In order to make MTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- MAIL
QUIT
NOOP
5.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the 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-MTP 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-MTP it
checks the 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.
5.5.3. SIZES
There are several objects that ought to have defined maximum
sizes.
user
The maximum total length of a user name is 40 characters.
host
The maximum total length of a host name or number is 20
characters.
RFC780 May 1981
Mail Transfer Protocol
path
The maximum total length of a sender-path or
receiver-path is 100 characters.
command line
The maximum total length of a command line including the
command word and the <CRLF> is 200 characters.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 65 characters.
text line
The maximum total length of a text line including the the
<CRLF> is 1000 characters.
To the maximum extent possible implementation techniques which
impose no limits at all to the length of these objects should
be used.
May 1981 RFC780
Mail Transfer Protocol
APPENDIX A
TCP Transport service
The Transmission Control Protocol [1] is used in the ARPA
Internet, and in any network following the US DoD standards for
internetwork protocols.
Connection Establishment
The MTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 57 (71 octal), that is L=57.
Data Transfer
The TCP connection supports the transmission of 8-bit bytes.
The MTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
RFC780 May 1981
Mail Transfer Protocol
APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [2] (implemented by the Network
Control Program) may be used in the ARPANET.
Connection Establishment
The MTP transmission channel is established via NCP between the
the sender process socket U and receiver process socket L. The
Initial Connection Protocol [3] is followed resulting in a pair
of simplex connections. This pair of connections is used as
the transmission channel. This protocol is assigned the
contact socket 57 (71 octal), that is L=57.
Data Transfer
The NCP data connections are established in 8-bit byte mode.
The MTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
May 1981 RFC780
Mail Transfer Protocol
APPENDIX C
NITS
The Network Independent Transport Service [4] may be used.
Connection Establishment
The MTP transmission channel is established via NITS between
the the sender process and receiver process. The sender
process executes the CONNECT primitive, and the waiting
receiver process executes the ACCEPT primitive.
Data Transfer
The NITS connection supports the transmission of 8-bit bytes.
The MTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
RFC780 May 1981
Mail Transfer Protocol
APPENDIX D
X.25 Transport service
It may be possible to use the X.25 service [5] as provided by the
Public Data Networks directly, but there are indications that it
is too error prone to qualify as a reliable channel. It is
suggested that a reliable end-to-end protocol such as TCP be used
on top of X.25 connections.
May 1981 RFC780
Mail Transfer Protocol
APPENDIX E
Theory of Reply Codes
The three digits of the reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated sender-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 command has been accepted, but the requested action
is being held in abeyance, pending confirmation of the
information in this reply. The sender-MTP should send
another command specifying whether to continue or abort
the action.
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
RFC780 May 1981
Mail Transfer Protocol
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 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
illustrates this. Each reply text is recommended rather than
mandatory, and may even change according to the command with
which it is associated. On the other hand, the reply codes
must strictly follow the specifications in this section.
May 1981 RFC780
Mail Transfer Protocol
Receiver implementations should not invent new codes for
slightly different situations from the ones described here, but
rather adapt codes already defined.
For example, a command such as NOOP whose successful execution
does not offer the sender-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.
The reply text may be longer than a single line; in these cases
the complete text must be marked so the sender-MTP knows when it
can stop reading the reply. This requires a special format to
indicate a multiple line reply.
The format for multi-line replies requires that every line,
except the last, begin with the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
The sender-MTP then simply needs to search for the reply code
followed by <SP> at the beginning of a line, and ignore all
preceding lines.
RFC780 May 1981
Mail Transfer Protocol
GLOSSARY
ASCII
American Standard Code for Information Interchange [6].
command
A request for a mail service action sent by the sender-MTP to the
receiver-MTP.
host
A computer in the internetwork environment on which mailboxes or
MTP processes reside.
line
A line of text ending with a <CRLF>.
A sequence of ASCII characters of arbitrary length, which conforms
to the standard set in RFC733 (Standard for the Format of ARPA
Network Text Messages [7]).
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.
receiver-MTP process
A process which transfers mail in cooperation with a sender-MTP
process. It waits for a connection to be established via the
transport service. It receives MTP commands from the sender-MTP,
sends replies, and governs the transfer of mail.
May 1981 RFC780
Mail Transfer Protocol
reply
A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel 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.
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 transport
service connection. It initiates MTP commands, receives replies,
and governs the transfer of mail.
transmission channel
A full-duplex communication path between a sender-MTP and a
receiver-MTP for the exchange of commands, replies, and mail text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
word
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
<CRLF>
The characters carriage return and line feed (in that order).
RFC780 May 1981
Mail Transfer Protocol
<SP>
The space character.
May 1981 RFC780
Mail Transfer Protocol
REFERENCES
[1] TCP
Postel, J., ed., "DOD Standard Transmission Control Protocol",
IEN 129, RFC761, USC/Information Sciences Institute,
NTIS ADA082609, January 1980. Appears in: Computer Communication
Review, Special Interest Group on Data Communications, ACM, V.10,
N.4, October 1980.
[2] NCP
McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[3] Initial Connection Protocol
Postel, J., "Official Initial Connection Protocol", NIC 7101,
11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[4] NITS
PSS/SG3, "A Network Independent Transport Service", Study Group 3,
The Post Office PSS Users Group, February 1980. Available from
the DCPU, National Physical Laboratory, Teddington, UK.
[5] X.25
CCITT, "Recommendation X.25 - Interface Between Data Terminal
Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
Terminals Operating in the Packet Mode on Public Data Networks,"
CCITT Orange Book, Vol. VIII.2, International Telephone and
Telegraph Consultative Committee, Geneva, 1976.
RFC780 May 1981
Mail Transfer Protocol
[6] ASCII
ASCII, "USA Code for Information Interchange", United States of
America Standards Institute, X3.4, 1968. Also in: Feinler, E.
and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
the Defense Communications Agency by SRI International, Menlo
Park, California, Revised January 1978.
[7] RFC733
Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
the Format of ARPA Network Text Messages," RFC733, NIC 41952,
November 1977. Also in: Feinler, E. and J. Postel, eds.,
"ARPANET Protocol Handbook", NIC 7104, for the Defense
Communications Agency by SRI International, Menlo Park,
California, Revised January 1978.