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RFC 821 August 1982
Simple Mail Transfer Protocol
TABLE OF CONTENTS
1. INTRODUCTION
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| 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
RFC 821 August 1982
Simple Mail Transfer Protocol
two host are connected to the same transport service, or via one or
more relay SMTP-servers when the source and destination hosts are not
connected to the same transport service.
To be able to provide the relay capability the SMTP-server must be
supplied with the name of the ultimate destination host as well as
the destination mailbox name.
The argument to the MAIL command is a reverse-path, which specifies
who the mail is from. The argument to the RCPT command is a
forward-path, which specifies who the mail is to. The forward-path
is a source route, while the reverse-path is a return route (which
may be used to return a message to the sender when an error occurs
with a relayed message).
When the same message is sent to multiple recipients the SMTP
encourages the transmission of only one copy of the data for all the
recipients at the same destination host.
The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears in Section 4 on specifications.
Commands and replies are not case sensitive. That is, a command or
reply word may be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive, and SMTP implementations
must take case to preserve the case of user names as they appear in
mailbox arguments. Host names are not case sensitive.
Commands and replies are composed of characters from the ASCII
character set [1]. When the transport service provides an 8-bit byte
(octet) transmission channel, each 7-bit character is transmitted
right justified in an octet with the high order bit cleared to zero.
When specifying the general form of a command or reply, an argument
(or special symbol) will be denoted by a meta-linguistic variable (or
constant), for example, "<string>" or "<reverse-path>". Here the
angle brackets indicate these are meta-linguistic variables.
However, some arguments use the angle brackets literally. For
example, an actual reverse-path is enclosed in angle brackets, i.e.,
"<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
angle brackets are actually transmitted in the command or reply).
<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.
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.
-------------------------------------------------------------
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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RFC 821 August 1982
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of Verifying a User Name
Either
S: VRFY Smith
R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
Or
S: VRFY Smith
R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
Or
S: VRFY Jones
R: 550 String does not match anything.
Or
S: VRFY Jones
R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
Or
S: VRFY Gourzenkyinplatz
R: 553 User ambiguous.
Example 3
-------------------------------------------------------------
-------------------------------------------------------------
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.
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.
RFC 821 August 1982
Simple Mail Transfer Protocol
3.5. OPENING AND CLOSING
At the time the transmission channel is opened there is an
exchange to ensure that the hosts are communicating with the hosts
they think they are.
The following two commands are used in transmission channel
opening and closing:
HELO <SP> <domain> <CRLF>
QUIT <CRLF>
In the HELO command the host sending the command identifies
itself; the command may be interpreted as saying "Hello, I am
<domain>".
-------------------------------------------------------------
Example of Connection Opening
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
Example 5
-------------------------------------------------------------
-------------------------------------------------------------
Example of Connection Closing
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Example 6
-------------------------------------------------------------
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.
-------------------------------------------------------------
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.
RFC 821 August 1982
Simple Mail Transfer Protocol
RFC 821 August 1982
Simple Mail Transfer Protocol
For example, mail received at relay host A with arguments
FROM:<USERX@HOSTY.ARPA>
TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>
will be relayed on to host B with arguments
FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.
This command causes its forward-path argument to be appended
to the forward-path buffer.
DATA (DATA)
The receiver treats the lines following the command as mail
data from the sender. This command causes the mail data
from this command to be appended to the mail data buffer.
The mail data may contain any of the 128 ASCII character
codes.
The mail data is terminated by a line containing only a
period, that is the character sequence "<CRLF>.<CRLF>" (see
Section 4.5.2 on Transparency). This is the end of mail
data indication.
The end of mail data indication requires that the receiver
must now process the stored mail transaction information.
This processing consumes the information in the reverse-path
buffer, the forward-path buffer, and the mail data buffer,
and on the completion of this command these buffers are
cleared. If the processing is successful the receiver must
send an OK reply. If the processing fails completely the
receiver must send a failure reply.
When the receiver-SMTP accepts a message either for relaying
or for final delivery it inserts at the beginning of the
mail data a time stamp line. The time stamp line indicates
the identity of the host that sent the message, and the
identity of the host that received the message (and is
inserting this time stamp), and the date and time the
message was received. Relayed messages will have multiple
time stamp lines.
When the receiver-SMTP makes the "final delivery" of a
message it inserts at the beginning of the mail data a
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
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.
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".
RFC 821 August 1982
Simple Mail Transfer Protocol
The following are the SMTP commands:
HELO <SP> <domain> <CRLF>
MAIL <SP> FROM:<reverse-path> <CRLF>
RCPT <SP> TO:<forward-path> <CRLF>
DATA <CRLF>
RSET <CRLF>
SEND <SP> FROM:<reverse-path> <CRLF>
SOML <SP> FROM:<reverse-path> <CRLF>
SAML <SP> FROM:<reverse-path> <CRLF>
VRFY <SP> <string> <CRLF>
EXPN <SP> <string> <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
QUIT <CRLF>
TURN <CRLF>
<reverse-path> ::= <path>
<forward-path> ::= <path>
<path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
<a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
<at-domain> ::= "@" <domain>
<domain> ::= <element> | <element> "." <domain>
<element> ::= <name> | "#" <number> | "[" <dotnum> "]"
<mailbox> ::= <local-part> "@" <domain>
<local-part> ::= <dot-string> | <quoted-string>
<name> ::= <a> <ldh-str> <let-dig>
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig> ::= <a> | <d>
<let-dig-hyp> ::= <a> | <d> | "-"
<dot-string> ::= <string> | <string> "." <dot-string>
<string> ::= <char> | <char> <string>
<quoted-string> ::= """ <qtext> """
<qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
<char> ::= <c> | "\" <x>
<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
<number> ::= <d> | <d> <number>
<CRLF> ::= <CR> <LF>
RFC 821 August 1982
Simple Mail Transfer Protocol
<CR> ::= the carriage return character (ASCII code 13)
<LF> ::= the line feed character (ASCII code 10)
<SP> ::= the space character (ASCII code 32)
<snum> ::= one, two, or three digits representing a decimal
integer value in the range 0 through 255
<a> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
<c> ::= any one of the 128 ASCII characters, but not any
<special> or <SP>
<d> ::= any one of the ten digits 0 through 9
<q> ::= any one of the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)
<x> ::= any one of the 128 ASCII characters (no exceptions)
<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | the control
characters (ASCII codes 0 through 31 inclusive and
127)
Note that the backslash, "\", is a quote character, which is
used to indicate that the next character is to be used
literally (instead of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with comma being the fourth character of the field.
Hosts are generally known by names which are translated to
addresses in each host. Note that the name elements of domains
are the official names -- no use of nicknames or aliases is
allowed.
Sometimes a host is not known to the translation function and
communication is blocked. To bypass this barrier two numeric
forms are also allowed for host "names". One form is a decimal
integer prefixed by a pound sign, "#", which indicates the
number is the address of the host. Another form is four small
decimal integers separated by dots and enclosed by brackets,
e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
Address in four 8-bit fields.
<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
<stamp> ::= <from-domain> <by-domain> <opt-info> ";"
<daytime>
<from-domain> ::= "FROM" <SP> <domain> <SP>
<by-domain> ::= "BY" <SP> <domain> <SP>
<opt-info> ::= [<via>] [<with>] [<id>] [<for>]
<via> ::= "VIA" <SP> <link> <SP>
<with> ::= "WITH" <SP> <protocol> <SP>
<id> ::= "ID" <SP> <string> <SP>
<for> ::= "FOR" <SP> <path> <SP>
<link> ::= The standard names for links are registered with
the Network Information Center.
<protocol> ::= The standard names for protocols are
registered with the Network Information Center.
<daytime> ::= <SP> <date> <SP> <time>
<date> ::= <dd> <SP> <mon> <SP> <yy>
<time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
<dd> ::= the one or two decimal integer day of the month in
the range 1 to 31.
<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
<yy> ::= the two decimal integer year of the century in the
range 00 to 99.
RFC 821 August 1982
Simple Mail Transfer Protocol
<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.
<zone> ::= "UT" for Universal Time (the default) or other
time zone designator (as in [2]).
-------------------------------------------------------------
Return Path Example
Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
Example 9
-------------------------------------------------------------
-------------------------------------------------------------
Time Stamp Line Example
Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
Example 10
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RFC 821 August 1982
Simple Mail Transfer Protocol
4.2.1. REPLY CODES BY FUNCTION GROUPS
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed
RFC 821 August 1982
Simple Mail Transfer Protocol
4.3. SEQUENCING OF COMMANDS AND REPLIES
The communication between the sender and receiver is intended to
be an alternating dialogue, controlled by the sender. As such,
the sender issues a command and the receiver responds with a
reply. The sender must wait for this response before sending
further commands.
One important reply is the connection greeting. Normally, a
receiver will send a 220 "Service ready" reply when the connection
is completed. The sender should wait for this greeting message
before sending any commands.
Note: all the greeting type replies have the official name of
the server host as the first word following the reply code.
For example,
220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
The table below lists alternative success and failure replies for
each command. These must be strictly adhered to; a receiver may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.
COMMAND-REPLY SEQUENCES
Each command is listed with its possible replies. The prefixes
used before the possible replies are "P" for preliminary (not
used in SMTP), "I" for intermediate, "S" for success, "F" for
failure, and "E" for error. The 421 reply (service not
available, closing transmission channel) may be given to any
command if the SMTP-receiver knows it must shut down. This
listing forms the basis for the State Diagrams in Section 4.4.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
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
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.
+---+ 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.
RFC 821 August 1982
Simple Mail Transfer Protocol
4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.
1. Before sending a line of mail text the sender-SMTP checks
the first character of the line. If it is a period, one
additional period is inserted at the beginning of the line.
2. When a line of mail text is received by the receiver-SMTP
it checks the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.
The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipient's mailbox
including format effectors and other control characters. If
the transmission channel provides an 8-bit byte (octets) data
stream, the 7-bit ASCII codes are transmitted right justified
in the octets with the high order bits cleared to zero.
In some systems it may be necessary to transform the data as
it is received and stored. This may be necessary for hosts
that use a different character set than ASCII as their local
character set, or that store data in records rather than
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
user
The maximum total length of a user name is 64 characters.
domain
The maximum total length of a domain name or number is 64
characters.
path
The maximum total length of a reverse-path or
forward-path is 256 characters (including the punctuation
and element separators).
command line
The maximum total length of a command line including the
command word and the <CRLF> is 512 characters.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters.
RFC 821 August 1982
Simple Mail Transfer Protocol
text line
The maximum total length of a text line including the
<CRLF> is 1000 characters (but not counting the leading
dot duplicated for transparency).
recipients buffer
The maximum total number of recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
Errors due to exceeding these limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
RFC 821 August 1982
Simple Mail Transfer Protocol
RFC 821 August 1982
Simple Mail Transfer Protocol
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
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
RFC 821 August 1982
Simple Mail Transfer Protocol
-------------------------------------------------------------
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
S: MAIL FROM:<Smith@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<Green@BBN-UNIX.ARPA>
R: 550 No such user here
S: RCPT TO:<Brown@BBN-UNIX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Scenario 1
-------------------------------------------------------------
-------------------------------------------------------------
R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
S: HELO ISI-VAXA.ARPA
R: 250 MIT-Multics.ARPA
S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<Jones@MIT-Multics.ARPA>
R: 250 OK
S: RCPT TO:<Green@MIT-Multics.ARPA>
R: 550 No such user here
S: RSET
R: 250 OK
S: QUIT
R: 221 MIT-Multics.ARPA Service closing transmission channel
Scenario 2
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RFC 821 August 1982
Simple Mail Transfer Protocol
Relayed Mail Scenario
-------------------------------------------------------------
Step 1 -- Source Host to Relay Host
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-AI.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<JQP@MIT-AI.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIE.ARPA
R: 250 BBN-VAX.ARPA
S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-VAX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
2 Nov 81 22:40:10 UT
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 3
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RFC 821 August 1982
Simple Mail Transfer Protocol
Verifying and Sending Scenario
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 4
-------------------------------------------------------------
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 450 User not active now
S: RSET
R: 250 OK
S: MAIL FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 5
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RFC 821 August 1982
Simple Mail Transfer Protocol
Doing the preceding scenario more efficiently.
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SOML FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 User not active now, so will do mail.
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 6
-------------------------------------------------------------
-------------------------------------------------------------
Step 1 -- Expanding the First List
R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-AI.ARPA
S: EXPN Example-People
R: 250-<ABC@MIT-MC.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-AI.ARPA Service closing transmission channel
RFC 821 August 1982
Simple Mail Transfer Protocol
Step 2 -- Expanding the Second List
R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-MC.ARPA
S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC.ARPA>
R: 250-<XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<fred@BBN-UNIX.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-MC.ARPA Service closing transmission channel
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
R: 250 OK
S: RCPT
TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 7
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RFC 821 August 1982
Simple Mail Transfer Protocol
Forwarding Scenarios
-------------------------------------------------------------
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF.ARPA>
R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
Scenario 8
-------------------------------------------------------------
-------------------------------------------------------------
Step 1 -- Trying the Mailbox at the First Host
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF.ARPA>
R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
S: RSET
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
Step 2 -- Delivering the Mail at the Second Host
R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISI.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<Jones@USC-ISI.ARPA>
R: OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISI.ARPA Service closing transmission channel
Scenario 9
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RFC 821 August 1982
Simple Mail Transfer Protocol
Too Many Recipients Scenario
-------------------------------------------------------------
R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BERKELEY.ARPA
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<fabry@BERKELEY.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY.ARPA Service closing transmission channel
Scenario 10
-------------------------------------------------------------
Note that a real implementation must handle many recipients as
specified in Section 4.5.3.
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.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
RFC 821 August 1982
Simple Mail Transfer Protocol