Mail Transfer Protocol
The same reply codes that are
used for the MAIL commands are used
for these commands.
[Page 12] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 13]
August 1982 RFC 821
Simple Mail Transfer Protocol
3.6. RELAYING
The forward-path may be a
source route of the form
"@ONE,@TWO:JOE@THREE",
where ONE, TWO, and THREE are hosts. This
form is used to emphasize the
distinction between an address and a
route. The mailbox is an
absolute address, and the route is
information about how to get
there. The two concepts should not
be confused.
Conceptually the elements of
the forward-path are moved to the
reverse-path as the message
is relayed from one server-SMTP to
another. The reverse-path is
a reverse source route, (i.e., a
source route from the current
location of the message to the
originator of the message).
When a server-SMTP deletes its
identifier from the
forward-path and inserts it into the
reverse-path, it must use the
name it is known by in the
environment it is sending
into, not the environment the mail came
from, in case the server-SMTP
is known by different names in
different environments.
If when the message arrives
at an SMTP the first element of the
forward-path is not the
identifier of that SMTP the element is not
deleted from the forward-path
and is used to determine the next
SMTP to send the message to.
In any case, the SMTP adds its own
identifier to the
reverse-path.
Using source routing the
receiver-SMTP receives mail to be relayed
to another server-SMTP The
receiver-SMTP may accept or reject the
task of relaying the mail in
the same way it accepts or rejects
mail for a local user. The
receiver-SMTP transforms the command
arguments by moving its own
identifier from the forward-path to
the beginning of the
reverse-path. The receiver-SMTP then becomes
a sender-SMTP, establishes a
transmission channel to the next SMTP
in the forward-path, and
sends it the mail.
The first host in the
reverse-path should be the host sending the
SMTP commands, and the first
host in the forward-path should be
the host receiving the SMTP
commands.
Notice that the forward-path
and reverse-path appear in the SMTP
commands and replies, but not
necessarily in the message. That
is, there is no need for
these paths and especially this syntax to
appear in the "To:"
, "From:", "CC:", etc. fields of the
message
header.
If a server-SMTP has accepted
the task of relaying the mail and
[Page 14] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
later finds that the
forward-path is incorrect or that the mail
cannot be delivered for
whatever reason, then it must construct an
"undeliverable
mail" notification message and send it to the
originator of the
undeliverable mail (as indicated by the
reverse-path).
This notification message
must be from the server-SMTP at this
host. Of course, server-SMTPs
should not send notification
messages about problems with
notification messages. One way to
prevent loops in error
reporting is to specify a null reverse-path
in the MAIL command of a
notification message. When such a
message is relayed it is
permissible to leave the reverse-path
null. A MAIL command with a
null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail
notification message is shown in example 7.
This notification is in
response to a message originated by JOE at
HOSTW and sent via HOSTX to
HOSTY with instructions to relay it on
to HOSTZ. What we see in the
example is the transaction between
HOSTY and HOSTX, which is the
first step in the return of the
notification message.
Postel [Page 15]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 16] Postel
RFC 821 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.
Postel [Page 17]
August 1982 RFC 821
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.
[Page 18] Postel
RFC 821 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.
Postel [Page 19]
August 1982 RFC 821
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.
[Page 20] Postel
RFC 821 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
Postel [Page 21]
August 1982 RFC 821
Simple Mail Transfer Protocol
return path line. The return
path line preserves the
information in the
<reverse-path> from the MAIL command.
Here, final delivery means
the message leaves the SMTP
world. Normally, this would
mean it has been delivered to
the destination user, but in
some cases it may be further
processed and transmitted by
another mail system.
It is possible for the
mailbox in the return path be
different from the actual
sender's mailbox, for example,
if error responses are to be
delivered a special error
handling mailbox rather than
the message senders.
The preceding two paragraphs
imply that the final mail data
will begin with a return path
line, followed by one or more
time stamp lines. These lines
will be followed by the mail
data header and body [2]. See
Example 8.
Special mention is needed of
the response and further action
required when the processing
following the end of mail data
indication is partially
successful. This could arise if
after accepting several
recipients and the mail data, the
receiver-SMTP finds that the
mail data can be successfully
delivered to some of the
recipients, but it cannot be to
others (for example, due to
mailbox space allocation
problems). In such a
situation, the response to the DATA
command must be an OK reply.
But, the receiver-SMTP must
compose and send an "undeliverable
mail" notification
message to the originator of
the message. Either a single
notification which lists all
of the recipients that failed
to get the message, or
separate notification messages must
be sent for each failed
recipient (see Example 7). All
undeliverable mail
notification messages are sent using the
MAIL command (even if they
result from processing a SEND,
SOML, or SAML command).
[Page 22] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of Return Path and
Received Time Stamps
Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
Received: from GHI.ARPA by
JKL.ARPA ; 27 Oct 81 15:27:39 PST
Received: from DEF.ARPA by
GHI.ARPA ; 27 Oct 81 15:15:13 PST
Received: from ABC.ARPA by
DEF.ARPA ; 27 Oct 81 15:01:59 PST
Date: 27 Oct 81 15:01:01 PST
From: JOE@ABC.ARPA
Subject: Improved Mailing
System Installed
To: SAM@JKL.ARPA
This is to inform you that
...
Example 8
-------------------------------------------------------------
SEND (SEND)
This command is used to
initiate a mail transaction in which
the mail data is delivered to
one or more terminals. The
argument field contains a
reverse-path. This command is
successful if the message is
delivered to a terminal.
The reverse-path consists of
an optional list of hosts and
the sender mailbox. When the
list of hosts is present, it
is a "reverse"
source route and indicates that the mail was
relayed through each host on
the list (the first host in the
list was the most recent
relay). This list is used as a
source route to return
non-delivery notices to the sender.
As each relay host adds
itself to the beginning of the list,
it must use its name as known
in the IPCE to which it is
relaying the mail rather than
the IPCE from which the mail
came (if they are different).
This command clears the
reverse-path buffer, the
forward-path buffer, and the
mail data buffer; and inserts
the reverse-path information
from this command into the
reverse-path buffer.
SEND OR MAIL (SOML)
This command is used to
initiate a mail transaction in which
the mail data is delivered to
one or more terminals or
Postel [Page 23]
August 1982 RFC 821
Simple Mail Transfer Protocol
mailboxes. For each recipient
the mail data is delivered to
the recipient's terminal if
the recipient is active on the
host (and accepting terminal
messages), otherwise to the
recipient's mailbox. The
argument field contains a
reverse-path. This command is
successful if the message is
delivered to a terminal or
the mailbox.
The reverse-path consists of
an optional list of hosts and
the sender mailbox. When the
list of hosts is present, it
is a "reverse"
source route and indicates that the mail was
relayed through each host on
the list (the first host in the
list was the most recent
relay). This list is used as a
source route to return
non-delivery notices to the sender.
As each relay host adds
itself to the beginning of the list,
it must use its name as known
in the IPCE to which it is
relaying the mail rather than
the IPCE from which the mail
came (if they are different).
This command clears the
reverse-path buffer, the
forward-path buffer, and the
mail data buffer; and inserts
the reverse-path information
from this command into the
reverse-path buffer.
SEND AND MAIL (SAML)
This command is used to
initiate a mail transaction in which
the mail data is delivered to
one or more terminals and
mailboxes. For each recipient
the mail data is delivered to
the recipient's terminal if
the recipient is active on the
host (and accepting terminal
messages), and for all
recipients to the recipient's
mailbox. The argument field
contains a reverse-path. This
command is successful if the
message is delivered to the
mailbox.
The reverse-path consists of
an optional list of hosts and
the sender mailbox. When the
list of hosts is present, it
is a "reverse"
source route and indicates that the mail was
relayed through each host on
the list (the first host in the
list was the most recent
relay). This list is used as a
source route to return
non-delivery notices to the sender.
As each relay host adds
itself to the beginning of the list,
it must use its name as known
in the IPCE to which it is
relaying the mail rather than
the IPCE from which the mail
came (if they are different).
This command clears the
reverse-path buffer, the
[Page 24] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
forward-path buffer, and the
mail data buffer; and inserts
the reverse-path information
from this command into the
reverse-path buffer.
RESET (RSET)
This command specifies that
the current mail transaction is
to be aborted. Any stored
sender, recipients, and mail data
must be discarded, and all
buffers and state tables cleared.
The receiver must send an OK
reply.
VERIFY (VRFY)
This command asks the
receiver to confirm that the argument
identifies a user. If it is a
user name, the full name of
the user (if known) and the
fully specified mailbox are
returned.
This command has no effect on
any of the reverse-path
buffer, the forward-path
buffer, or the mail data buffer.
EXPAND (EXPN)
This command asks the
receiver to confirm that the argument
identifies a mailing list,
and if so, to return the
membership of that list. The
full name of the users (if
known) and the fully
specified mailboxes are returned in a
multiline reply.
This command has no effect on
any of the reverse-path
buffer, the forward-path
buffer, or the mail data buffer.
HELP (HELP)
This command causes the
receiver to send helpful information
to the sender of the HELP
command. The command may take an
argument (e.g., any command
name) and return more specific
information as a response.
This command has no effect on
any of the reverse-path
buffer, the forward-path
buffer, or the mail data buffer.
Postel [Page 25]
August 1982 RFC 821
Simple Mail Transfer Protocol
NOOP (NOOP)
This command does not affect
any parameters or previously
entered commands. It
specifies no action other than that
the receiver send an OK
reply.
This command has no effect on
any of the reverse-path
buffer, the forward-path
buffer, or the mail data buffer.
QUIT (QUIT)
This command specifies that
the receiver must send an OK
reply, and then close the
transmission channel.
The receiver should not close
the transmission channel until
it receives and replies to a
QUIT command (even if there was
an error). The sender should
not close the transmission
channel until it send a QUIT
command and receives the reply
(even if there was an error
response to a previous command).
If the connection is closed
prematurely the receiver should
act as if a RSET command had
been received (canceling any
pending transaction, but not
undoing any previously
completed transaction), the
sender should act as if the
command or transaction in
progress had received a temporary
error (4xx).
TURN (TURN)
This command specifies that
the receiver must either (1)
send an OK reply and then
take on the role of the
sender-SMTP, or (2) send a
refusal reply and retain the role
of the receiver-SMTP.
If program-A is currently the
sender-SMTP and it sends the
TURN command and receives an
OK reply (250) then program-A
becomes the receiver-SMTP.
Program-A is then in the initial
state as if the transmission
channel just opened, and it
then sends the 220 service
ready greeting.
If program-B is currently the
receiver-SMTP and it receives
the TURN command and sends an
OK reply (250) then program-B
becomes the sender-SMTP.
Program-B is then in the initial
state as if the transmission
channel just opened, and it
then expects to receive the
220 service ready greeting.
To refuse to change roles the
receiver sends the 502 reply.
[Page 26] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
There are restrictions on the
order in which these command may
be used.
The first command in a
session must be the HELO command.
The HELO command may be used
later in a session as well. If
the HELO command argument is
not acceptable a 501 failure
reply must be returned and
the receiver-SMTP must stay in
the same state.
The NOOP, HELP, EXPN, and
VRFY commands can be used at any
time during a session.
The MAIL, SEND, SOML, or SAML
commands begin a mail
transaction. Once started a
mail transaction consists of
one of the transaction
beginning commands, one or more RCPT
commands, and a DATA command,
in that order. A mail
transaction may be aborted by
the RSET command. There may
be zero or more transactions
in a session.
If the transaction beginning
command argument is not
acceptable a 501 failure
reply must be returned and the
receiver-SMTP must stay in
the same state. If the commands
in a transaction are out of
order a 503 failure reply must
be returned and the
receiver-SMTP must stay in the same
state.
The last command in a session
must be the QUIT command. The
QUIT command can not be used
at any other time in a session.
4.1.2. COMMAND SYNTAX
The commands consist of a
command code followed by an argument
field. Command codes are four
alphabetic characters. Upper
and lower case alphabetic
characters are to be treated
identically. Thus, any of the
following may represent the mail
command:
MAIL Mail mail MaIl mAIl
This also applies to any
symbols representing parameter values,
such as "TO" or
"to" for the forward-path. Command codes and
the argument fields are
separated by one or more spaces.
However, within the
reverse-path and forward-path arguments
case is important. In
particular, in some hosts the user
"smith" is
different from the user "Smith".
Postel [Page 27]
August 1982 RFC 821
Simple Mail Transfer Protocol
The argument field consists
of a variable length character
string ending with the
character sequence <CRLF>. The receiver
is to take no action until
this sequence is received.
Square brackets denote an
optional argument field. If the
option is not taken, the
appropriate default is implied.
[Page 28] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
The following are the SMTP
commands:
HELO <SP>
<domain> <CRLF>
MAIL <SP>
FROM:<reverse-path> <CRLF>
RCPT <SP>
TO:<forward-path> <CRLF>
DATA <CRLF>
RSET <CRLF>
SEND <SP>
FROM:<reverse-path> <CRLF>
SOML <SP>
FROM:<reverse-path> <CRLF>
SAML <SP>
FROM:<reverse-path> <CRLF>
VRFY <SP>
<string> <CRLF>
EXPN <SP>
<string> <CRLF>
HELP [<SP>
<string>] <CRLF>
NOOP <CRLF>
QUIT <CRLF>
TURN <CRLF>
Postel [Page 29]
August 1982 RFC 821
Simple Mail Transfer Protocol
The syntax of the above
argument fields (using BNF notation
where applicable) is given
below. The "..." notation indicates
that a field may be repeated
one or more times.
<reverse-path> ::=
<path>
<forward-path> ::=
<path>
<path> ::=
"<" [ <a-d-l> ":" ]
<mailbox> ">"
<a-d-l> ::=
<at-domain> | <at-domain> ","
<a-d-l>
<at-domain> ::=
"@" <domain>
<domain> ::=
<element> | <element> "." <domain>
<element> ::=
<name> | "#" <number> | "["
<dotnum> "]"
<mailbox> ::=
<local-part> "@" <domain>
<local-part> ::=
<dot-string> | <quoted-string>
<name> ::= <a>
<ldh-str> <let-dig>
<ldh-str> ::=
<let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig> ::= <a>
| <d>
<let-dig-hyp> ::=
<a> | <d> | "-"
<dot-string> ::=
<string> | <string> "."
<dot-string>
<string> ::=
<char> | <char> <string>
<quoted-string> ::=
""" <qtext> """
<qtext> ::=
"\" <x> | "\" <x>
<qtext> | <q> | <q> <qtext>
<char> ::= <c> |
"\" <x>
<dotnum> ::=
<snum> "." <snum> "."
<snum> "." <snum>
<number> ::= <d>
| <d> <number>
<CRLF> ::= <CR>
<LF>
[Page 30] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
<CR> ::= the carriage
return character (ASCII code 13)
<LF> ::= the line feed
character (ASCII code 10)
<SP> ::= the space
character (ASCII code 32)
<snum> ::= one, two, or
three digits representing a decimal
integer value in the range 0
through 255
<a> ::= any one of the
52 alphabetic characters A through Z
in upper case and a through z
in lower case
<c> ::= any one of the
128 ASCII characters, but not any
<special> or <SP>
<d> ::= any one of the
ten digits 0 through 9
<q> ::= any one of the
128 ASCII characters except <CR>,
<LF>, quote ("),
or backslash (\)
<x> ::= any one of the
128 ASCII characters (no exceptions)
<special> ::=
"<" | ">" | "(" |
")" | "[" | "]" | "\"
| "."
| "," |
";" | ":" | "@"
""" | the control
characters (ASCII codes 0
through 31 inclusive and
127)
Note that the backslash,
"\", is a quote character, which is
used to indicate that the
next character is to be used
literally (instead of its
normal interpretation). For example,
"Joe\,Smith" could
be used to indicate a single nine character
user field with comma being
the fourth character of the field.
Hosts are generally known by
names which are translated to
addresses in each host. Note
that the name elements of domains
are the official names -- no
use of nicknames or aliases is
allowed.
Sometimes a host is not known
to the translation function and
communication is blocked. To
bypass this barrier two numeric
forms are also allowed for
host "names". One form is a decimal
integer prefixed by a pound
sign, "#", which indicates the
number is the address of the
host. Another form is four small
decimal integers separated by
dots and enclosed by brackets,
e.g.,
"[123.255.37.2]", which indicates a 32-bit ARPA
Internet
Address in four 8-bit fields.
Postel [Page 31]
August 1982 RFC 821
Simple Mail Transfer Protocol
The time stamp line and the
return path line are formally
defined as follows:
<return-path-line> ::=
"Return-Path:"
<SP><reverse-path><CRLF>
<time-stamp-line> ::=
"Received:" <SP> <stamp> <CRLF>
<stamp> ::=
<from-domain> <by-domain> <opt-info>
";"
<daytime>
<from-domain> ::=
"FROM" <SP> <domain> <SP>
<by-domain> ::=
"BY" <SP> <domain> <SP>
<opt-info> ::=
[<via>] [<with>] [<id>] [<for>]
<via> ::=
"VIA" <SP> <link> <SP>
<with> ::=
"WITH" <SP> <protocol> <SP>
<id> ::= "ID"
<SP> <string> <SP>
<for> ::=
"FOR" <SP> <path> <SP>
<link> ::= The standard
names for links are registered with
the Network Information
Center.
<protocol> ::= The
standard names for protocols are
registered with the Network
Information Center.
<daytime> ::=
<SP> <date> <SP> <time>
<date> ::= <dd>
<SP> <mon> <SP> <yy>
<time> ::= <hh>
":" <mm> ":" <ss> <SP>
<zone>
<dd> ::= the one or two
decimal integer day of the month in
the range 1 to 31.
<mon> ::=
"JAN" | "FEB" | "MAR" |
"APR" | "MAY" | "JUN" |
"JUL" |
"AUG" | "SEP" | "OCT" |
"NOV" | "DEC"
<yy> ::= the two
decimal integer year of the century in the
range 00 to 99.
[Page 32] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
<hh> ::= the two
decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two
decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two
decimal integer second of the minute in the
range 00 to 59.
<zone> ::=
"UT" for Universal Time (the default) or other
time zone designator (as in
[2]).
-------------------------------------------------------------
Return Path Example
Return-Path:
<@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
Example 9
-------------------------------------------------------------
-------------------------------------------------------------
Time Stamp Line Example
Received: FROM ABC.ARPA BY
XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
Received: from ABC.ARPA by
XYZ.ARPA via TELENET with X25
id M12345 for Smith@PDQ.ARPA
; 22 OCT 81 09:23:59 PDT
Example 10
-------------------------------------------------------------
Postel [Page 33]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2. SMTP REPLIES
Replies to SMTP commands are
devised to ensure the synchronization
of requests and actions in
the process of mail transfer, and to
guarantee that the
sender-SMTP always knows the state of the
receiver-SMTP. Every command
must generate exactly one reply.
The details of the
command-reply sequence are made explicit in
Section 5.3 on Sequencing and
Section 5.4 State Diagrams.
An SMTP reply consists of a
three digit number (transmitted as
three alphanumeric
characters) followed by some text. The number
is intended for use by
automata to determine what state to enter
next; the text is meant for
the human user. It is intended that
the three digits contain
enough encoded information that the
sender-SMTP need not examine
the text and may either discard it or
pass it on to the user, as
appropriate. In particular, the text
may be receiver-dependent and
context dependent, so there are
likely to be varying texts
for each reply code. A discussion of
the theory of reply codes is
given in Appendix E. Formally, a
reply is defined to be the
sequence: a three-digit code, <SP>,
one line of text, and
<CRLF>, or a multiline reply (as defined in
Appendix E). Only the EXPN
and HELP commands are expected to
result in multiline replies
in normal circumstances, however
multiline replies are allowed
for any command.
[Page 34] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.2.1. REPLY CODES BY
FUNCTION GROUPS
500 Syntax error, command
unrecognized
[This may include errors such
as command line too long]
501 Syntax error in
parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not
implemented
211 System status, or system
help reply
214 Help message
[Information on how to use
the receiver or the meaning of a
particular non-standard
command; this reply is useful only
to the human user]
220 <domain> Service
ready
221 <domain> Service
closing transmission channel
421 <domain> Service
not available,
closing transmission channel
[This may be a reply to any
command if the service knows it
must shut down]
250 Requested mail action
okay, completed
251 User not local; will
forward to <forward-path>
450 Requested mail action not
taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not
taken: mailbox unavailable
[E.g., mailbox not found, no
access]
451 Requested action aborted:
error in processing
551 User not local; please
try <forward-path>
452 Requested action not
taken: insufficient system storage
552 Requested mail action
aborted: exceeded storage allocation
553 Requested action not
taken: mailbox name not allowed
[E.g., mailbox syntax
incorrect]
354 Start mail input; end
with <CRLF>.<CRLF>
554 Transaction failed
Postel [Page 35]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2.2. NUMERIC ORDER LIST OF
REPLY CODES
211 System status, or system
help reply
214 Help message
[Information on how to use
the receiver or the meaning of a
particular non-standard
command; this reply is useful only
to the human user]
220 <domain> Service
ready
221 <domain> Service
closing transmission channel
250 Requested mail action
okay, completed
251 User not local; will
forward to <forward-path>
354 Start mail input; end
with <CRLF>.<CRLF>
421 <domain> Service
not available,
closing transmission channel
[This may be a reply to any
command if the service knows it
must shut down]
450 Requested mail action not
taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted:
local error in processing
452 Requested action not
taken: insufficient system storage
500 Syntax error, command
unrecognized
[This may include errors such
as command line too long]
501 Syntax error in
parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not
implemented
550 Requested action not
taken: mailbox unavailable
[E.g., mailbox not found, no
access]
551 User not local; please
try <forward-path>
552 Requested mail action
aborted: exceeded storage allocation
553 Requested action not
taken: mailbox name not allowed
[E.g., mailbox syntax
incorrect]
554 Transaction failed
[Page 36] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.3. SEQUENCING OF COMMANDS
AND REPLIES
The communication between the
sender and receiver is intended to
be an alternating dialogue,
controlled by the sender. As such,
the sender issues a command
and the receiver responds with a
reply. The sender must wait
for this response before sending
further commands.
One important reply is the
connection greeting. Normally, a
receiver will send a 220
"Service ready" reply when the connection
is completed. The sender
should wait for this greeting message
before sending any commands.
Note: all the greeting type
replies have the official name of
the server host as the first
word following the reply code.
For example,
220 <SP> USC-ISIF.ARPA
<SP> Service ready <CRLF>
The table below lists
alternative success and failure replies for
each command. These must be
strictly adhered to; a receiver may
substitute text in the
replies, but the meaning and action implied
by the code numbers and by
the specific command reply sequence
cannot be altered.
COMMAND-REPLY SEQUENCES
Each command is listed with
its possible replies. The prefixes
used before the possible
replies are "P" for preliminary (not
used in SMTP), "I"
for intermediate, "S" for success, "F" for
failure, and "E"
for error. The 421 reply (service not
available, closing
transmission channel) may be given to any
command if the SMTP-receiver
knows it must shut down. This
listing forms the basis for
the State Diagrams in Section 4.4.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
Postel [Page 37]
August 1982 RFC 821
Simple Mail Transfer Protocol
RCPT
S: 250, 251
F: 550, 551, 552, 553, 450,
451, 452
E: 500, 501, 503, 421
DATA
I: 354 -> data -> S:
250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 503, 421
RSET
S: 250
E: 500, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251
F: 550, 551, 553
E: 500, 501, 502, 504, 421
EXPN
S: 250
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500, 421
QUIT
S: 221
E: 500
TURN
S: 250
F: 502
E: 500, 503
[Page 38] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.4. STATE DIAGRAMS
Following are state diagrams
for a simple-minded SMTP
implementation. Only the
first digit of the reply codes is used.
There is one state diagram
for each group of SMTP commands. The
command groupings were
determined by constructing a model for each
command and then collecting
together the commands with
structurally identical
models.
For each command there are
three possible outcomes: "success"
(S), "failure" (F),
and "error" (E). In the state diagrams below
we use the symbol B for
"begin", and the symbol W for "wait for
reply".
First, the diagram that
represents most of the SMTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W
|---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the
commands:
HELO, MAIL, RCPT, RSET, SEND,
SOML, SAML, VRFY, EXPN, HELP,
NOOP, QUIT, TURN.
Postel [Page 39]
August 1982 RFC 821
Simple Mail Transfer Protocol
A more complex diagram models
the DATA command:
+---+ DATA +---+ 1,2 +---+
| B |---------->| W
|-------------------->| E |
+---+ +---+
------------>+---+
3| |4,5 |
| | |
-------------- ----- |
| | | +---+
| ---------- -------->| S
|
| | | | +---+
| | ------------
| | | |
V 1,3| |2 |
+---+ data +---+
--------------->+---+
| |---------->| W | | F |
+---+
+---+-------------------->+---+
4,5
Note that the
"data" here is a series of lines sent from the
sender to the receiver with
no response expected until the last
line is sent.
[Page 40] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP
workable, the following minimum
implementation is required
for all receivers:
COMMANDS -- HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for
data transparency the character
sequence
"<CRLF>.<CRLF>" ends the mail text and
cannot be sent
by the user. In general,
users are not aware of such
"forbidden"
sequences. To allow all user composed text to be
transmitted transparently the
following procedures are used.
1. Before sending a line of
mail text the sender-SMTP checks
the first character of the
line. If it is a period, one
additional period is inserted
at the beginning of the line.
2. When a line of mail text
is received by the receiver-SMTP
it checks the line. If the
line is composed of a single
period it is the end of mail.
If the first character is a
period and there are other
characters on the line, the first
character is deleted.
The mail data may contain any
of the 128 ASCII characters. All
characters are to be
delivered to the recipient's mailbox
including format effectors
and other control characters. If
the transmission channel
provides an 8-bit byte (octets) data
stream, the 7-bit ASCII codes
are transmitted right justified
in the octets with the high
order bits cleared to zero.
In some systems it may be
necessary to transform the data as
it is received and stored.
This may be necessary for hosts
that use a different
character set than ASCII as their local
character set, or that store
data in records rather than
Postel [Page 41]
August 1982 RFC 821
Simple Mail Transfer Protocol
strings. If such transforms
are necessary, they must be
reversible -- especially if
such transforms are applied to
mail being relayed.
4.5.3. SIZES
There are several objects
that have required minimum maximum
sizes. That is, every
implementation must be able to receive
objects of at least these
sizes, but must not send objects
larger than these sizes.
****************************************************
* *
* TO THE MAXIMUM EXTENT
POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO
LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE
USED. *
* *
****************************************************
user
The maximum total length of a
user name is 64 characters.
domain
The maximum total length of a
domain name or number is 64
characters.
path
The maximum total length of a
reverse-path or
forward-path is 256
characters (including the punctuation
and element separators).
command line
The maximum total length of a
command line including the
command word and the
<CRLF> is 512 characters.
reply line
The maximum total length of a
reply line including the
reply code and the
<CRLF> is 512 characters.
[Page 42] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
text line
The maximum total length of a
text line including the
<CRLF> is 1000
characters (but not counting the leading
dot duplicated for
transparency).
recipients buffer
The maximum total number of
recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT
POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO
LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE
USED. *
* *
****************************************************
Errors due to exceeding these
limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
Postel [Page 43]
August 1982 RFC 821
Simple Mail Transfer Protocol
APPENDIX A
TCP Transport service
The Transmission Control
Protocol [3] is used in the ARPA
Internet, and in any network
following the US DoD standards for
internetwork protocols.
Connection Establishment
The SMTP transmission channel
is a TCP connection established
between the sender process
port U and the receiver process port
L. This single full duplex
connection is used as the
transmission channel. This
protocol is assigned the service
port 25 (31 octal), that is
L=25.
Data Transfer
The TCP connection supports
the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII
characters. Each character is
transmitted as an 8-bit byte
with the high-order bit cleared to
zero.
[Page 44] Postel
RFC 821 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.
Postel [Page 45]
August 1982 RFC 821
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.
[Page 46] Postel
RFC 821 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.
Postel [Page 47]
August 1982 RFC 821
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
[Page 48] Postel
RFC 821 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
Postel [Page 49]
August 1982 RFC 821
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.
[Page 50] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 51]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 52] Postel
RFC 821 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
Postel [Page 53]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 54] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 55]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 56] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 57]
August 1982 RFC 821
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
[Page 58] Postel
RFC 821 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
Postel [Page 59]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 60] Postel
RFC 821 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
-------------------------------------------------------------
Postel [Page 61]
August 1982 RFC 821
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
-------------------------------------------------------------
[Page 62] Postel
RFC 821 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.
Postel [Page 63]
August 1982 RFC 821
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 (RFC
822 [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.
[Page 64] Postel
RFC 821 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.
Postel [Page 65]
August 1982 RFC 821
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.
[Page 66] Postel
RFC 821 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] RFC 822
Crocker, D., "Standard
for the Format of ARPA Internet Text
Messages," RFC 822,
Department of Electrical Engineering,
University of Delaware,
August 1982.
[3] TCP
Postel, J., ed.,
"Transmission Control Protocol - DARPA Internet
Program Protocol
Specification", RFC 793, 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.
Postel [Page 67]
August 1982 RFC 821
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.
[Page 68] Postel
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