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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.

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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.


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August 1982 RFC 821
Simple Mail Transfer Protocol

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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:<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

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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

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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.


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

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

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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.

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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.

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Simple Mail Transfer Protocol

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

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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

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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

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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.

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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

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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.

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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.

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