brian@sdcsvax.UCSD.EDU (Brian Kantor) (08/28/87)
-- RFC 821 August 1982 Simple Mail Transfer Protocol forward-path buffer, and the mail data buffer; and inserts the reverse-path information from this command into the reverse-path buffer. RESET (RSET) This command specifies that the current mail transaction is to be aborted. Any stored sender, recipients, and mail data must be discarded, and all buffers and state tables cleared. The receiver must send an OK reply. VERIFY (VRFY) This command asks the receiver to confirm that the argument identifies a user. If it is a user name, the full name of the user (if known) and the fully specified mailbox are returned. This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer. EXPAND (EXPN) This command asks the receiver to confirm that the argument identifies a mailing list, and if so, to return the membership of that list. The full name of the users (if known) and the fully specified mailboxes are returned in a multiline reply. This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer. HELP (HELP) This command causes the receiver to send helpful information to the sender of the HELP command. The command may take an argument (e.g., any command name) and return more specific information as a response. This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer. Postel [Page 25] August 1982 RFC 821 Simple Mail Transfer Protocol NOOP (NOOP) This command does not affect any parameters or previously entered commands. It specifies no action other than that the receiver send an OK reply. This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer. QUIT (QUIT) This command specifies that the receiver must send an OK reply, and then close the transmission channel. The receiver should not close the transmission channel until it receives and replies to a QUIT command (even if there was an error). The sender should not close the transmission channel until it send a QUIT command and receives the reply (even if there was an error response to a previous command). If the connection is closed prematurely the receiver should act as if a RSET command had been received (canceling any pending transaction, but not undoing any previously completed transaction), the sender should act as if the command or transaction in progress had received a temporary error (4xx). TURN (TURN) This command specifies that the receiver must either (1) send an OK reply and then take on the role of the sender-SMTP, or (2) send a refusal reply and retain the role of the receiver-SMTP. If program-A is currently the sender-SMTP and it sends the TURN command and receives an OK reply (250) then program-A becomes the receiver-SMTP. Program-A is then in the initial state as if the transmission channel just opened, and it then sends the 220 service ready greeting. If program-B is currently the receiver-SMTP and it receives the TURN command and sends an OK reply (250) then program-B becomes the sender-SMTP. Program-B is then in the initial state as if the transmission channel just opened, and it then expects to receive the 220 service ready greeting. To refuse to change roles the receiver sends the 502 reply. [Page 26] Postel RFC 821 August 1982 Simple Mail Transfer Protocol There are restrictions on the order in which these command may be used. The first command in a session must be the HELO command. The HELO command may be used later in a session as well. If the HELO command argument is not acceptable a 501 failure reply must be returned and the receiver-SMTP must stay in the same state. The NOOP, HELP, EXPN, and VRFY commands can be used at any time during a session. The MAIL, SEND, SOML, or SAML commands begin a mail transaction. Once started a mail transaction consists of one of the transaction beginning commands, one or more RCPT commands, and a DATA command, in that order. A mail transaction may be aborted by the RSET command. There may be zero or more transactions in a session. If the transaction beginning command argument is not acceptable a 501 failure reply must be returned and the receiver-SMTP must stay in the same state. If the commands in a transaction are out of order a 503 failure reply must be returned and the receiver-SMTP must stay in the same state. The last command in a session must be the QUIT command. The QUIT command can not be used at any other time in a session. 4.1.2. COMMAND SYNTAX The commands consist of a command code followed by an argument field. Command codes are four alphabetic characters. Upper and lower case alphabetic characters are to be treated identically. Thus, any of the following may represent the mail command: MAIL Mail mail MaIl mAIl This also applies to any symbols representing parameter values, such as "TO" or "to" for the forward-path. Command codes and the argument fields are separated by one or more spaces. However, within the reverse-path and forward-path arguments case is important. In particular, in some hosts the user "smith" is different from the user "Smith". Postel [Page 27] August 1982 RFC 821 Simple Mail Transfer Protocol The argument field consists of a variable length character string ending with the character sequence <CRLF>. The receiver is to take no action until this sequence is received. Square brackets denote an optional argument field. If the option is not taken, the appropriate default is implied. [Page 28] Postel RFC 821 August 1982 Simple Mail Transfer Protocol The following are the SMTP commands: HELO <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> Postel [Page 29] August 1982 RFC 821 Simple Mail Transfer Protocol The syntax of the above argument fields (using BNF notation where applicable) is given below. The "..." notation indicates that a field may be repeated one or more times. <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> [Page 30] Postel 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. Postel [Page 31] August 1982 RFC 821 Simple Mail Transfer Protocol The time stamp line and the return path line are formally defined as follows: <return-path-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. [Page 32] Postel 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 ------------------------------------------------------------- Postel [Page 33] August 1982 RFC 821 Simple Mail Transfer Protocol 4.2. SMTP REPLIES Replies to SMTP commands are devised to ensure the synchronization of requests and actions in the process of mail transfer, and to guarantee that the sender-SMTP always knows the state of the receiver-SMTP. Every command must generate exactly one reply. The details of the command-reply sequence are made explicit in Section 5.3 on Sequencing and Section 5.4 State Diagrams. An SMTP reply consists of a three digit number (transmitted as three alphanumeric characters) followed by some text. The number is intended for use by automata to determine what state to enter next; the text is meant for the human user. It is intended that the three digits contain enough encoded information that the sender-SMTP need not examine the text and may either discard it or pass it on to the user, as appropriate. In particular, the text may be receiver-dependent and context dependent, so there are likely to be varying texts for each reply code. A discussion of the theory of reply codes is given in Appendix E. Formally, a reply is defined to be the sequence: a three-digit code, <SP>, one line of text, and <CRLF>, or a multiline reply (as defined in Appendix E). Only the EXPN and HELP commands are expected to result in multiline replies in normal circumstances, however multiline replies are allowed for any command. [Page 34] Postel RFC 821 August 1982 Simple Mail Transfer Protocol 4.2.1. REPLY CODES BY FUNCTION GROUPS 500 Syntax error, command unrecognized [This may include errors such as command line too long] 501 Syntax error in parameters or arguments 502 Command not implemented 503 Bad sequence of commands 504 Command parameter not implemented 211 System status, or system help reply 214 Help message [Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user] 220 <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 Postel [Page 35] August 1982 RFC 821 Simple Mail Transfer Protocol 4.2.2. NUMERIC ORDER LIST OF REPLY CODES 211 System status, or system help reply 214 Help message [Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user] 220 <domain> Service ready 221 <domain> Service closing transmission channel 250 Requested mail action okay, completed 251 User not local; will forward to <forward-path> 354 Start mail input; end with <CRLF>.<CRLF> 421 <domain> Service not available, closing transmission channel [This may be a reply to any command if the service knows it must shut down] 450 Requested mail action not taken: mailbox unavailable [E.g., mailbox busy] 451 Requested action aborted: local error in processing 452 Requested action not taken: insufficient system storage 500 Syntax error, command unrecognized [This may include errors such as command line too long] 501 Syntax error in parameters or arguments 502 Command not implemented 503 Bad sequence of commands 504 Command parameter not implemented 550 Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access] 551 User not local; please try <forward-path> 552 Requested mail action aborted: exceeded storage allocation 553 Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect] 554 Transaction failed [Page 36] Postel RFC 821 August 1982 Simple Mail Transfer Protocol 4.3. SEQUENCING OF COMMANDS AND REPLIES The communication between the sender and receiver is intended to be an alternating dialogue, controlled by the sender. As such, the sender issues a command and the receiver responds with a reply. The sender must wait for this response before sending further commands. One important reply is the connection greeting. Normally, a receiver will send a 220 "Service ready" reply when the connection is completed. The sender should wait for this greeting message before sending any commands. Note: all the greeting type replies have the official name of the server host as the first word following the reply code. For example, 220 <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 Postel [Page 37] August 1982 RFC 821 Simple Mail Transfer Protocol RCPT S: 250, 251 F: 550, 551, 552, 553, 450, 451, 452 E: 500, 501, 503, 421 DATA I: 354 -> data -> S: 250 F: 552, 554, 451, 452 F: 451, 554 E: 500, 501, 503, 421 RSET S: 250 E: 500, 501, 504, 421 SEND S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 SOML S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 SAML S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 VRFY S: 250, 251 F: 550, 551, 553 E: 500, 501, 502, 504, 421 EXPN S: 250 F: 550 E: 500, 501, 502, 504, 421 HELP S: 211, 214 E: 500, 501, 502, 504, 421 NOOP S: 250 E: 500, 421 QUIT S: 221 E: 500 TURN S: 250 F: 502 E: 500, 503 [Page 38] Postel RFC 821 August 1982 Simple Mail Transfer Protocol 4.4. STATE DIAGRAMS Following are state diagrams for a simple-minded SMTP implementation. Only the first digit of the reply codes is used. There is one state diagram for each group of SMTP commands. The command groupings were determined by constructing a model for each command and then collecting together the commands with structurally identical models. For each command there are three possible outcomes: "success" (S), "failure" (F), and "error" (E). In the state diagrams below we use the symbol B for "begin", and the symbol W for "wait for reply". First, the diagram that represents most of the SMTP commands: 1,3 +---+ ----------->| E | | +---+ | +---+ cmd +---+ 2 +---+ | B |---------->| W |---------->| S | +---+ +---+ +---+ | | 4,5 +---+ ----------->| F | +---+ This diagram models the commands: HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP, NOOP, QUIT, TURN. Postel [Page 39] August 1982 RFC 821 Simple Mail Transfer Protocol A more complex diagram models the DATA command: +---+ DATA +---+ 1,2 +---+ | B |---------->| W |-------------------->| E | +---+ +---+ ------------>+---+ 3| |4,5 | | | | -------------- ----- | | | | +---+ | ---------- -------->| S | | | | | +---+ | | ------------ | | | | V 1,3| |2 | +---+ data +---+ --------------->+---+ | |---------->| W | | F | +---+ +---+-------------------->+---+ 4,5 Note that the "data" here is a series of lines sent from the sender to the receiver with no response expected until the last line is sent. [Page 40] Postel RFC 821 August 1982 Simple Mail Transfer Protocol 4.5. DETAILS 4.5.1. MINIMUM IMPLEMENTATION In order to make SMTP workable, the following minimum implementation is required for all receivers: COMMANDS -- HELO MAIL RCPT DATA RSET NOOP QUIT 4.5.2. TRANSPARENCY Without some provision for data transparency the character sequence "<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 Postel [Page 41] August 1982 RFC 821 Simple Mail Transfer Protocol strings. If such transforms are necessary, they must be reversible -- especially if such transforms are applied to mail being relayed. 4.5.3. SIZES There are several objects that have required minimum maximum sizes. That is, every implementation must be able to receive objects of at least these sizes, but must not send objects larger than these sizes. **************************************************** * * * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION * * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH * * OF THESE OBJECTS SHOULD BE USED. * * * **************************************************** user The maximum total length of a user name is 64 characters. domain The maximum total length of a domain name or number is 64 characters. path The maximum total length of a reverse-path or forward-path is 256 characters (including the punctuation and element separators). command line The maximum total length of a command line including the command word and the <CRLF> is 512 characters. reply line The maximum total length of a reply line including the reply code and the <CRLF> is 512 characters. [Page 42] Postel RFC 821 August 1982 Simple Mail Transfer Protocol text line The maximum total length of a text line including the <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. Postel [Page 43] August 1982 RFC 821 Simple Mail Transfer Protocol APPENDIX A TCP Transport service The Transmission Control Protocol [3] is used in the ARPA Internet, and in any network following the US DoD standards for internetwork protocols. Connection Establishment The SMTP transmission channel is a TCP connection established between the sender process port U and the receiver process port L. This single full duplex connection is used as the transmission channel. This protocol is assigned the service port 25 (31 octal), that is L=25. Data Transfer The TCP connection supports the transmission of 8-bit bytes. The SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero. [Page 44] Postel RFC 821 August 1982 Simple Mail Transfer Protocol APPENDIX B NCP Transport service The ARPANET Host-to-Host Protocol [4] (implemented by the Network Control Program) may be used in the ARPANET. Connection Establishment The SMTP transmission channel is established via NCP between the sender process socket U and receiver process socket L. The Initial Connection Protocol [5] is followed resulting in a pair of simplex connections. This pair of connections is used as the transmission channel. This protocol is assigned the contact socket 25 (31 octal), that is L=25. Data Transfer The NCP data connections are established in 8-bit byte mode. The SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero. Postel [Page 45] August 1982 RFC 821 Simple Mail Transfer Protocol APPENDIX C NITS The Network Independent Transport Service [6] may be used. Connection Establishment The SMTP transmission channel is established via NITS between the sender process and receiver process. The sender process executes the CONNECT primitive, and the waiting receiver process executes the ACCEPT primitive. Data Transfer The NITS connection supports the transmission of 8-bit bytes. The SMTP data is 7-bit ASCII characters. Each character is transmitted as an 8-bit byte with the high-order bit cleared to zero. [Page 46] Postel RFC 821 August 1982 Simple Mail Transfer Protocol APPENDIX D X.25 Transport service It may be possible to use the X.25 service [7] as provided by the Public Data Networks directly, however, it is suggested that a reliable end-to-end protocol such as TCP be used on top of X.25 connections. Postel [Page 47] August 1982 RFC 821 Simple Mail Transfer Protocol APPENDIX E Theory of Reply Codes The three digits of the reply each have a special significance. The first digit denotes whether the response is good, bad or incomplete. An unsophisticated sender-SMTP will be able to determine its next action (proceed as planned, redo, retrench, etc.) by simply examining this first digit. A sender-SMTP that wants to know approximately what kind of error occurred (e.g., mail system error, command syntax error) may examine the second digit, reserving the third digit for the finest gradation of information. There are five values for the first digit of the reply code: 1yz Positive Preliminary reply The command has been accepted, but the requested action is being held in abeyance, pending confirmation of the information in this reply. The sender-SMTP should send another command specifying whether to continue or abort the action. [Note: SMTP does not have any commands that allow this type of reply, and so does not have the continue or abort commands.] 2yz Positive Completion reply The requested action has been successfully completed. A new request may be initiated. 3yz Positive Intermediate reply The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information. The sender-SMTP should send another command specifying this information. This reply is used in command sequence groups. 4yz Transient Negative Completion reply The command was not accepted and the requested action did not occur. However, the error condition is temporary and the action may be requested again. The sender should [Page 48] Postel RFC 821 August 1982 Simple Mail Transfer Protocol return to the beginning of the command sequence (if any). It is difficult to assign a meaning to "transient" when two different sites (receiver- and sender- SMTPs) must agree on the interpretation. Each reply in this category might have a different time value, but the sender-SMTP is encouraged to try again. A rule of thumb to determine if a reply fits into the 4yz or the 5yz category (see below) is that replies are 4yz if they can be repeated without any change in command form or in properties of the sender or receiver. (E.g., the command is repeated identically and the receiver does not put up a new implementation.) 5yz Permanent Negative Completion reply The command was not accepted and the requested action did not occur. The sender-SMTP is discouraged from repeating the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct the sender-SMTP to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status). The second digit encodes responses in specific categories: x0z Syntax -- These replies refer to syntax errors, syntactically correct commands that don't fit any functional category, and unimplemented or superfluous commands. x1z Information -- These are replies to requests for information, such as status or help. x2z Connections -- These are replies referring to the transmission channel. x3z Unspecified as yet. x4z Unspecified as yet. x5z Mail system -- These replies indicate the status of the receiver mail system vis-a-vis the requested transfer or other mail system action. The third digit gives a finer gradation of meaning in each category specified by the second digit. The list of replies Postel [Page 49] August 1982 RFC 821 Simple Mail Transfer Protocol illustrates this. Each reply text is recommended rather than mandatory, and may even change according to the command with which it is associated. On the other hand, the reply codes must strictly follow the specifications in this section. Receiver implementations should not invent new codes for slightly different situations from the ones described here, but rather adapt codes already defined. For example, a command such as NOOP whose successful execution does not offer the sender-SMTP any new information will return a 250 reply. The response is 502 when the command requests an unimplemented non-site-specific action. A refinement of that is the 504 reply for a command that is implemented, but that requests an unimplemented parameter. The reply text may be longer than a single line; in these cases the complete text must be marked so the sender-SMTP knows when it can stop reading the reply. This requires a special format to indicate a multiple line reply. The format for multiline replies requires that every line, except the last, begin with the reply code, followed immediately by a hyphen, "-" (also known as minus), followed by text. The last line will begin with the reply code, followed immediately by <SP>, optionally some text, and <CRLF>. For example: 123-First line 123-Second line 123-234 text beginning with numbers 123 The last line In many cases the sender-SMTP then simply needs to search for the reply code followed by <SP> at the beginning of a line, and ignore all preceding lines. In a few cases, there is important data for the sender in the reply "text". The sender will know these cases from the current context.