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.
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
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, "<string>" or "<reverse-path>". 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.,
 "<Smith@ISIA>" is an instance of <reverse-path> (the angle brackets
 are actually transmitted in the command or reply).
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
 <reverse-path> contains the source mailbox.
 MAIL <SP> FROM:<reverse-path> <CRLF>
 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 <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.
RFC 788 November 1981
 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
 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 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.
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:<Smith@Alpha>
 R: 250 OK
 S: RCPT TO:<Jones@Beta>
 R: 250 OK
 S: RCPT TO:<Green@Beta>
 R: 550 No such user here
 S: RCPT TO:<Brown@Beta>
 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
 -------------------------------------------------------------
RFC 788 November 1981
 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 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@ISI>
 R: 251 User not local; will forward to <Postel@ISIF>
 Or
 S: RCPT TO:<Paul@ISIB>
 R: 551 User not local; please try <Mockapetris@ISIF>
 Example 2
 -------------------------------------------------------------
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 <Postel@ISIF>
 Or
 S: VRFY Jones
 R: 550 String does not match anything.
 Example 3
 -------------------------------------------------------------
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 <Postel@ISIF>
 R: 250-Fred Fonebone <Fonebone@ISIQ>
 R: 250-Sam Q. Smith <SQSmith@ISIQ>
 R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
 R: 250-<joe@foo-unix>
 R: 250 <xyz@bar-unix>
 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.
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 <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.
RFC 788 November 1981
 Simple Mail Transfer Protocol
 The same reply codes that are used for the MAIL commands are used
 for these commands.
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 <SP> <host> <CRLF>
 QUIT <CRLF>
 In the HELO command the host sending the command identifies
 itself; the command may be interpreted as saying "Hello, i am
 <host>".
 -------------------------------------------------------------
 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
 -------------------------------------------------------------
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).
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
 -------------------------------------------------------------
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.
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 <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.
 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
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:<X@Y>
 TO:<@A,@B,C@D>
 will be relayed on to host B with arguments
 FROM:<@A,X@Y>
 TO:<@B,C@D>.
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 "<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
 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.
 The preceding two paragraphs imply that the final mail data
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.
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
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.
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.
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 <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.
November 1981 RFC 788
Simple Mail Transfer Protocol 
 The following are the SMTP commands:
 HELO <SP> <host> <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>
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.
 <reverse-path> ::= <path>
 <forward-path> ::= <path>
 <path> ::= "<" ["@" <host> "," ...] <mailbox> ">"
 <host> ::= <a> <string> | "#" <number> | "[" <dotnum> "]"
 <mailbox> ::= <user> "@" <host>
 <user> ::= <string>
 <string> ::= <char> | <char> <string>
 <char> ::= <c> | '\' <c> | '\' <s>
 <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
 <number> ::= <d> | <d> <number>
 <snum> ::= 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 except
 <specials>
 <d> ::= any one of the ten digits 0 through 9
 <s> ::= any one of <specials>
 <specials> ::= '<', '>', '(', ')', '\', ',', ';', ':', '@',
 '"', 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,円Smith" could be used to indicate a single nine character
 user field with comma being the fourth character of the field.
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-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
 <time-stamp-line> ::= "Mail-From:" <SP> <stamp> <CRLF>
 <stamp> ::= [<ptcl>] <from-host> <this-host> <daytime>
 <ptcl> ::= <protocol> <SP> "host" <SP>
 <from-host> ::= <host> <SP>
 <this-host> ::= "received by" <SP> <host> <SP>
 <protocol> ::= "TCP" | "NCP" | "NITS" | "X25" | "INTERNET" |
 "ARPANET"
 Note: INTERNET = TCP, ARPANET = NCP, and if the <ptcl> is
 not present INTERNET is assumed.
 <daytime> ::= "at" <SP> <date> <SP> <time>
 <date> ::= <dd> "-" <mon> "-" <yy>
 <time> ::= <hh> ":" <mm> ":" <ss> "-" <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 01 to 99.
RFC 788 November 1981
 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> ::= a time zone designator (as in [2]) or "UT" for
 Universal Time (the default).
 Return Path Example:
 Return-Path: <@CHARLIE,@BAKER,JOE@ABLE>
 Mail From Example:
 Mail-From: ABC received by XYZ at 22-OCT-81 09:23:59-PDT
November 1981 RFC 788
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, so there are likely to be varying texts
 for each reply code. A discussion of the theory of reply codes is
 given in the 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 command are expected to result in multiline replies in
 normal circumstances, however multiline replies are allowed for
 any command.
RFC 788 November 1981
 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 <host> Service ready
 221 <host> Service closing transmission channel
 421 <host> 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
November 1981 RFC 788
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 <host> Service ready
 221 <host> 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 <host> 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
RFC 788 November 1981
 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 "Awaiting input" 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 <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
November 1981 RFC 788
Simple Mail Transfer Protocol 
 RCPT
 S: 250, 251
 F: 550, 551, 552, 553, 450, 451, 452
 E: 500, 501, 421
 DATA
 I: 354 -> data -> S: 250
 F: 552, 554, 451, 452
 F: 451, 554
 E: 500, 501, 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
 F: 550
 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
RFC 788 November 1981
 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.
November 1981 RFC 788
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.
RFC 788 November 1981
 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 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 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 recipients mailbox
 including format effectors and other control characters. The
 7-bit ASCII codes are transmitted right justified in 8-bit
 bytes (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
 strings. If such transforms are necessary, they must be
 reversible -- especially if such transforms are applied to
 mail being relayed.
November 1981 RFC 788
Simple Mail Transfer Protocol 
 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.
 host
 The maximum total length of a host name or number is 40
 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.
 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).
RFC 788 November 1981
 Simple Mail Transfer Protocol
 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.
November 1981 RFC 788
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 a 8-bit byte with the high-order bit cleared to
 zero.
RFC 788 November 1981
 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 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 a 8-bit byte with the high-order bit cleared to
 zero.
November 1981 RFC 788
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 a 8-bit byte with the high-order bit cleared to
 zero.
RFC 788 November 1981
 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, but there are indications that it
 is too error prone to qualify as a reliable channel. It is
 suggested that a reliable end-to-end protocol such as TCP be used
 on top of X.25 connections.
November 1981 RFC 788
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
RFC 788 November 1981
 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
November 1981 RFC 788
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 multi-line replies requires that every line,
 except the last, begin with the reply code, followed
 immediately by a hyphen, "-" (also known as minus), followed by
 text. The last line will begin with the reply code, followed
 immediately by <SP>, optionally some text, and <CRLF>.
 For example:
 123-First line
 123-Second line
 123-234 text beginning with numbers
 123 The last line
 The sender-SMTP then simply needs to search for the reply code
 followed by <SP> at the beginning of a line, and ignore all
 preceding lines.
RFC 788 November 1981
 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 Simple Mail Transfer Service Ready
 S: HELO USC-ISIF
 R: 250 BBN-UNIX
 S: MAIL FROM:<Smith@USC-ISIF>
 R: 250 OK
 S: RCPT TO:<Jones@BBN-UNIX>
 R: 250 OK
 S: RCPT TO:<Green@BBN-UNIX>
 R: 550 No such user here
 S: RCPT TO:<Brown@BBN-UNIX>
 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 Service closing transmission channel
 Scenario 1
 -------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol 
 Aborted SMTP Transaction Scenario
 -------------------------------------------------------------
 R: 220 MIT-Multics Simple Mail Transfer Service Ready
 S: HELO ISI-VAXA
 R: 250 MIT-Multics
 S: MAIL FROM:<Smith@ISI-VAXA>
 R: 250 OK
 S: RCPT TO:<Jones@MIT-Multics>
 R: 250 OK
 S: RCPT TO:<Green@MIT-Multics>
 R: 550 No such user here
 S: RSET
 R: 250 OK
 S: QUIT
 R: 221 MIT-Multics Service closing transmission channel
 Scenario 2
 -------------------------------------------------------------
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Relayed Mail Scenario
 -------------------------------------------------------------
 Step 1 -- Source Host to Relay Host
 R: 220 USC-ISIE Simple Mail Transfer Service Ready
 S: HELO MIT-AI
 R: 250 USC-ISIE
 S: MAIL FROM:<JQP@MIT-AI>
 R: 250 OK
 S: RCPT TO:<@ISIE,Jones@BBN-VAX>
 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 at MIT-AI>
 S: Subject: The Next Meeting of the Board
 S: To: Jones at BBN-Vax
 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 Service closing transmission channel
November 1981 RFC 788
Simple Mail Transfer Protocol 
 Step 2 -- Relay Host to Destination Host
 R: 220 BBN-VAX Simple Mail Transfer Service Ready
 S: HELO USC-ISIE
 R: 250 BBN-VAX
 S: MAIL FROM:<@ISIE,JQP@MIT-AI>
 R: 250 OK
 S: RCPT TO:<Jones@BBN-VAX>
 R: 250 OK
 S: DATA
 R: 354 Start mail input; end with <CRLF>.<CRLF>
 S: Mail-From: NCP host MIT-AI received by USC-ISIE at
 2-Nov-81 22:40:10
 S: Date: 2-Nov-81 22:33:44
 S: From: John Q. Public <JQP at MIT-AI>
 S: Subject: The Next Meeting of the Board
 S: To: Jones at BBN-Vax
 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 Service closing transmission channel
 Scenario 3
 -------------------------------------------------------------
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Verifying and Sending Scenario
 -------------------------------------------------------------
 R: 220 SU-SCORE Simple Mail Transfer Service Ready
 S: HELO MIT-MC
 R: 250 SU-SCORE
 S: VRFY Crispin
 R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
 S: SEND FROM:<EAK@MIT-MC>
 R: 250 OK
 S: RCPT TO:<Admin.MRC@SU-SCORE>
 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 Service closing transmission channel
 Scenario 4
 -------------------------------------------------------------
November 1981 RFC 788
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 Simple Mail Transfer Service Ready
 S: HELO MIT-MC
 R: 250 SU-SCORE
 S: VRFY Crispin
 R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
 S: SEND FROM:<EAK@MIT-MC>
 R: 250 OK
 S: RCPT TO:<Admin.MRC@SU-SCORE>
 R: 450 User not active now
 S: RSET
 R: 250 OK
 S: MAIL FROM:<EAK@MIT-MC>
 R: 250 OK
 S: RCPT TO:<Admin.MRC@SU-SCORE>
 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 Service closing transmission channel
 Scenario 5
 -------------------------------------------------------------
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Doing the preceding scenario more efficiently.
 -------------------------------------------------------------
 R: 220 SU-SCORE Simple Mail Transfer Service Ready
 S: HELO MIT-MC
 R: 250 SU-SCORE
 S: VRFY Crispin
 R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
 S: SOML FROM:<EAK@MIT-MC>
 R: 250 OK
 S: RCPT TO:<Admin.MRC@SU-SCORE>
 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 Service closing transmission channel
 Scenario 6
 -------------------------------------------------------------
November 1981 RFC 788
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 Simple Mail Transfer Service Ready
 S: HELO SU-SCORE
 R: 250 MIT-AI
 S: EXPN Example-People
 R: 250-<ABC@MIT-MC>
 R: 250-Fred Fonebone <Fonebone@ISIQ>
 R: 250-Xenon Y. Zither <XYZ@MIT-AI>
 R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
 R: 250-<joe@foo-unix>
 R: 250 <xyz@bar-unix>
 S: QUIT
 R: 221 MIT-AI Service closing transmission channel
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Step 2 -- Expanding the Second List
 R: 220 MIT-MC Simple Mail Transfer Service Ready
 S: HELO SU-SCORE
 R: 250 MIT-MC
 S: EXPN Interested-Parties
 R: 250-Al Calico <ABC@MIT-MC>
 R: 250-<XYZ@MIT-AI>
 R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
 R: 250-<fred@BBN-UNIX>
 R: 250 <xyz@bar-unix>
 S: QUIT
 R: 221 MIT-MC Service closing transmission channel
November 1981 RFC 788
Simple Mail Transfer Protocol 
 Step 3 -- Mailing to All via a Relay Host
 R: 220 USC-ISIE Simple Mail Transfer Service Ready
 S: HELO SU-SCORE
 R: 250 USC-ISIE
 S: MAIL FROM:<Account.Person@SU-SCORE>
 R: 250 OK
 S: RCPT TO:<@ISIE,ABC@MIT-MC>
 R: 250 OK
 S: RCPT TO:<@ISIE,Fonebone@ISIQ>
 R: 250 OK
 S: RCPT TO:<@ISIE,XYZ@MIT-AI>
 R: 250 OK
 S: RCPT TO:<@ISIE,@ISIF,Q-Smith@ISI-VAXA>
 R: 250 OK
 S: RCPT TO:<@ISIE,joe@FOO-UNIX>
 R: 250 OK
 S: RCPT TO:<@ISIE,xyz@BAR-UNIX>
 R: 250 OK
 S: RCPT TO:<@ISIE,fred@BBN-UNIX>
 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 Service closing transmission channel
 Scenario 7
 -------------------------------------------------------------
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Forwarding Scenarios
 -------------------------------------------------------------
 R: 220 USC-ISIF Simple Mail Transfer Service Ready
 S: HELO LBL-UNIX
 R: 250 USC-ISIF
 S: MAIL FROM:<mo@LBL-UNIX>
 R: 250 OK
 S: RCPT TO:<fred@USC-ISIF>
 R: 251 User not local; will forward to <Jones@USC-ISIA>
 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 Service closing transmission channel
 Scenario 8
 -------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol 
 -------------------------------------------------------------
 Step 1 -- Trying the Mailbox at the First Host
 R: 220 USC-ISIF Simple Mail Transfer Service Ready
 S: HELO LBL-UNIX
 R: 250 USC-ISIF
 S: MAIL FROM:<mo@LBL-UNIX>
 R: 250 OK
 S: RCPT TO:<fred@USC-ISIF>
 R: 251 User not local; will forward to <Jones@USC-ISIA>
 S: RSET
 R: 250 OK
 S: QUIT
 R: 221 USC-ISIF Service closing transmission channel
 Step 2 -- Delivering the Mail at the Second Host
 R: 220 USC-ISIA Simple Mail Transfer Service Ready
 S: HELO LBL-UNIX
 R: 250 USC-ISIA
 S: MAIL FROM:<mo@LBL-UNIX>
 R: 250 OK
 S: RCPT TO:<Jones@USC-ISIA>
 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-ISIA Service closing transmission channel
 Scenario 9
 -------------------------------------------------------------
RFC 788 November 1981
 Simple Mail Transfer Protocol
 Too Many Recipients Scenario
 -------------------------------------------------------------
 R: 220 BERKELEY Simple Mail Transfer Service Ready
 S: HELO USC-ISIF
 R: 250 BERKELEY
 S: MAIL FROM:<Postel@USC-ISIF>
 R: 250 OK
 S: RCPT TO:<fabry@BERKELEY>
 R: 250 OK
 S: RCPT TO:<eric@BERKELEY>
 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>
 R: 250 OK
 S: RCPT TO:<eric@BERKELEY>
 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 Service closing transmission channel
 Scenario 10
 -------------------------------------------------------------
November 1981 RFC 788
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.
 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 line of text 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 Network
 Text Messages (RFC 733 [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@host". Additionally, the "container" in which mail is
 stored.
 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.
RFC 788 November 1981
 Simple Mail Transfer Protocol
 reply
 A reply is an acknowledgment (positive or negative) sent from
 receiver to sender via the transmission channel in response to a
 SMTP 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.
 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.
November 1981 RFC 788
Simple Mail Transfer Protocol 
 word
 A sequence of printing characters.
 <CRLF>
 The characters carriage return and line feed (in that order).
 <SP>
 The space character.
RFC 788 November 1981
 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 733
 Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
 the Format of ARPA Network Text Messages," RFC 733, NIC 41952,
 November 1977. Also in: Feinler, E. and J. Postel, eds.,
 "ARPANET Protocol Handbook", NIC 7104, for the Defense
 Communications Agency by SRI International, Menlo Park,
 California, Revised January 1978.
 [3] TCP
 Postel, J., ed., "Transmission Control Protocol - DARPA Internet
 Program Protocol Specification", RFC 793, USC/Information Sciences
 Institute, September 1981.
 [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.
November 1981 RFC 788
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.