Simple Mail Transfer Protocol
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.
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Simple Mail Transfer Protocol
2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as
the result of a user mail request, the sender-SMTP establishes a
two-way transmission channel to a receiver-SMTP. The receiver-SMTP
may be either the ultimate destination or an intermediate. SMTP
commands are generated by the sender-SMTP and sent to the
receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a
MAIL command indicating the sender of the mail. If the SMTP-receiver
can accept mail it responds with an OK reply. The SMTP-sender then
sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an
OK reply; if not, it responds with a reply rejecting that recipient
(but not the whole mail transaction). The SMTP-sender and
SMTP-receiver may negotiate several recipients. When the recipients
have been negotiated the SMTP-sender sends the mail data, terminating
with a special sequence. If the SMTP-receiver successfully processes
the mail data it responds with an OK reply. The dialog is purposely
lock-step, one-at-a-time.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
-------------------------------------------------------------
The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
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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, "" or "". 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.,
"" is an instance of (the
angle brackets are actually transmitted in the command or reply).
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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
contains the source mailbox.
MAIL FROM:
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 can contain more than just a mailbox. The
is a reverse source routing list of hosts and
source mailbox. The first host in the should be
the host sending this command.
The second step in the procedure is the RCPT command.
RCPT TO:
This command gives a forward-path identifying one recipient.
If accepted, the receiver-SMTP returns a 250 OK reply, and
stores the forward-path. If the recipient is unknown the
receiver-SMTP returns a 550 Failure reply. This second step of
the procedure can be repeated any number of times.
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Simple Mail Transfer Protocol
The can contain more than just a mailbox. The
is a source routing list of hosts and the
destination mailbox. The first host in the
should be the host receiving this command.
The third step in the procedure is the DATA command.
DATA
If accepted, the receiver-SMTP returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.
Since the mail data is sent on the transmission channel the end
of the mail data must be indicated so that the command and
reply dialog can be resumed. SMTP indicates the end of the
mail data by sending a line containing only a period. A
transparency procedure is used to prevent this from interfering
with the user's text (see Section 4.5.2).
Please note that the mail data includes the memo header
items such as Date, Subject, To, Cc, From [2].
The end of mail data indicator also confirms the mail
transaction and tells the receiver-SMTP to now process the
stored recipients and mail data. If accepted, the
receiver-SMTP returns a 250 OK reply. The DATA command should
fail only if the mail transaction was incomplete (for example,
no recipients), or if resources are not available.
The above procedure is an example of a mail transaction. These
commands must be used only in the order discussed above.
Example 1 (below) illustrates the use of these commands in a mail
transaction.
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-------------------------------------------------------------
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:
R: 250 OK
S: RCPT TO:
R: 250 OK
S: RCPT TO:
R: 550 No such user here
S: RCPT TO:
R: 250 OK
S: DATA
R: 354 Start mail input; end with .
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
The mail has now been accepted for Jones and Brown. Green did
not have a mailbox at host Beta.
Example 1
-------------------------------------------------------------
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3.2. FORWARDING
There are some cases where the destination information in the
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
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
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:
R: 251 User not local; will forward to
Or
S: RCPT TO:
R: 551 User not local; please try
Example 2
-------------------------------------------------------------
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3.3. VERIFYING AND EXPANDING
SMTP provides as additional features, commands to verify a user
name or expand a mailing list. This is done with the VRFY and
EXPN commands, which have character string arguments. For the
VRFY command, the string is a user name, and the response may
include the full name of the user and must include the mailbox of
the user. For the EXPN command, the string identifies a mailing
list, and the multiline response may include the full name of the
users and must give the mailboxes on the mailing list.
"User name" is a fuzzy term and used purposely. If a host
implements the VRFY or EXPN commands then at least local mailboxes
must be recognized as "user names". If a host chooses to
recognize other strings as "user names" that is allowed.
In some hosts the distinction between a mailing list and an alias
for a single mailbox is a bit fuzzy, since a common data structure
may hold both types of entries, and it is possible to have mailing
lists of one mailbox. If a request is made to verify a mailing
list a positive response can be given if on receipt of a message
so addressed it will be delivered to everyone on the list,
otherwise an error should be reported (e.g., "550 That is a
mailing list, not a user"). If a request is made to expand a user
name a positive response can be formed by returning a list
containing one name, or an error can be reported (e.g., "550 That
is a user name, not a mailing list").
In the case of a multiline reply (normal for EXPN) exactly one
mailbox is to be specified on each line of the reply. In the case
of an ambiguous request, for example, "VRFY Smith", where there
are two Smith's the response must be "553 User ambiguous".
The case of verifying a user name is straightforward as shown in
example 3.
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-------------------------------------------------------------
Example of Verifying a User Name
Either
S: VRFY Smith
R: 250 Fred Smith
Or
S: VRFY Smith
R: 251 User not local; will forward to
Or
S: VRFY Jones
R: 550 String does not match anything.
Or
S: VRFY Jones
R: 551 User not local; please try
Or
S: VRFY Gourzenkyinplatz
R: 553 User ambiguous.
Example 3
-------------------------------------------------------------
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The case of expanding a mailbox list requires a multiline reply as
shown in example 4.
-------------------------------------------------------------
Example of Expanding a Mailing List
Either
S: EXPN Example-People
R: 250-Jon Postel
R: 250-Fred Fonebone
R: 250-Sam Q. Smith
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-
R: 250
Or
S: EXPN Executive-Washroom-List
R: 550 Access Denied to You.
Example 4
-------------------------------------------------------------
The character string arguments of the VRFY and EXPN commands
cannot be further restricted due to the variety of implementations
of the user name and mailbox list concepts. On some systems it
may be appropriate for the argument of the EXPN command to be a
file name for a file containing a mailing list, but again there is
a variety of file naming conventions in the Internet.
The VRFY and EXPN commands are not included in the minimum
implementation (Section 4.5.1), and are not required to work
across relays when they are implemented.
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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 FROM:
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 FROM:
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 FROM:
The Send And MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. In any case the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered the
mailbox.
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The same reply codes that are used for the MAIL commands are used
for these commands.
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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
QUIT
In the HELO command the host sending the command identifies
itself; the command may be interpreted as saying "Hello, I am
".
-------------------------------------------------------------
Example of Connection Opening
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
Example 5
-------------------------------------------------------------
-------------------------------------------------------------
Example of Connection Closing
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Example 6
-------------------------------------------------------------
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3.6. RELAYING
The forward-path may be a source route of the form
"@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. This
form is used to emphasize the distinction between an address and a
route. The mailbox is an absolute address, and the route is
information about how to get there. The two concepts should not
be confused.
Conceptually the elements of the forward-path are moved to the
reverse-path as the message is relayed from one server-SMTP to
another. The reverse-path is a reverse source route, (i.e., a
source route from the current location of the message to the
originator of the message). When a server-SMTP deletes its
identifier from the forward-path and inserts it into the
reverse-path, it must use the name it is known by in the
environment it is sending into, not the environment the mail came
from, in case the server-SMTP is known by different names in
different environments.
If when the message arrives at an SMTP the first element of the
forward-path is not the identifier of that SMTP the element is not
deleted from the forward-path and is used to determine the next
SMTP to send the message to. In any case, the SMTP adds its own
identifier to the reverse-path.
Using source routing the receiver-SMTP receives mail to be relayed
to another server-SMTP The receiver-SMTP may accept or reject the
task of relaying the mail in the same way it accepts or rejects
mail for a local user. The receiver-SMTP transforms the command
arguments by moving its own identifier from the forward-path to
the beginning of the reverse-path. The receiver-SMTP then becomes
a sender-SMTP, establishes a transmission channel to the next SMTP
in the forward-path, and sends it the mail.
The first host in the reverse-path should be the host sending the
SMTP commands, and the first host in the forward-path should be
the host receiving the SMTP commands.
Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That
is, there is no need for these paths and especially this syntax to
appear in the "To:" , "From:", "CC:", etc. fields of the message
header.
If a server-SMTP has accepted the task of relaying the mail and
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Simple Mail Transfer Protocol
later finds that the forward-path is incorrect or that the mail
cannot be delivered for whatever reason, then it must construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the
reverse-path).
This notification message must be from the server-SMTP at this
host. Of course, server-SMTPs should not send notification
messages about problems with notification messages. One way to
prevent loops in error reporting is to specify a null reverse-path
in the MAIL command of a notification message. When such a
message is relayed it is permissible to leave the reverse-path
null. A MAIL command with a null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
to HOSTZ. What we see in the example is the transaction between
HOSTY and HOSTX, which is the first step in the return of the
notification message.
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-------------------------------------------------------------
Example Undeliverable Mail Notification Message
S: MAIL FROM:<>
R: 250 ok
S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
R: 250 ok
S: DATA
R: 354 send the mail data, end with .
S: Date: 23 Oct 81 11:22:33
S: From: SMTP@HOSTY.ARPA
S: To: JOE@HOSTW.ARPA
S: Subject: Mail System Problem
S:
S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
S: HOSTZ.ARPA said this:
S: "550 No Such User"
S: .
R: 250 ok
Example 7
-------------------------------------------------------------
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3.7. DOMAINS
Domains are a recently introduced concept in the ARPA Internet
mail system. The use of domains changes the address space from a
flat global space of simple character string host names to a
hierarchically structured rooted tree of global addresses. The
host name is replaced by a domain and host designator which is a
sequence of domain element strings separated by periods with the
understanding that the domain elements are ordered from the most
specific to the most general.
For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
"PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
Whenever domain names are used in SMTP only the official names are
used, the use of nicknames or aliases is not allowed.
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3.8. CHANGING ROLES
The TURN command may be used to reverse the roles of the two
programs communicating over the transmission channel.
If program-A is currently the sender-SMTP and it sends the TURN
command and receives an ok reply (250) then program-A becomes the
receiver-SMTP.
If program-B is currently the receiver-SMTP and it receives the
TURN command and sends an ok reply (250) then program-B becomes
the sender-SMTP.
To refuse to change roles the receiver sends the 502 reply.
Please note that this command is optional. It would not normally
be used in situations where the transmission channel is TCP.
However, when the cost of establishing the transmission channel is
high, this command may be quite useful. For example, this command
may be useful in supporting be mail exchange using the public
switched telephone system as a transmission channel, especially if
some hosts poll other hosts for mail exchanges.
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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 . The command codes themselves are
alphabetic characters terminated by if parameters follow
and otherwise. The syntax of mailboxes must conform to
receiver site conventions. The SMTP commands are discussed
below. The SMTP replies are discussed in the Section 4.2.
A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the argument of the MAIL command, the
forward-path is the argument of the RCPT command, and the mail
data is the argument of the DATA command. These arguments or
data objects must be transmitted and held pending the
confirmation communicated by the end of mail data indication
which finalizes the transaction. The model for this is that
distinct buffers are provided to hold the types of data
objects, that is, there is a reverse-path buffer, a
forward-path buffer, and a mail data buffer. Specific commands
cause information to be appended to a specific buffer, or cause
one or more buffers to be cleared.
HELLO (HELO)
This command is used to identify the sender-SMTP to the
receiver-SMTP. The argument field contains the host name of
the sender-SMTP.
The receiver-SMTP identifies itself to the sender-SMTP in
the connection greeting reply, and in the response to this
command.
This command and an OK reply to it confirm that both the
sender-SMTP and the receiver-SMTP are in the initial state,
that is, there is no transaction in progress and all state
tables and buffers are cleared.
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MAIL (MAIL)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more mailboxes. The
argument field contains a reverse-path.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different). In some types of error
reporting messages (for example, undeliverable mail
notifications) the reverse-path may be null (see Example 7).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
RECIPIENT (RCPT)
This command is used to identify an individual recipient of
the mail data; multiple recipients are specified by multiple
use of this command.
The forward-path consists of an optional list of hosts and a
required destination mailbox. When the list of hosts is
present, it is a source route and indicates that the mail
must be relayed to the next host on the list. If the
receiver-SMTP does not implement the relay function it may
user the same reply it would for an unknown local user
(550).
When mail is relayed, the relay host must remove itself from
the beginning forward-path and put itself at the beginning
of the reverse-path. When mail reaches its ultimate
destination (the forward-path contains only a destination
mailbox), the receiver-SMTP inserts it into the destination
mailbox in accordance with its host mail conventions.
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For example, mail received at relay host A with arguments
FROM:
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 "." (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
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Simple Mail Transfer Protocol
return path line. The return path line preserves the
information in the 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 . 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
MAIL FROM:
RCPT TO:
DATA
RSET
SEND FROM:
SOML FROM:
SAML FROM:
VRFY
EXPN
HELP [ ]
NOOP
QUIT
TURN
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.
::=
::=
::= "<" [ ":" ] ">"
::= | ","
::= "@"
::= | "."
::= | "#" | "[" "]"
::= "@"
::= |
::=
::= |
::= |
::= | | "-"
::= | "."
::= |
::= """ """
::= "\" | "\" | |
::= | "\"
::= "." "." "."
::= |
::=
[Page 30] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
::= the carriage return character (ASCII code 13)
::= the line feed character (ASCII code 10)
::= the space character (ASCII code 32)
::= one, two, or three digits representing a decimal
integer value in the range 0 through 255
::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
::= any one of the 128 ASCII characters, but not any
or
::= any one of the ten digits 0 through 9
::= any one of the 128 ASCII characters except ,
, quote ("), or backslash (\)
::= any one of the 128 ASCII characters (no exceptions)
::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | 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:"
::= "Received:"
::= ";"
::= "FROM"
::= "BY"
::= [] [] [] []
::= "VIA"
::= "WITH"
::= "ID"
::= "FOR"
::= The standard names for links are registered with
the Network Information Center.
::= The standard names for protocols are
registered with the Network Information Center.
::=
::=
::= ":" ":"
::= the one or two decimal integer day of the month in the
range 1 to 31. ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC" ::= 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 ::= the two
decimal integer hour of the day in the range 00 to 24. ::= the two
decimal integer minute of the hour in the range 00 to 59. ::= the
two decimal integer second of the minute in the range 00 to 59. ::=
"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, , one line of text, and , 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 Service ready 221 Service closing transmission
channel 421 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 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 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 .
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 Service ready 221 Service closing transmission channel 250
Requested mail action okay, completed 251 User not local; will
forward to 354 Start mail input; end with . 421 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
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
USC-ISIF.ARPA Service ready 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 "." 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 is 512 characters. reply line The maximum
total length of a reply line including the reply code and the 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 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 , optionally some text, and
. 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 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: R: 250 OK S: RCPT
TO: R: 250 OK S: RCPT TO: R: 550 No such user here S: RCPT TO: R:
250 OK S: DATA R: 354 Start mail input; end with . 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: R: 250 OK S:
RCPT TO: R: 250 OK S: RCPT TO: 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: 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 . S: Date: 2 Nov 81 22:33:44 S:
From: John Q. Public 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: R: 250 OK S: DATA R: 354 Start mail input; end with . 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 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 S: SEND FROM: R: 250 OK S: RCPT TO: R: 250 OK S: DATA R:
354 Start mail input; end with . 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 S: SEND FROM: R: 250 OK S: RCPT TO: R: 450 User not active
now S: RSET R: 250 OK S: MAIL FROM: R: 250 OK S: RCPT TO: R: 250 OK
S: DATA R: 354 Start mail input; end with . 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 S: SOML FROM: R: 250 OK S: RCPT TO: R: 250 User not active
now, so will do mail. S: DATA R: 354 Start mail input; end with .
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- R: 250-Fred Fonebone R: 250-Xenon Y.
Zither R: 250-Quincy Smith
<@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250- R: 250 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 R: 250- R: 250-Quincy Smith
<@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250- R: 250 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: 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 . 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: R: 250 OK S: RCPT
TO: R: 251 User not local; will forward to S: DATA R: 354 Start
mail input; end with . 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: R: 250 OK S: RCPT TO: R: 251 User not
local; will forward to 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: R: 250 OK S: RCPT TO: R: OK S: DATA R:
354 Start mail input; end with . 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: R: 250 OK S: RCPT
TO: R: 250 OK S: RCPT TO: R: 552 Recipient storage full, try again
in another transaction S: DATA R: 354 Start mail input; end with .
S: Blah blah blah... S: ...etc. etc. etc. S: . R: 250 OK S: MAIL
FROM: R: 250 OK S: RCPT TO: R: 250 OK S: DATA R: 354 Start mail
input; end with . 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 . 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.
The characters carriage return and line feed (in that order). 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