RFC772 - Mail Transfer Protocol

Network Working Group S. Sluizer

Request for Comments: 772 J. Postel

ISI

September 1980

MAIL TRANSFER PROTOCOL

PREFACE

This is a first draft of this protocol and comments are very

definitely requested.

INTRODUCTION

The objective of Mail Transfer Protocol (MTP) is to transfer mail

reliably and efficiently.

This paper assumes knowledge of the following protocols described in

the ARPA Internet Protocol Handbook. The reader will note strong

similarities to portions of the File Transfer Protocol; in part, this

is due to the original ARPA Network implementation of computer mail

as a feature of FTP.

The ARPANET Host-to-Host Protocol [Network Control Protocol] (NCP)

The Transmission Control Protocol (TCP)

The TELNET Protocol (TELNET)

The File Transfer Protocol (FTP)

DISCUSSION

In this section, the terminology and the MTP model are discussed.

The terms defined in this section are only those that have special

significance in MTP. Some of the terminology is very specific to the

MTP model; some readers may wish to turn to the section on the MTP

model while reviewing the terminology.

TERMINOLOGY

ASCII

The ASCII character set as defined in the ARPA Internet

Protocol Handbook. In MTP, ASCII characters are defined to be

the lower half of an eight-bit code set (i.e., the most

significant bit is zero) and is called NVT-ASCII.

September 1980 RFC772

Mail Transfer Protocol

control connection

The TCP full-duplex communication path or two NCP simplex

communication paths between a sender-MTP and a receiver-MTP for

the exchange of commands, replies, and mail text. The control

connection operates according to the TELNET Protocol.

data mode

The mail is transmitted over the control connection as a stream

of octets. (In FTP terminology this is called stream mode.)

data structure

The internal structure of mail is considered to be a continuous

sequence of data octets. (In FTP terminology this is called

file-structure.)

data representation

The internal representation of all data (i.e., mail) is in

NVT-ASCII.

host

A computer in the internetwork environment on which mailboxes

reside.

MTP commands

A set of commands which comprise the control information

flowing from the sender-MTP to the receiver-MTP.

mail

An ordered set of computer data of arbitrary length, which

conforms to the standard set in RFC733 (Standard for the

Format of ARPA Network Text Messages).

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@host". Additionally, the "container" in

which mail is stored.

RFC772 September 1980

Mail Transfer Protocol

NVT

The Network Virtual Terminal as defined in the TELNET Protocol.

octet

Bytes in MTP are octets (8 bits). This is not necessarily the

same byte size in which data is stored in a host.

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

receiver to sender via the control connection in response to a

MTP 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.

receiver-MTP process

A process which transfers mail in cooperation with a sender-MTP

process. It "listens" on its port/socket L for a connection

from a sender-MTP and establishes a control connection using

the TELNET Protocol. It receives MTP commands from the

sender-MTP, sends replies, and governs the transfer of mail.

sender-MTP process

A process which transfers mail in cooperation with a

receiver-MTP process. A local language may be used in the user

interface command/reply dialogue. The sender-MTP initiates the

control connection from its port/socket U to the receiver-MTP

process. It initiates MTP commands, receives replies, and

governs the transfer of mail.

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.

September 1980 RFC772

Mail Transfer Protocol

THE MTP MODEL

With the above definitions in mind, the following model (shown in

Figure 1) may be diagrammed for an MTP service.

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

--------

MTP <--> User

Receiver-Commands/Replies Sender- --------

-------- MTP <--------------> MTP --------

Mail <--> Mail <--> Mail

System System

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

Receiver-MTP Sender-MTP

Model for MTP Use

Figure 1

In the model described in Figure 1, the sender-MTP initiates the

TCP/NCP control connection which follows the TELNET Protocol. At

the initiation of the user, standard MTP commands are generated by

the sender-MTP and transmitted to the receiver-MTP via the control

connection. Standard replies are sent from the receiver-MTP to

the sender-MTP over the control connection in response to the

commands. In addition, mail is sent over the control connection.

MAIL TRANSFER FUNCTIONS

The control connection is used for the transfer of commands which

describe the functions to be performed, the replies to commands, as

well as the actual transfer of mail. Mail is transferred only via

the control connection.

The communication channel from the sender-MTP to the receiver-MTP is

established by a TCP/NCP control connection from the sender to a

standard receiver port/socket. The sender-MTP is responsible for

sending MTP commands, interpreting the replies received, and sending

the mail; the receiver-MTP interprets commands, sends replies, and

receives the mail.

RFC772 September 1980

Mail Transfer Protocol

MAIL REPRESENTATION AND STORAGE

Mail is transferred from a storage device in the sending host to a

storage device in the receiving host. It may be necessary to

perform certain transformations on the mail because data storage

representations in the two systems are different. For example,

NVT-ASCII has different data storage representations in different

systems. PDP-10's generally store NVT-ASCII as five 7-bit ASCII

characters, left-justified in a 36-bit Word. 360's store

NVT-ASCII as four 8-bit EBCDIC codes in a 32-bit word. Multics

stores NVT-ASCII as four 9-bit characters in a 36-bit word.

For the sake of simplicity, all data must be represented in MTP as

NVT-ASCII. This means that characters must be converted into the

standard NVT-ASCII representation when transmitting text,

regardless of whether the sending and receiving hosts are

dissimilar. The sender converts the data from its internal

character representation to the standard 8-bit NVT-ASCII

representation (see the TELNET specification). The receiver

converts the data from the standard form to its own internal form.

In accordance with this standard, the <CRLF> sequence should be

used to denote the end of a line of text.

The mail in MTP has no internal structure and is considered to be

a continuous sequence of data octets.

ERROR RECOVERY AND RESTART

There is no provision for detecting bits lost or scrambled in data

transfer; this level of error control is handled by the TCP/NCP.

In addition, there is no restart procedure provided to protect

senders from gross system failures (including failures of a host,

an MTP-process, or the underlying network).

MTP COMMANDS

COMMAND SEMANTICS

The MTP commands define the mail transfer or the mail system

function requested by the user. The syntax of mailboxes must

conform to receiver site conventions (with standard defaults

applicable). In response to an MTP transfer command, the mail

shall always be transferred over the control connection.

The Mail Transfer Protocol follows the specifications of the

TELNET Protocol for all communications over the control

September 1980 RFC772

Mail Transfer Protocol

connection. Although the language used for TELNET communication

can be a negotiated option, the "TELNET language" and the

corresponding "TELNET end of line code" are required to be

NVT-ASCII and <CRLF> respectively. No other specifications of the

TELNET Protocol will be cited.

MTP commands are NVT-ASCII strings terminated by <CRLF>. The

command codes themselves are alphabetic characters terminated by

the character <SP> (space) if parameters follow and <CRLF>

otherwise.

The MTP commands are discussed below. In the description of a few

of the commands in this section the possible replies are given

eXPlicitly. MTP replies are discussed in the next section.

MAIL (MAIL)

This command allows a sender-MTP to send mail over the

control connection. The argument field contains a sender

and optional path sequence. If the path sequence is

present, it consists of an optional list of hosts and a

destination mailbox. When the list of hosts is present, it

is source routing information and indicates that the mail

must be forwarded to the first host on the list. Following

this command line the receiver treats all subsequent

characters as mail text from the sender. The mail text is

terminated by the character sequence "CRLF.CRLF".

As mail is forwarded along the path sequence, each

forwarding host must remove itself from the list. When mail

reaches its ultimate destination (the path sequence has only

a (possibly empty) destination mailbox), the receiver

inserts it into the destination mailbox in accordance with

its host mail conventions. If the second argument field is

blank (one or more spaces) or empty (<CRLF>), the mail is

destined for a printer or other designated place for site

general delivery mail. The mail may be marked as sent from

the sender as specified by the first argument field.

MAIL RECIPIENT SCHEME QUESTION (MRSQ)

This MTP command is used to select a scheme for the

transmission of mail to several users at the same host. The

schemes are to list the recipients first, or to send the

mail first.

RFC772 September 1980

Mail Transfer Protocol

MAIL RECIPIENT (MRCP)

This command is used to identify the individual recipients

of the mail in the transmission of mail for multiple users

at one host.

HELP (HELP)

This command causes the receiver to send helpful information

regarding its implementation status over the control

connection to the receiver. The command may take an

argument (e.g., any command name) and return more specific

information as a response. The reply is type 211 or 214.

QUIT (QUIT)

This command specifies that the receiver must close the

control connection.

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.

COMMAND SYNTAX

The commands (and their functions and semantics) are TELNET

NVT-ASCII strings transmitted over the control connection. The

functions and semantics of commands are described in the section

on MTP Commands. The reply sequences are discussed in the section

on Sequencing of Commands and Replies. Scenarios illustrating the

use of commands are provided in the section on Typical MTP

Scenarios. The command syntax is specified in this section.

The commands begin with a command code followed by an argument

field. The 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 R or r for RECIPIENT first. The command codes and the

argument fields are separated by one or more spaces.

September 1980 RFC772

Mail Transfer Protocol

The argument field consists of a variable length character string

ending with the character sequence <CRLF>. It should be noted

that the receiver is to take no action until the end of line code

is received.

The syntax is specified below in NVT-ASCII. All characters in the

argument field are ASCII characters. Square brackets denote an

optional argument field. If the option is not taken, the

appropriate default is implied.

The following are the MTP commands:

MAIL <SP> FROM:<sender> [<SP> TO:<path>] <CRLF>

MRSQ [<SP> <scheme>] <CRLF>

MRCP <SP> TO:<path> <CRLF>

HELP [<SP> <string>] <CRLF>

QUIT <CRLF>

NOOP <CRLF>

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.

<sender> ::= "<" <mailbox> ">"

<path> ::= "<" ["@" <host> "," ...] <mailbox> ">"

<scheme> ::= "R" "T" "?"

<char> ::= any of the 128 ASCII characters except <CR> and <LF>

RFC772 September 1980

Mail Transfer Protocol

CONTROL FUNCTIONS

Most time-sharing systems provide mechanisms to allow a terminal

user to regain control of a "runaway" process. When used locally,

such systems have Access to all user-supplied signals, whether

these are normal characters or special "out of band" signals.

When terminals are connected to the system through the network,

the system does not necessarily have access to all user signals;

the network's flow control mechanisms may cause such signals to be

buffered elsewhere, for example in the user's host.

To counter this problem, the TELNET "Synch" mechanism is used. A

Synch signal consists of a TCP Urgent or an NCP Interrupt

notification, coupled with the TELNET command DATA MARK (DM).

This notification, which is not subject to the flow control

pertaining to the TELNET connection, is used to invoke special

handling of the data stream by the process which receives it. In

this mode the data stream is immediately scanned for a TELNET

Interrupt Process (IP) command. (The rationale for the use of the

TELNET IP command is to allow an existing server TELNET module to

sit "under" the MTP. If this code were directly implemented in

the MTP the IP command would be unnecessary.) The TELNET command

DM is the synchronizing mark in the data stream which indicates

that any special signal has already occurred and the recipient can

return to normal processing of the data stream. For a more

complete understanding of this mechanism, see the TELNET Protocol

Specification in the Internet Protocol Handbook.

The effect of this mechanism is to to discard all characters (up

to the DM) between the sender of the Synch and its recipeint.

Thus, all characters in the control connection are ignored until

the TELNET command DM is received. The full sequence is

illustrated below. Each vertical bar () represents the boundary

between data octets; IAC refers to the TELNET command code

Interpret As Command.

Old New

-+-+-+-+-+-----+---+--+---+--+-

...MAIL ... IACIPIACDM...

-+-+-+-+-+-----+---+--+---+--+-

September 1980 RFC772

Mail Transfer Protocol

MTP REPLIES

Replies to Mail Transfer Protocol commands are devised to ensure the

synchronization of requests and actions in the process of mail

transfer, and to guarantee that the sender-MTP always knows the state

of the receiver. Every command must generate at least one reply,

although there may be more than one. In the latter case, the

multiple replies must be easily distinguished. Additionally, some

commands must occur sequentially, such as MRSQ T->MAIL->MRCP or

MRSQ R->MRCP->MAIL. Replies to these sequences show the existence of

an intermediate state if all preceding commands have been successful.

A failure at any point in the sequence necessitates the repetition of

the entire sequence from the beginning.

The details of the command-reply sequence are made explicit in the

section on State Diagrams.

An MTP 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-MTP will

not need to 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, so there are likely to be varying texts for each

reply code.

Formally, a reply is defined to be the sequence: a three-digit code,

space <SP>, one line of text (where the maximum line length is 65),

and a terminal <CRLF>. Occasionally the text is longer than a single

line; in these cases the complete text must be bracketed so the

sender-MTP knows when it can stop reading the reply. This requires a

special first line format to indicate a multiple line reply, and

another on the last line to so designate it. Both lines will contain

the appropriate reply code which indicates the transaction state.

Thus the format for multi-line replies is that the first line will

begin with the exact required reply code, followed immediately by

a Hyphen, "-" (also known as minus), followed by text. The last

line will begin with the same code, followed immediately by space

<SP>, optionally some text, and <CRLF>.

RFC772 September 1980

Mail Transfer Protocol

For example:

123-First line

Second line

234 A line beginning with numbers

123 The last line

The sender-MTP then simply needs to search for the second

occurrence of the same reply code followed by <SP> (space> at the

beginning of a line, and ignore all intermediary lines. If an

intermediary line begins with a three-digit number, the receiver

must pad the front to avoid confusion.

This scheme allows standard system routines to be used for

reply information, with "artificial" first and last lines

tacked on. In the rare cases where these routines are able to

generate three digits and a space at the beginning of any line,

the beginning of each text line should be offset by some

neutral text, like space.

This scheme assumes that multi-line replies may not be nested. In

general, reply nesting will not occur except for random system

messages (also called spontaneous replies) which may interrupt

another reply. System messages (i.e., those not processed by the

receiver-MTP) will NOT carry reply codes and may occur anywhere in

the command-reply sequence. They may be ignored by the sender-MTP

as they are only information for the human user.

The three digits of the reply each have a special significance. This

is intended to allow a range of very simple to very sophisticated

response by the sender-MTP. The first digit denotes whether the

response is good, bad or incomplete. (Referring to the state

diagram) an unsophisticated sender-MTP will be able to determine its

next action (proceed as planned, redo, retrench, etc.) by simply

examining this first digit. A sender-MTP 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 requested action is being initiated; expect another

reply before proceeding with a new command. (The sender-MTP

sending another command before the completion reply would be

September 1980 RFC772

Mail Transfer Protocol

in violation of protocol. However, receiver-MTP processes

should queue any commands that arrive while a preceding

command is in progress.)

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-MTP 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 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- MTPs) must agree on

the interpretation. Each reply in this category might have

a different time value, but the sender-MTP 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; 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-MTP 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-MTP 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

his/her Directory status.)

RFC772 September 1980

Mail Transfer Protocol

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 control

connection.

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 below

will illustrate 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.

A command such as NOOP whose successful execution does not

offer the sender-MTP any new information will return a 200

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.

REPLY CODES BY FUNCTION GROUPS

200 Command okay

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

September 1980 RFC772

Mail Transfer Protocol

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]

215 <scheme> is the preferred scheme

120 <host> Service ready in nnn minutes

220 <host> Service ready for new user

221 <host> Service closing control connection

421 <host> Service not available, closing control connection

[This may be a reply to any command if the service knows it

must shut down]

151 User not local; will forward to <user>@<host>

152 User unknown; mail will be forwarded by the operator

250 Requested mail action okay, completed

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: local error in processing

452 Requested action not taken: insufficient system storage space

552 Requested mail action aborted: exceeded storage allocation

[For current mailbox location]

553 Requested action not taken: mailbox name not allowed

354 Start mail input; end with <CR><LF>.<CR><LF>

NUMERIC ORDER LIST OF REPLY CODES

120 <host> Service ready in nnn minutes

151 User not local; will forward to <user>@<host>

152 User unknown; mail will be forwarded by the operator

200 Command okay

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]

215 <scheme> is the preferred scheme

220 <host> Service ready for new user

221 <host> Service closing control connection

250 Requested mail action okay, completed

354 Start mail input; end with <CR><LF>.<CR><LF>

RFC772 September 1980

Mail Transfer Protocol

421 <host> Service not available, closing control connection

[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 space

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

550 Requested action not taken: mailbox unavailable

[E.g., mailbox not found, no access]

552 Requested mail action aborted: exceeded storage allocation

[For current mailbox location]

553 Requested action not taken: mailbox name not allowed

DISCUSSION OF MAIL TRANSFER

The basic command for transmitting mail is MAIL. This command causes

the transmitted data to be entered into the recipient's mailbox.

MAIL <SP> "FROM:" <sender> [<SP> "TO:" <path>] <CRLF>

<sender> is a mailbox and <path> is a source routing list of

hosts and destination mailbox. If accepted, it returns a 354

reply and considers all succeeding lines to be the message

text. It is terminated by a line containing only a period,

upon which a 250 completion reply is returned. Various errors

are possible.

There are two possible preliminary replies that a receiver may use to

indicate that it is accepting mail for a user whose mailbox is not at

that receiver.

151 User not local; will forward to <user>@<host>

This reply indicates that the receiver knows the user's mailbox

is on another host and will take responsibility for forwarding

the mail to that host. For example, at BBN (or ISI) there are

several hosts. Each has a list of many of the users on the

hosts. Each host can accept mail for any user on their list

and forward it to the correct host.

September 1980 RFC772

Mail Transfer Protocol

152 User Unknown; mail will be forwarded by the operator

This reply indicates that the host does not recognize the user

name, but that it will accept the mail and have the operator

attempt to deliver it. This is useful if the user name is

misspelled, but may be a disservice if the mail is really

undeliverable.

If forwarding by the operator is unacceptable or if the user would

prefer to send the mail directly to the recipient's actual host, the

dialogue may be terminated upon receipt of one of these preliminary

responses.

There are two MTP commands which allow the text of a message to be

mailed to several recipients simultaneously; such message

transmission is far more efficient than the practice of sending the

text again and again for each additional recipient at a site. In

one, all recipients are specified first, and then the text is sent.

In the other, the order is reversed and the text is sent first,

followed by the recipients. Both schemes are necessary because

neither by itself is optimal for all systems, as will be explained

later. To select a particular scheme, the MRSQ command is used; to

specify recipients after a scheme is chosen, MRCP commands are given;

and to furnish text, the MAIL command is used.

SCHEME SELECTION: MRSQ

MRSQ is the means by which a sender-MTP can test for MRSQ/MRCP

implementation, select a particular scheme, reset its state, and

even do some rudimentary negotiation. Its format is as follows:

MRSQ [<SP> <scheme>] <CRLF>

<scheme> is a single character. The following are defined:

R Recipients first. If this is not implemented, T must be.

T Text first. If this is not implemented, R must be.

? Request for preference. This must always be implemented.

No argument means a "selection" of none of the schemes (the

default).

Possible replies are:

200 OK, we'll use specified scheme

215 <scheme> This is the scheme I prefer

501 I understand MRSQ but can't use that scheme

5xx Command unrecognized or unimplemented

RFC772 September 1980

Mail Transfer Protocol

There are three ASPects of MRSQ. The first is that an MRSQ with

no argument must always return a 200 reply and restore the default

state of having no scheme selected. Any other reply implies that

MRSQ and hence MRCP are not understood or cannot be performed

correctly.

The second is that the use of "?" as a <scheme> asks the MTP

receiver to return a 215 reply in which the receiver specifies a

"preferred" scheme. The format of this reply is simple:

215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>

Any other reply (e.g., 4xx or 5xx) implies that MRSQ and MRCP

are not implemented, because "?" must always be implemented if

MRSQ is.

The third important point about MRSQ is that it always has the

side effect of resetting all schemes to their initial state. This

reset must be done no matter what the reply will be -- 200, 215,

or 501. The actions necessary for a reset will be explained when

discussing how each scheme actually works.

MESSAGE TEXT SPECIFICATION: MAIL

Regardless of which scheme (if any) has been selected, a MAIL

command with a non-null "TO" argument will behave exactly as

before; the MRSQ/MRCP commands have no effect on it. However, a

normal MAIL command does have the same side effect as MRSQ; it

"resets" the current scheme to its initial state.

It is only when the "TO" argument is null (e.g., MAIL FROM:<X@Y>

<CRLF>) that the particular scheme chosen is important. Rather

than producing an error (as most receivers currently do), the

receiver will accept message text for this "null" specification.

What it does with it depends on which scheme is in effect, and

will be described in the section on Scheme Mechanics.

September 1980 RFC772

Mail Transfer Protocol

RECIPIENT SPECIFICATION: MRCP

In order to specify recipient names (i.e., mailboxes) and receive

some acknowledgment (or refusal) for each name, the following

command is used:

MRCP <SP> TO:<path> <CRLF>

Reply for no scheme:

503 No scheme specified yet; use MRSQ

Replies for scheme T are identical to those for MAIL.

Replies for scheme R (recipients first):

200 OK, name stored

452 Recipient table full, this name not stored

553 Recipient name rejected

4xx Temporary error, try this name again later

5xx Permanent error, report to sender

Note that use of this command is an error if no scheme has been

selected yet; an MRSQ <scheme> must have been given if MRCP is to

be used.

SCHEME MECHANICS: MRSQ R (RECIPIENTS-FIRST)

In the recipients-first scheme, MRCP is used to specify names

which the MTP receiver stores in a list or table. Normally the

reply for each MRCP will be either a 200 for acceptance or a

4xx/5xx rejection code. All 5xx codes are permanent rejections

(e.g., user not known) which should be reported to the human user,

whereas 4xx codes in general connote some temporary error that may

be rectified later. None of the 4xx/5xx replies impinge on

previous or succeeding MRCP commands, except for 452 which

indicates that no further MRCPs will succeed unless a message is

sent to the already stored recipients or a reset is done.

RFC772 September 1980

Mail Transfer Protocol

Sending message text to stored recipients is done by giving a MAIL

command with no "TO" argument; that is, just MAIL <SP> <sender>

<CRLF>. Transmission of the message text is exactly the same as

for normal MAIL. However, a positive acknowledgment at the end of

transmission means the message has been sent to ALL recipients

that were remembered with MRCP, and a failure code means that it

should be considered to have failed for ALL of these specified

recipients. This applies regardless of the actual error code.

Regardless of what the reply signifies, all stored recipient names

are flushed and forgotten -- in other words, things are reset to

their initial state. This purging of the recipient name list must

also be done as the reset side effect of any use of MRSQ.

A 452 reply to an MRCP can be handled by using MAIL to specify the

message for currently stored recipients, and then sending more

MRCPs and another MAIL, as many times as necessary. For example,

if a receiver only had room for 10 names this would result in a

50-recipient message being sent 5 times, to 10 different

recipients each time.

If a sender attempts to specify message text (MAIL with no "TO"

argument) before any successful MRCPs have been given, this should

be treated exactly as a "normal" MAIL with a null recipient would

be; some receivers return an error, such as "550 Null recipient".

See the example in Appendix A for a mail transfer using MRSQ R.

SCHEME MECHANICS: MRSQ T (TEXT-FIRST)

In the text-first scheme, MAIL with no "TO" argument is used to

specify message text, which the receiver stores away. Succeeding

MRCPs are then treated as if they were MAIL commands, except that

none of the text transfer manipulations are done; the stored

message text is sent to the specified recipient, and a reply code

is returned identical to that which an actual MAIL would invoke.

(Note that ANY 2xx code indicates success.)

The stored message text is not forgotten until the next MAIL or

MRSQ, which will either replace it with new text or flush it

entirely. Any use of MRSQ will reset this scheme by flushing

stored text, as will any use of MAIL with a non-null argument.

If an MRCP is seen before any message text has been stored, the

sender in effect is trying to send a null message; some receivers

might allow this, others would return an error code.

September 1980 RFC772

Mail Transfer Protocol

See the example in Appendix B for a mail transfer using MRSQ T.

WHY TWO SCHEMES ANYWAY?

Because neither by itself is optimal for all systems. MRSQ R

allows more of a "bulk" mailing because everything is saved up and

then mailed simultaneously. This is very useful for systems such

as ITS where the MTP-receiver does not itself write mail directly,

but hands it on to a central mailer demon of great power. The

more information (e.g., recipients) associated with a single

"hand-off", the more efficiently mail can be delivered.

By contrast, MRSQ T is geared to receiver-MTPs which want to

deliver mail directly, in one-by-one incremental fashion. For

each given recipient this scheme returns an individual

success/failure reply code which may depend on variable mail

system factors such as exceeding disk allocation, mailbox access

conflicts, and so forth. If these receiver-MTPs tried to emulate

MRSQ Rs bulk mailing, they would have to ensure that a success

reply to the MAIL indeed meant that it had been delivered to ALL

recipients specified -- not just some.

NOTES:

* Because these commands are not required in the minimum

implementation of MTP, one must be prepared to deal with sites

which don't recognize either MRSQ or MRCP. "MRSQ" and "MRSQ ?"

are explicitly designed as tests to see whether either scheme is

implemented. MRCP is not designed as a test, and a failure

return of the "unimplemented" variety could be confused with "No

scheme selected yet", or even with "Recipient unknown".

* There is no way to indicate in a positive response to "MRSQ ?"

that the preferred "scheme" for a receiver is that of the

default state; i.e., none of the multi-recipient schemes. The

rationale is that in this case, it would be pointless to

implement MRSQ/MRCP at all, and the response would therefore be

negative.

RFC772 September 1980

Mail Transfer Protocol

* One reason that the use of MAIL is restricted to null "TO"

arguments with this multi-recipient extension is the ambiguity

that would result if a non-null "TO" argument were allowed. For

example, if MRSQ R was in effect and some MRCPs had been given,

and a MAIL FROM:<X@Y> TO:<FOO><CRLF> was done, there would be no

way to distinguish a failure reply for mailbox "FOO" from a

global failure for all recipients specified. A similar

situation exists for MRSQ T; it would not be clear whether the

text was stored and the mailbox failed, or vice versa, or both.

* "Resets" are done by all MRSQs and "normal" MAILs to avoid

confusion and overly complicated implementation. The MRSQ

command implies a change or uncertainty of status, and the MAIL

command would otherwise have to use some independent mechanisms

to avoid clobbering the data bases (e.g., message text storage

area) used by the T/R schemes. However, once a scheme is

selected, it remains "in effect" just as an FTP "TYPE A" remains

selected. The recommended way for doing a reset, without

changing the current selection, is with "MRSQ ?". Remember that

"MRSQ" alone reverts to the no-scheme state.

* It is permissible to intersperse other MTP commands among the

MRSQ/MRCP/MAIL sequences.

September 1980 RFC772

Mail Transfer Protocol

DECLARATIVE SPECIFICATIONS

MINIMUM IMPLEMENTATION

In order to make MTP workable without needless error messages, the

following minimum implementation is required for all receivers:

COMMANDS -- QUIT

MAIL

NOOP

In terms of FTP, the values of the transfer parameters must be:

TYPE -- ASCII

MODE -- STREAM

STRU -- FILE-STRUCTURE

All hosts must use the above values for mail transfer.

CONNECTIONS

The receiver-MTP shall "listen" on Port L. The sender-MTP shall

initiate the TCP/NCP control connection. The control connection

consists of a full-duplex connection under TCP; it is two simplex

connections under NCP. Receiver- and sender- MTPs should follow

the conventions of the TELNET Protocol as specified in the ARPA

Internet Protocol Handbook. Receivers are under no obligation to

provide for editing of command lines and may specify that it be

done in the sender host. The control connection shall be closed

by the receiver at the sender's request after all transfers and

replies are completed.

SEQUENCING OF COMMANDS AND REPLIES

The communication between the sender and receiver is intended to

be an alternating dialogue. As such, the sender issues an MTP

command and the receiver responds with a prompt primary reply.

The sender should wait for this initial primary success or failure

response before sending further commands.

Certain commands require a second reply for which the sender

should also wait. These replies may, for example, report on the

progress or completion of mail transfer. They are secondary

replies to mail transfer commands.

One important group of informational replies is the connection

RFC772 September 1980

Mail Transfer Protocol

greetings. Under normal circumstances, a receiver will send a 220

reply, "awaiting input", when the connection is completed. The

sender should wait for this greeting message before sending any

commands. If the receiver is unable to accept input right away,

it should send a 120 "expected delay" reply immediately and a 220

reply when ready. The sender will then know not to hang up if

there is a delay.

Note: all the greeting type replies have the official name of

the server host as the first word following the reply code.

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

In this section, the command-reply sequence is presented. Each

command is listed with its possible replies; command groups are

listed together. Preliminary replies are listed first (with

their succeeding replies indented under them), then positive

and negative completion, and finally intermediary replies with

the remaining commands from the sequence following. The 421

reply (service not available, closing control connection) may

be given at any point if the MTP-receiver knows it must shut

down. This listing forms the basis for the state diagrams,

which will be presented separately.

CONNECTION ESTABLISHMENT

120

220

421

MAIL ACTION COMMANDS

MAIL

151, 152

354

250

451, 552

354

250

451, 552

450, 550, 452, 553

500, 501, 502, 421

September 1980 RFC772

Mail Transfer Protocol

MRSQ

200, 215

500, 501, 502, 421

MRCP

151, 152

200

450, 550, 452, 553

500, 501, 502, 503, 421

QUIT

221

INFORMATIONAL COMMANDS

HELP

211, 214

500, 501, 502, 421

MISCELLANEOUS COMMANDS

NOOP

200

500 421

STATE DIAGRAMS

Here we present state diagrams for a very simple minded MTP

implementation. Only the first digit of the reply codes is used.

There is one state diagram for each group of MTP 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".

RFC772 September 1980

Mail Transfer Protocol

We first present the diagram that represents the most MTP commands:

1,3 +---+

-----------> E

+---+

+---+ cmd +---+ 2 +---+

B ----------> W ----------> S

+---+ +---+ +---+

4,5 +---+

-----------> F

+---+

This diagram models the commands:

HELP, MRCP, MRSQ, NOOP, QUIT.

September 1980 RFC772

Mail Transfer Protocol

A more complex diagram models the MAIL command:

---- 1

+---+ cmd -->+---+ 2 +---+

B ----------> W ----------> E

+---+ +---+ -->+---+

3 4,5

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

+---+

-------------> S

1,3 +---+

2 --------

+---+ text +---+ 4,5 ----->+---+

----------> W ----------> F

+---+ +---+ +---+

Note that the "text" here is a series of lines sent from the

sender to the receiver with no response expected until the last

line is sent. (The last line must consist of only a single

period.)

RFC772 September 1980

Mail Transfer Protocol

Finally we present a generalized diagram that could be used to model

the command and reply interchange:

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

Begin

+---+ cmd +---+ 2 +---+

--> -------> ---------->

W S -----

--> --> -----

+---+ +---+ 4,5 +---+

1 3 +---+

---- ----> F -----

+---+

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

End

September 1980 RFC772

Mail Transfer Protocol

CONNECTION ESTABLISHMENT

The MTP control connection is established via TCP/NCP between the

receiver process port/socket L and the sender process port/socket U.

This protocol is assigned the service port/socket 57 (71 octal), that

is L=57.

RFC772 September 1980

Mail Transfer Protocol

APPENDIX A

Example of MRSQ R (Recipients-first)

This is an example of how MRSQ R is used. First the sender must

establish that the receiver in fact implements MRSQ.

S: MRSQ <CRLF>

R: 200 OK, no scheme selected

An MRSQ with a null argument always returns a 200 if implemented,

selecting the default "scheme", i.e., none of them. If MRSQ were not

implemented, a code of 4xx or 5xx would be returned.

S: MRSQ R <CRLF>

R: 200 OK, using that scheme

All is well; now the recipients can be specified.

S: MRCP TO:<Foo@Y> <CRLF>

R: 200 OK

S: MRCP TO:<Raboof@Y> <CRLF>

R: 553 No such user here

S: MRCP TO:<bar@Y> <CRLF>

R: 200 OK

S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>

R: 200 OK

Note that the failure of "Raboof" has no effect on the storage of

mail for "Foo", "bar" or the mail to be forwarded to "fubar@Z"

through host "X". Now the message text is furnished, by giving a

MAIL command with no "TO" argument.

S: MAIL FROM:<waldo@A><CRLF>

R: 354 Type mail, ended by <CRLF>.<CRLF>

S: Blah blah blah blah....etc. etc. etc.

S: <CRLF>.<CRLF>

R: 250 Mail sent

The mail text has now been sent to "Foo" and "bar" as well as

forwarded to "fubar@Z".

September 1980 RFC772

Mail Transfer Protocol

APPENDIX B

Example of MRSQ T (Text-first)

Using the same message as the previous example to establish that the

receiver implements MRSQ.

S: MRSQ ? <CRLF>

R: 215 T Text first, please

MRSQ is indeed implemented, and the receiver says that it prefers

"T", but that needn't stop the sender from trying something else.

S: MRSQ R <CRLF>

R: 501 Sorry, I really can't do that

It's possible that it could have understood "R" also, but in general

it's best to use the "preferred" scheme, since the receiver knows

which is most efficient for its particular site.

S: MRSQ T <CRLF>

R: 200 OK, using that scheme

Scheme "T" is now selected, and the message text is sent by giving a

mail command with no "TO" argument.

S: MAIL FROM:<WALDO@A><CRLF>

R: 354 Type mail, ended by <CRLF>.<CRLF>

S: Blah blah blah blah....etc. etc. etc.

S: <CRLF>.<CRLF>

R: 250 Mail stored

Now recipients can be specified.

S: MRCP TO:<Foo@Y> <CRLF>

R: 250 Stored mail sent

S: MRCP TO:<Raboof@Y> <CRLF>

R: 553 No such user here

S: MRCP TO:<bar@Y> <CRLF>

R: 250 Stored mail sent

S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>

R: 200 OK

RFC772 September 1980

Mail Transfer Protocol

The text has now been sent to "Foo" and "bar" at host "Y" and will be

forwarded to "fubar@Z" through host "X", and still remains stored. A

new message can be sent with another MAIL/MRCP ... sequence, but a

careful sender would reset the state using the exchange below.

S: MRSQ ? <CRLF>

R: 215 T Text first, please

Which resets the state without altering the scheme in effect.