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diff --git a/RFC/rfc821.txt b/RFC/rfc821.txt deleted file mode 100644 index d877b72c..00000000 --- a/RFC/rfc821.txt +++ /dev/null @@ -1,4050 +0,0 @@ - - - - RFC 821 - - - - - - SIMPLE MAIL TRANSFER PROTOCOL - - - - Jonathan B. Postel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - August 1982 - - - - Information Sciences Institute - University of Southern California - 4676 Admiralty Way - Marina del Rey, California 90291 - - (213) 822-1511 - - - - -RFC 821 August 1982 - Simple Mail Transfer Protocol - - - - TABLE OF CONTENTS - - 1. INTRODUCTION .................................................. 1 - - 2. THE SMTP MODEL ................................................ 2 - - 3. THE SMTP PROCEDURE ............................................ 4 - - 3.1. Mail ..................................................... 4 - 3.2. Forwarding ............................................... 7 - 3.3. Verifying and Expanding .................................. 8 - 3.4. Sending and Mailing ..................................... 11 - 3.5. Opening and Closing ..................................... 13 - 3.6. Relaying ................................................ 14 - 3.7. Domains ................................................. 17 - 3.8. Changing Roles .......................................... 18 - - 4. THE SMTP SPECIFICATIONS ...................................... 19 - - 4.1. SMTP Commands ........................................... 19 - 4.1.1. Command Semantics ..................................... 19 - 4.1.2. Command Syntax ........................................ 27 - 4.2. SMTP Replies ............................................ 34 - 4.2.1. Reply Codes by Function Group ......................... 35 - 4.2.2. Reply Codes in Numeric Order .......................... 36 - 4.3. Sequencing of Commands and Replies ...................... 37 - 4.4. State Diagrams .......................................... 39 - 4.5. Details ................................................. 41 - 4.5.1. Minimum Implementation ................................ 41 - 4.5.2. Transparency .......................................... 41 - 4.5.3. Sizes ................................................. 42 - - APPENDIX A: TCP ................................................. 44 - APPENDIX B: NCP ................................................. 45 - APPENDIX C: NITS ................................................ 46 - APPENDIX D: X.25 ................................................ 47 - APPENDIX E: Theory of Reply Codes ............................... 48 - APPENDIX F: Scenarios ........................................... 51 - - GLOSSARY ......................................................... 64 - - REFERENCES ....................................................... 67 - - - - -Network Working Group J. Postel -Request for Comments: DRAFT ISI -Replaces: RFC 788, 780, 772 August 1982 - - SIMPLE MAIL TRANSFER PROTOCOL - - -1. INTRODUCTION - - The objective of Simple Mail Transfer Protocol (SMTP) is to transfer - mail reliably and efficiently. - - SMTP is independent of the particular transmission subsystem and - requires only a reliable ordered data stream channel. Appendices A, - B, C, and D describe the use of SMTP with various transport services. - A Glossary provides the definitions of terms as used in this - document. - - An important feature of SMTP is its capability to relay mail across - transport service environments. A transport service provides an - interprocess communication environment (IPCE). An IPCE may cover one - network, several networks, or a subset of a network. It is important - to realize that transport systems (or IPCEs) are not one-to-one with - networks. A process can communicate directly with another process - through any mutually known IPCE. Mail is an application or use of - interprocess communication. Mail can be communicated between - processes in different IPCEs by relaying through a process connected - to two (or more) IPCEs. More specifically, mail can be relayed - between hosts on different transport systems by a host on both - transport systems. - - - - - - - - - - - - - - - - - - - - - - - - -Postel [Page 1] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - -2. THE SMTP MODEL - - The SMTP design is based on the following model of communication: as - the result of a user mail request, the sender-SMTP establishes a - two-way transmission channel to a receiver-SMTP. The receiver-SMTP - may be either the ultimate destination or an intermediate. SMTP - commands are generated by the sender-SMTP and sent to the - receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the - sender-SMTP in response to the commands. - - Once the transmission channel is established, the SMTP-sender sends a - MAIL command indicating the sender of the mail. If the SMTP-receiver - can accept mail it responds with an OK reply. The SMTP-sender then - sends a RCPT command identifying a recipient of the mail. If the - SMTP-receiver can accept mail for that recipient it responds with an - OK reply; if not, it responds with a reply rejecting that recipient - (but not the whole mail transaction). The SMTP-sender and - SMTP-receiver may negotiate several recipients. When the recipients - have been negotiated the SMTP-sender sends the mail data, terminating - with a special sequence. If the SMTP-receiver successfully processes - the mail data it responds with an OK reply. The dialog is purposely - lock-step, one-at-a-time. - - ------------------------------------------------------------- - - - +----------+ +----------+ - +------+ | | | | - | 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 - - - -[Page 2] Postel - - - -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). - - - - - -Postel [Page 3] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - -3. THE SMTP PROCEDURES - - This section presents the procedures used in SMTP in several parts. - First comes the basic mail procedure defined as a mail transaction. - Following this are descriptions of forwarding mail, verifying mailbox - names and expanding mailing lists, sending to terminals instead of or - in combination with mailboxes, and the opening and closing exchanges. - At the end of this section are comments on relaying, a note on mail - domains, and a discussion of changing roles. Throughout this section - are examples of partial command and reply sequences, several complete - scenarios are presented in Appendix F. - - 3.1. MAIL - - There are three steps to SMTP mail transactions. The transaction - is started with a MAIL command which gives the sender - identification. A series of one or more RCPT commands follows - giving the receiver information. Then a DATA command gives the - mail data. And finally, the end of mail data indicator confirms - the transaction. - - The first step in the procedure is the MAIL command. The - <reverse-path> contains the source mailbox. - - MAIL <SP> FROM:<reverse-path> <CRLF> - - This command tells the SMTP-receiver that a new mail - transaction is starting and to reset all its state tables and - buffers, including any recipients or mail data. It gives the - reverse-path which can be used to report errors. If accepted, - the receiver-SMTP returns a 250 OK reply. - - The <reverse-path> can contain more than just a mailbox. The - <reverse-path> is a reverse source routing list of hosts and - source mailbox. The first host in the <reverse-path> should be - the host sending this command. - - The second step in the procedure is the RCPT command. - - RCPT <SP> TO:<forward-path> <CRLF> - - This command gives a forward-path identifying one recipient. - If accepted, the receiver-SMTP returns a 250 OK reply, and - stores the forward-path. If the recipient is unknown the - receiver-SMTP returns a 550 Failure reply. This second step of - the procedure can be repeated any number of times. - - - -[Page 4] Postel - - - -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. - - - - - - - - - - - - - - -Postel [Page 5] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - ------------------------------------------------------------- - - 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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - -[Page 6] Postel - - - -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 - - ------------------------------------------------------------- - - - - -Postel [Page 7] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - 3.3. VERIFYING AND EXPANDING - - SMTP provides as additional features, commands to verify a user - name or expand a mailing list. This is done with the VRFY and - EXPN commands, which have character string arguments. For the - VRFY command, the string is a user name, and the response may - include the full name of the user and must include the mailbox of - the user. For the EXPN command, the string identifies a mailing - list, and the multiline response may include the full name of the - users and must give the mailboxes on the mailing list. - - "User name" is a fuzzy term and used purposely. If a host - implements the VRFY or EXPN commands then at least local mailboxes - must be recognized as "user names". If a host chooses to - recognize other strings as "user names" that is allowed. - - In some hosts the distinction between a mailing list and an alias - for a single mailbox is a bit fuzzy, since a common data structure - may hold both types of entries, and it is possible to have mailing - lists of one mailbox. If a request is made to verify a mailing - list a positive response can be given if on receipt of a message - so addressed it will be delivered to everyone on the list, - otherwise an error should be reported (e.g., "550 That is a - mailing list, not a user"). If a request is made to expand a user - name a positive response can be formed by returning a list - containing one name, or an error can be reported (e.g., "550 That - is a user name, not a mailing list"). - - In the case of a multiline reply (normal for EXPN) exactly one - mailbox is to be specified on each line of the reply. In the case - of an ambiguous request, for example, "VRFY Smith", where there - are two Smith's the response must be "553 User ambiguous". - - The case of verifying a user name is straightforward as shown in - example 3. - - - - - - - - - - - - - - -[Page 8] Postel - - - -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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - -Postel [Page 9] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - The case of expanding a mailbox list requires a multiline reply as - shown in example 4. - - ------------------------------------------------------------- - - Example of Expanding a Mailing List - - Either - - S: EXPN Example-People - R: 250-Jon Postel <Postel@USC-ISIF.ARPA> - R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA> - R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA> - R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> - R: 250-<joe@foo-unix.ARPA> - R: 250 <xyz@bar-unix.ARPA> - - Or - - S: EXPN Executive-Washroom-List - R: 550 Access Denied to You. - - Example 4 - - ------------------------------------------------------------- - - The character string arguments of the VRFY and EXPN commands - cannot be further restricted due to the variety of implementations - of the user name and mailbox list concepts. On some systems it - may be appropriate for the argument of the EXPN command to be a - file name for a file containing a mailing list, but again there is - a variety of file naming conventions in the Internet. - - The VRFY and EXPN commands are not included in the minimum - implementation (Section 4.5.1), and are not required to work - across relays when they are implemented. - - - - - - - - - - - - - -[Page 10] Postel - - - -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. - - - -Postel [Page 11] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - The same reply codes that are used for the MAIL commands are used - for these commands. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 12] Postel - - - -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 - - ------------------------------------------------------------- - - - - - - - - - - -Postel [Page 13] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - 3.6. RELAYING - - The forward-path may be a source route of the form - "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. This - form is used to emphasize the distinction between an address and a - route. The mailbox is an absolute address, and the route is - information about how to get there. The two concepts should not - be confused. - - Conceptually the elements of the forward-path are moved to the - reverse-path as the message is relayed from one server-SMTP to - another. The reverse-path is a reverse source route, (i.e., a - source route from the current location of the message to the - originator of the message). When a server-SMTP deletes its - identifier from the forward-path and inserts it into the - reverse-path, it must use the name it is known by in the - environment it is sending into, not the environment the mail came - from, in case the server-SMTP is known by different names in - different environments. - - If when the message arrives at an SMTP the first element of the - forward-path is not the identifier of that SMTP the element is not - deleted from the forward-path and is used to determine the next - SMTP to send the message to. In any case, the SMTP adds its own - identifier to the reverse-path. - - Using source routing the receiver-SMTP receives mail to be relayed - to another server-SMTP The receiver-SMTP may accept or reject the - task of relaying the mail in the same way it accepts or rejects - mail for a local user. The receiver-SMTP transforms the command - arguments by moving its own identifier from the forward-path to - the beginning of the reverse-path. The receiver-SMTP then becomes - a sender-SMTP, establishes a transmission channel to the next SMTP - in the forward-path, and sends it the mail. - - The first host in the reverse-path should be the host sending the - SMTP commands, and the first host in the forward-path should be - the host receiving the SMTP commands. - - Notice that the forward-path and reverse-path appear in the SMTP - commands and replies, but not necessarily in the message. That - is, there is no need for these paths and especially this syntax to - appear in the "To:" , "From:", "CC:", etc. fields of the message - header. - - If a server-SMTP has accepted the task of relaying the mail and - - - -[Page 14] Postel - - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - -Postel [Page 15] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - ------------------------------------------------------------- - - Example Undeliverable Mail Notification Message - - S: MAIL FROM:<> - R: 250 ok - S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA> - R: 250 ok - S: DATA - R: 354 send the mail data, end with . - S: Date: 23 Oct 81 11:22:33 - S: From: SMTP@HOSTY.ARPA - S: To: JOE@HOSTW.ARPA - S: Subject: Mail System Problem - S: - S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost. - S: HOSTZ.ARPA said this: - S: "550 No Such User" - S: . - R: 250 ok - - Example 7 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 16] Postel - - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Postel [Page 17] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - 3.8. CHANGING ROLES - - The TURN command may be used to reverse the roles of the two - programs communicating over the transmission channel. - - If program-A is currently the sender-SMTP and it sends the TURN - command and receives an ok reply (250) then program-A becomes the - receiver-SMTP. - - If program-B is currently the receiver-SMTP and it receives the - TURN command and sends an ok reply (250) then program-B becomes - the sender-SMTP. - - To refuse to change roles the receiver sends the 502 reply. - - Please note that this command is optional. It would not normally - be used in situations where the transmission channel is TCP. - However, when the cost of establishing the transmission channel is - high, this command may be quite useful. For example, this command - may be useful in supporting be mail exchange using the public - switched telephone system as a transmission channel, especially if - some hosts poll other hosts for mail exchanges. - - - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 18] Postel - - - -RFC 821 August 1982 - Simple Mail Transfer Protocol - - - -4. THE SMTP SPECIFICATIONS - - 4.1. SMTP COMMANDS - - 4.1.1. COMMAND SEMANTICS - - The SMTP commands define the mail transfer or the mail system - function requested by the user. SMTP commands are character - strings terminated by <CRLF>. The command codes themselves are - alphabetic characters terminated by <SP> if parameters follow - and <CRLF> otherwise. The syntax of mailboxes must conform to - receiver site conventions. The SMTP commands are discussed - below. The SMTP replies are discussed in the Section 4.2. - - A mail transaction involves several data objects which are - communicated as arguments to different commands. The - reverse-path is the argument of the MAIL command, the - forward-path is the argument of the RCPT command, and the mail - data is the argument of the DATA command. These arguments or - data objects must be transmitted and held pending the - confirmation communicated by the end of mail data indication - which finalizes the transaction. The model for this is that - distinct buffers are provided to hold the types of data - objects, that is, there is a reverse-path buffer, a - forward-path buffer, and a mail data buffer. Specific commands - cause information to be appended to a specific buffer, or cause - one or more buffers to be cleared. - - HELLO (HELO) - - This command is used to identify the sender-SMTP to the - receiver-SMTP. The argument field contains the host name of - the sender-SMTP. - - The receiver-SMTP identifies itself to the sender-SMTP in - the connection greeting reply, and in the response to this - command. - - This command and an OK reply to it confirm that both the - sender-SMTP and the receiver-SMTP are in the initial state, - that is, there is no transaction in progress and all state - tables and buffers are cleared. - - - - - - - -Postel [Page 19] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - MAIL (MAIL) - - This command is used to initiate a mail transaction in which - the mail data is delivered to one or more mailboxes. The - argument field contains a reverse-path. - - 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). In some types of error - reporting messages (for example, undeliverable mail - notifications) the reverse-path may be null (see Example 7). - - 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. - - RECIPIENT (RCPT) - - This command is used to identify an individual recipient of - the mail data; multiple recipients are specified by multiple - use of this command. - - The forward-path consists of an optional list of hosts and a - required destination mailbox. When the list of hosts is - present, it is a source route and indicates that the mail - must be relayed to the next host on the list. If the - receiver-SMTP does not implement the relay function it may - user the same reply it would for an unknown local user - (550). - - When mail is relayed, the relay host must remove itself from - the beginning forward-path and put itself at the beginning - of the reverse-path. When mail reaches its ultimate - destination (the forward-path contains only a destination - mailbox), the receiver-SMTP inserts it into the destination - mailbox in accordance with its host mail conventions. - - - - - -[Page 20] Postel - - - -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 - - - -Postel [Page 21] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - return path line. The return path line preserves the - information in the <reverse-path> from the MAIL command. - Here, final delivery means the message leaves the SMTP - world. Normally, this would mean it has been delivered to - the destination user, but in some cases it may be further - processed and transmitted by another mail system. - - It is possible for the mailbox in the return path be - different from the actual sender's mailbox, for example, - if error responses are to be delivered a special error - handling mailbox rather than the message senders. - - The preceding two paragraphs imply that the final mail data - will begin with a return path line, followed by one or more - time stamp lines. These lines will be followed by the mail - data header and body [2]. See Example 8. - - Special mention is needed of the response and further action - required when the processing following the end of mail data - indication is partially successful. This could arise if - after accepting several recipients and the mail data, the - receiver-SMTP finds that the mail data can be successfully - delivered to some of the recipients, but it cannot be to - others (for example, due to mailbox space allocation - problems). In such a situation, the response to the DATA - command must be an OK reply. But, the receiver-SMTP must - compose and send an "undeliverable mail" notification - message to the originator of the message. Either a single - notification which lists all of the recipients that failed - to get the message, or separate notification messages must - be sent for each failed recipient (see Example 7). All - undeliverable mail notification messages are sent using the - MAIL command (even if they result from processing a SEND, - SOML, or SAML command). - - - - - - - - - - - - - - - -[Page 22] Postel - - - -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 - - - -Postel [Page 23] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - mailboxes. For each recipient the mail data is delivered to - the recipient's terminal if the recipient is active on the - host (and accepting terminal messages), otherwise to the - recipient's mailbox. The argument field contains a - reverse-path. This command is successful if the message is - delivered to a terminal or the mailbox. - - 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 AND MAIL (SAML) - - This command is used to initiate a mail transaction in which - the mail data is delivered to one or more terminals and - mailboxes. For each recipient the mail data is delivered to - the recipient's terminal if the recipient is active on the - host (and accepting terminal messages), and for all - recipients to the recipient's mailbox. The argument field - contains a reverse-path. This command is successful if the - message is delivered to the mailbox. - - 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 - - - -[Page 24] Postel - - - -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. - - - - - - - - - - - - -[Page 50] Postel - - - -RFC 821 August 1982 - Simple Mail Transfer Protocol - - - -APPENDIX F - - Scenarios - - This section presents complete scenarios of several types of SMTP - sessions. - - A Typical SMTP Transaction Scenario - - This SMTP example shows mail sent by Smith at host USC-ISIF, to - Jones, Green, and Brown at host BBN-UNIX. Here we assume that - host USC-ISIF contacts host BBN-UNIX directly. The mail is - accepted for Jones and Brown. Green does not have a mailbox at - host BBN-UNIX. - - ------------------------------------------------------------- - - 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 - - ------------------------------------------------------------- - - - -Postel [Page 51] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - Aborted SMTP Transaction Scenario - - ------------------------------------------------------------- - - 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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - - - - - - - -[Page 52] Postel - - - -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 - - - - - - - - - - - - - - - - - -Postel [Page 53] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - Step 2 -- Relay Host to Destination Host - - 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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - -[Page 54] Postel - - - -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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - - - -Postel [Page 55] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - Sending and Mailing Scenarios - - First the user's name is verified, then an attempt is made to - send to the user's terminal. When that fails, the messages is - mailed to the user's mailbox. - - ------------------------------------------------------------- - - 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 - - ------------------------------------------------------------- - - - - - - -[Page 56] Postel - - - -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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - - - -Postel [Page 57] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - Mailing List Scenario - - First each of two mailing lists are expanded in separate sessions - with different hosts. Then the message is sent to everyone that - appeared on either list (but no duplicates) via a relay host. - - ------------------------------------------------------------- - - 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 - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 58] Postel - - - -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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Postel [Page 59] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - Step 3 -- Mailing to All via a Relay Host - - 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 - - ------------------------------------------------------------- - - - - - - - - - - - - -[Page 60] Postel - - - -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 - - ------------------------------------------------------------- - - - - - - - - - - - - - - - - - - - - - - -Postel [Page 61] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - ------------------------------------------------------------- - - 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 - - ------------------------------------------------------------- - - - - -[Page 62] Postel - - - -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. - - - -Postel [Page 63] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - -GLOSSARY - - ASCII - - American Standard Code for Information Interchange [1]. - - command - - A request for a mail service action sent by the sender-SMTP to the - receiver-SMTP. - - domain - - The hierarchially structured global character string address of a - host computer in the mail system. - - end of mail data indication - - A special sequence of characters that indicates the end of the - mail data. In particular, the five characters carriage return, - line feed, period, carriage return, line feed, in that order. - - host - - A computer in the internetwork environment on which mailboxes or - SMTP processes reside. - - line - - A a sequence of ASCII characters ending with a <CRLF>. - - mail data - - A sequence of ASCII characters of arbitrary length, which conforms - to the standard set in the Standard for the Format of ARPA - Internet Text Messages (RFC 822 [2]). - - 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@domain". Additionally, the "container" in which mail - is stored. - - - - - -[Page 64] Postel - - - -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. - - - - - -Postel [Page 65] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - 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 sequence of printing characters. - - <CRLF> - - The characters carriage return and line feed (in that order). - - <SP> - - The space character. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 66] Postel - - - -RFC 821 August 1982 - Simple Mail Transfer Protocol - - - -REFERENCES - - [1] 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. - - [2] RFC 822 - - Crocker, D., "Standard for the Format of ARPA Internet Text - Messages," RFC 822, Department of Electrical Engineering, - University of Delaware, August 1982. - - [3] TCP - - Postel, J., ed., "Transmission Control Protocol - DARPA Internet - Program Protocol Specification", RFC 793, USC/Information Sciences - Institute, NTIS AD Number A111091, September 1981. Also in: - Feinler, E. and J. Postel, eds., "Internet Protocol Transition - Workbook", SRI International, Menlo Park, California, March 1982. - - [4] 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. - - [5] 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. - - [6] 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. - - - - -Postel [Page 67] - - - -August 1982 RFC 821 -Simple Mail Transfer Protocol - - - - [7] 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -[Page 68] Postel - |