RFC788 SIMPLE MAIL TRANSFER PROTOCOL Jonathan B. Postel November 1981 Information Sciences Institute University of Southern California 4676 Admiralty Way Marina del Rey, California 90291 (213) 822-1511  RFC 788 November 1981 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 ..................................... 10 3.5. Opening and Closing ..................................... 12 3.6. Relaying ................................................ 13 3.7. Domains ................................................. 15 4. THE SMTP SPECIFICATIONS ...................................... 16 4.1. SMTP Commands ........................................... 16 4.1.1. Command Semantics ..................................... 16 4.1.2. Command Syntax ........................................ 23 4.2. SMTP Replies ............................................ 28 4.2.1. Reply Codes by Function Group ......................... 29 4.2.2. Reply Codes in Numeric Order .......................... 30 4.3. Sequencing of Commands and Replies ...................... 31 4.4. State Diagrams .......................................... 33 4.5. Details ................................................. 35 4.5.1. Minimum Implementation ................................ 35 4.5.2. Transparency .......................................... 35 4.5.3. Sizes ................................................. 36 APPENDIX A: TCP ................................................. 38 APPENDIX B: NCP ................................................. 39 APPENDIX C: NITS ................................................ 40 APPENDIX D: X.25 ................................................ 41 APPENDIX E: Theory of Reply Codes ............................... 42 APPENDIX F: Scenarios ........................................... 45 GLOSSARY ......................................................... 58 REFERENCES ....................................................... 61  Network Working Group J. Postel Request for Comments: 788 ISI Replaces: RFC 780, 772 November 1981 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]  November 1981 RFC 788 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 full-duplex 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 788 November 1981 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]. Each 7-bit character is transmitted right justified in an 8-bit byte (or 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, "" or "". Here the angle brackets indicate these are a meta-linguistic variables. However, some arguments use the angle brackets literally. For example, an actual reverse-path is enclosed in angle brackets, i.e., "" is an instance of (the angle brackets are actually transmitted in the command or reply). Postel [Page 3]  November 1981 RFC 788 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, and a note on mail domains. 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 a SMTP mail transaction. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follow 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 contains the source mailbox. MAIL FROM: This command tells the 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 can contain more than just a mailbox. The is a reverse source routing list of hosts and source mailbox. The first host in the should be the host sending this command. The second step in the procedure is the RCPT command. RCPT TO: 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 788 November 1981 Simple Mail Transfer Protocol The can contain more than just a mailbox. The is a source routing list of hosts and destination mailbox. The first host in the should be the host receiving this command. The third step in the procedure is the DATA command. DATA 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 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 SMTP 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]  November 1981 RFC 788 Simple Mail Transfer Protocol ------------------------------------------------------------- Example of the SMTP Procedure This SMTP example shows mail sent by Smith at host Alpha, to Jones, Green, and Brown at host Beta. Here we assume that host Alpha contacts host Beta directly. S: MAIL FROM: R: 250 OK S: RCPT TO: R: 250 OK S: RCPT TO: R: 550 No such user here S: RCPT TO: R: 250 OK S: DATA R: 354 Start mail input; end with . S: Blah blah blah... S: ...etc. etc. etc. S: . 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 788 November 1981 Simple Mail Transfer Protocol 3.2. FORWARDING There are some cases where the destination information in the is incorrect, but the receiver-SMTP knows the correct destination. In such cases, one the following replies should be used to allow the sender to contact the correct destination. 251 User not local; will forward to 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 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: R: 251 User not local; will forward to Or S: RCPT TO: R: 551 User not local; please try Example 2 ------------------------------------------------------------- Postel [Page 7]  November 1981 RFC 788 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 a character string arguments. For the VRFY command, the string is a user name, and the 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. The case of verifying a user name is straightforward as shown in example 3. ------------------------------------------------------------- Example of Verifying a User Name Either S: VRFY Postel R: 250 Jon Postel Or S: VRFY Jones R: 550 String does not match anything. Example 3 ------------------------------------------------------------- [Page 8] Postel  RFC 788 November 1981 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 R: 250-Fred Fonebone R: 250-Sam Q. Smith R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA> R: 250- R: 250 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. Postel [Page 9]  November 1981 RFC 788 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). User's should have the ability to control the writing of messages on their terminals. Most hosts permit the user's 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 FROM: 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 FROM: 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 FROM: 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. [Page 10] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol The same reply codes that are used for the MAIL commands are used for these commands. Postel [Page 11]  November 1981 RFC 788 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 QUIT In the HELO command the host sending the command identifies itself; the command may be interpreted as saying "Hello, i am ". ------------------------------------------------------------- Example of Connection Opening R: 220 BBN-UNIX Simple Mail Transfer Service Ready S: HELO USC-ISIF R: 250 BBN-UNIX Example 5 ------------------------------------------------------------- ------------------------------------------------------------- Example of Connection Closing S: QUIT R: 221 BBN-UNIX Service closing transmission channel Example 6 ------------------------------------------------------------- [Page 12] Postel  RFC 788 November 1981 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. 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. 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 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). Postel [Page 13]  November 1981 RFC 788 Simple Mail Transfer Protocol This notification message must be from the server-SMTP at this host. Of course, server-SMTPs should not send notification messages about problems with notification messages. One way to prevent loops in error reporting is to specify a null reverse-path in the MAIL command of a notification message. When such a message is relayed it is permissible to leave the reverse-path null. A MAIL command with a null reverse-path appears as follows: MAIL FROM:<> An undeliverable mail notification message is shown in example 7. This notification is in response to a message originated by JOE at HOSTW and sent via HOSTX to HOSTY with instructions to relay it on to HOSTZ. What we see in the example is the transaction between HOSTY and HOSTX, which is the first step in the return of the notification message. ------------------------------------------------------------- Example Undeliverable Mail Notification Message S: MAIL FROM:<> R: 250 ok S: RCPT TO:<@HOSTX,JOE@HOSTW> R: 250 ok S: DATA R: 354 send the mail data, end with . S: Date: 23 Oct 81 S: Sender: SMTP@HOSTY S: Subject: Mail System Problem S: S: Sorry JOE, your message to SAM@HOSTZ lost. S: HOSTZ said this: S: "550 No Such User" S: . R: 250 ok Example 7 ------------------------------------------------------------- [Page 14] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol 3.7. DOMAINS At some not too distant future time it might be necessary to expand the mailbox format to include a region or name domain identifier. There is quite a bit of discussion on this at present, and is likely that SMTP will be revised in the future to take into account naming domains. The examples in this document do not show mail domains. Postel [Page 15]  November 1981 RFC 788 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 . The command codes themselves are alphabetic characters terminated by if parameters follow and 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. 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 [Page 16] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol 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 is 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. For example, mail received at relay host A with arguments FROM: TO:<@A,@B,C@D> will be relayed on to host B with arguments FROM:<@A,X@Y> TO:<@B,C@D>. Postel [Page 17]  November 1981 RFC 788 Simple Mail Transfer Protocol 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 "." (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 return path line. The return path line preserves the information in the 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. The preceding two paragraphs imply that the final mail data [Page 18] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol 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]. For example: Return-Path: <@GHI,@DEF,@ABC,JOE@ABC> Mail-From: GHI received by JKL at 27-Oct-81 15:27:39-PST Mail-From: DEF received by GHI at 27-Oct-81 15:15:13-PST Mail-From: ABC received by DEF at 27-Oct-81 15:01:59-PST Date: 27-Oct-81 15:01:01-PST From: JOE@ABC Subject: Improved Mailing System Installed To: SAM@JKL This is to inform you that ... 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). 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 the 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. Postel [Page 19]  November 1981 RFC 788 Simple Mail Transfer Protocol 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 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 the 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 [Page 20] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol 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 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. Postel [Page 21]  November 1981 RFC 788 Simple Mail Transfer Protocol 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. 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). 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. [Page 22] Postel  RFC 788 November 1981 Simple Mail Transfer Protocol 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. 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". The argument field consists of a variable length character string ending with the character sequence . 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. Postel [Page 23]  November 1981 RFC 788 Simple Mail Transfer Protocol The following are the SMTP commands: HELO MAIL FROM: RCPT TO: DATA RSET SEND FROM: SOML FROM: SAML FROM: VRFY EXPN HELP [ ] NOOP QUIT [Page 24] Postel  RFC 788 November 1981 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. ::= ::= ::= "<" ["@" "," ...] ">" ::= | "#" | "[" "]" ::= "@" ::= ::= | ::= | '\' | '\' ::= "." "." "." ::= | ::= three digits representing a decimal integer value in the range 0 through 255 ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case ::= any one of the 128 ASCII characters except ::= any one of the ten digits 0 through 9 ::= any one of ::= '<', '>', '(', ')', '\', ',', ';', ':', '@', '"', and the control characters (ASCII codes 0 through 37 octal inclusive and 177 octal) 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. Postel [Page 25]  November 1981 RFC 788 Simple Mail Transfer Protocol Hosts are generally known by names which are translated to addresses in each host. 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. The time stamp line and the return path line are formally defined as follows: ::= "Return-Path:" ::= "Mail-From:" ::= [] ::= "host" ::= ::= "received by" ::= "TCP" | "NCP" | "NITS" | "X25" | "INTERNET" | "ARPANET" Note: INTERNET = TCP, ARPANET = NCP, and if the is not present INTERNET is assumed. ::= "at" ::=