INTERNET-DRAFT John C. Klensin, Editor Expires in six months Dawn P. Mann, Co-Editor July 30, 1997 Simple Mail Transfer Protocol draft-ietf-drums-smtpupd-06.txt Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet-Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). If consensus is reached on this document, it will be forwarded to the IESG with the recommendation that it be processed onto the Standards track. [[Sections marked with doubled brackets (e.g., "<<") are explicit placeholders or known major loose ends. The marking ## is a note in the draft to recheck a section number and should be ignored.]] [[As discussed in the WG, most of the syntax and examples have not been changed from 821 -- those changes are a last- step item, after the ABNF document completely stabilizes and differences with 822bis have been resolved. Similarly, the numbers of appendices will be rationalized (and Appendix X removed) before the document is submitted to the IESG. Please check appendix X.2 for additional problems of which the editor is already painfully aware before complaining about things that are missing.]] TABLE OF CONTENTS 0. ABSTRACT 1. INTRODUCTION 2. THE SMTP MODEL 2.1 Basic structure 2.2 The extension model 2.3 Other terminology 2.4 Syntax Principles 3. THE SMTP PROCEDURES: AN OVERVIEW 3.1 Session initiation 3.2 Client initiation 3.3 Mail transactions 3.4 Forwarding for Address Correction or Updating 3.5 Commands for Debugging Addresses 3.6 Domains 3.7 Relaying 3.8 Mail Gatewaying 3.9 Terminating sessions and connections 3.10 Mailing lists and Aliases 4. THE SMTP SPECIFICATIONS 4.1. SMTP Commands 4.1.1. Command Semantics and Syntax 4.1.2. Lower-level Syntax 4.1.3. Address literals 4.1.4. Order of commands 4.1.5. Private-use commands 4.2. SMTP Replies 4.2.1. Reply Code Severities and Theory 4.2.2. Reply Codes by Function Group 4.2.3. Reply Codes in Numeric Order 4.2.4. Reply code 502 4.2.5 Reply codes after DATA and the subsequent CRLF.CRLF. 4.3. Sequencing of Commands and Replies 4.4 Trace information 4.5. Details 4.5.1. Minimum Implementation 4.5.2. Transparency 4.5.3. Sizes and Timeouts 4.5.4 SMTP Queuing Strategies 5. Address resolution and mail handling 6. Problem detection and handling 6.1 Reliable delivery and replies by email 6.2 Loop detection 6.3 Compensating for irregularities 7. Security Considerations 7.1 Mail security and spoofing 7.2 "Blind" copies 7.3 VRFY, EXPN, and security 7.4 Information disclosure 7.5 Scope of operation of SMTP servers 8. IANA Considerations 9. References 10. Editor's addresses 11. Acknowledgments APPENDIX A: TCP APPENDIX B: Generating SMTP commands from RFC 822 headers APPENDIX C: Source routes APPENDIX F: Scenarios APPENDIX G: Other gateway issues. APPENDIX I: Deprecated features of RFC 821 APPENDIX X: Change summary and Loose ends (temporary) 0. Abstract This document is a self-contained specification of the basic protocol for the Internet electronic mail transport, consolidating and updating * the original SMTP specification of RFC 821 [RFC-821], * Domain name system requirements and implications for mail transport from RFC 1035 [RFC-DNS] and RFC 974 [RFC974], * the clarifications and applicability statements in RFC 1123 [RFC-1123], and * material drawn from the SMTP Extension mechanisms [SMTPEXT]. It replaces RFC 821, RFC 974, and the mail transport materials of RFC 1123. However, RFC 821 specifies some features that are not in significant use in the Internet of the mid-1990s and (in appendices) some additional transport models. Those sections are omitted here in the interest of clarity and brevity; readers needing them should refer to RFC 821. It also includes some additional material from RFC 1123 that required amplification. This material has been identified in multiple ways, mostly by tracking flaming on the header-people list [HEADER-PEOPLE] and problems of unusual readings or interpretations that have turned up as the SMTP extensions have been deployed. Where this specification moves beyond consolidation and actually differs from earlier documents, it supersedes them technically as well as textually. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a 'mail posting' protocol, as recommended for POP [RFC-POP2, RFC-POP3] and IMAP [RFC-IMAP4]. Section ##2.3 provides definitions of terms specific to this document. Except when the historical terminology is necessary for clarity, this document uses the current 'client' and 'server' terminology to identify the sending and receiving SMTP processes, respectively. A companion document discusses message bodies and formats RFC 822, MIME, and their relationship - [MSGFMT]. 1. INTRODUCTION The objective of the 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. While this document specifically discusses transport over TCP, other transports are possible. Appendices to RFC 821 describe some of them. An important feature of SMTP is its capability to transport mail across transport service environments, usually referred to as "mail gatewaying" (see section 3.8). A transport service environment might consist of the mutually-TCP-accessible hosts on the public Internet, a firewall-isolated private TCP/IP LAN, or a LAN or WAN environment utilizing an entirely different transport-level protocol. It is important to realize that "transport systems" are one-to-one with usual definitions of "networks". A process can communicate directly with another process, and transport mail using this protocol, through any mutually known transport layer. Conversely, mail can be relayed (actually gatewayed) between hosts on different transport systems by a host on both transport systems. The Mail eXchanger mechanisms of the domain name system [RFC-DNS, and section ##5 of this document] usually permit relaying and gatewaying to occur invisibly to the user. 2. THE SMTP MODEL 2.1 Basic structure The SMTP design is based on the following model of communication: as the result of a user mail request (or transfer from a mail user agent (see section ##2.3)), the SMTP client establishes a two-way transmission channel to an SMTP server. A fully-capable SMTP client determines the address of an appropriate host running an SMTP server by resolving the domain name given in the SMTP request to either an intermediate mail exchanger host or a final target host. In other cases, common with clients associated with implementations of the POP [RFC-POP2, RFC-POP3] or IMAP [RFC-IMAP4] protocols, or when the client is inside an isolated transport service environment, the SMTP client may send all of its traffic to a single SMTP server which, in turn, relays the mail to final (or other intermediate) destinations. The relay and those destinations are expected to support all of the queuing, retrying, and alternate address functions discussed in this specification. The SMTP server may be either the ultimate destination or an intermediate "relay" (i.e., may assume the role of an SMTP client after receiving the message). SMTP commands are generated by the SMTP client and sent to the SMTP server. SMTP replies are sent from the SMTP server to the SMTP client in response to the commands. Once the transmission channel is established and initial handshaking completed, the SMTP client normally initiates a mail transaction. Such a transaction consists of a series of commands to specify the originator and destination of the mail and transmission of the message content (including any headers or other structure) itself. When the same message is sent to multiple recipients, this protocol encourages the transmission of only one copy of the data for all recipients at the same destination (or intermediate relay) host. The server responds to each command with a reply; replies may indicate that the command was accepted, that additional commands are expected, or that a temporary or permanent error condition exists. Commands specifying the sender or recipients may include server-permitted SMTP service extension requests as discussed in section ##2.2. The dialog is purposely lock-step, one-at-a-time, although this can be modified by mutually-agreed extension requests (e.g., [RFC-Pipeline]). Once a given mail message has been transmitted, the client may either request that the connection be shut down or may initiate other mail transactions. ------------------------------------------------------------- +----------+ +----------+ +------+ | | | | | User |<-->| | SMTP | | +------+ | Sender- |Commands/Replies| Receiver-| +------+ | SMTP |<-------------->| SMTP | +------+ | File |<-->| | and Mail | |<-->| File | |System| | | | | |System| +------+ +----------+ +----------+ +------+ SMTP client SMTP server Model for SMTP Use Figure 1 ------------------------------------------------------------- In addition, an SMTP client may use a connection to an SMTP server for ancillary services such as verification of email addresses or retrieval of mailing list subscriber addresses. As suggested above, this protocol provides mechanisms for the transmission of mail. This transmission normally occurs directly from the sending user's host to the receiving user's host when the two hosts are connected to the same transport service. When they are not connected to the same transport service, transmission occurs via one or more relay SMTP servers. An intermediate host that acts as either an SMTP relay or as a gateway into some other transmission environment may also be selected through the use of the domain name service (DNS) Mail eXchanger mechanism. To provide relay capability, the SMTP server is supplied with the name of the ultimate destination host as well as the destination mailbox name. Usually, intermediate hosts are determined via the DNS MX record, not by explicit "source" routing (see Appendices ##C and ##I). 2.2 The Extension Model 2.2.1 Background In an effort that started in 1990, approximately a decade after RFC 821 was completed, the protocol was modified with a "service extensions" model permitting the client and server to agree to utilize shared functionality beyond the original SMTP requirements. Contemporary SMTP implementations MUST support the basic extension mechanisms (see below for details), i.e., servers MUST support the EHLO command even if they do not implement any specific extensions and clients MUST preferentially utilize EHLO rather than HELO. (However, for compatibility with older conforming implementations, SMTP clients and servers MUST support the original HELO mechanisms as a fallback.) Unless the different characteristics of HELO must be identified for interoperability purposes, this document discusses only EHLO. SMTP is widely and deployed and high-quality implementations have proven to be very robust., However, the Internet community now considers some services to be important that were not anticipated when the protocol was first designed. If support for those services is to be added, it must be done in a way that permits older implementations to continue working acceptably. In an effort that started in 1990, approximately a decade after RFC 821 was completed, the protocol was modified with a "service extensions" model permitting the client and server to agree to utilize shared functionality beyond the original SMTP requirements. The SMTP extension mechanism defines a means whereby an extended SMTP client and server may recognize each other, and the server can inform the client as to the service extensions that it supports. The extension framework consists of: (1) The SMTP command EHLO, superseding the earlier HELO, (2) a registry of SMTP service extensions, (3) additional parameters to the SMTP MAIL FROM and RCPT TO commands, and (4) optional replacements for verbs defined in this protocol, such as for DATA (e.g., see [RFC-BDAT]). SMTP's strength comes primarily from its simplicity. Experience with many protocols has shown that: -- protocols with few options tend towards ubiquity, whereas -- protocols with many options tend towards obscurity. Each and every extension, regardless of its benefits, must be carefully scrutinized with respect to its implementation, deployment, and interoperability costs. In many cases, the cost of extending the SMTP service will likely outweigh the benefit. Contemporary SMTP implementations MUST support the basic extension mechanism (see below for details), i.e., servers MUST support the EHLO command even if they do not implement any specific extensions and clients MUST preferentially utilize EHLO rather than HELO. 2.2.2 Definition and Registration of Extensions The IANA maintains a registry of SMTP service extensions. A corresponding EHLO keyword value is associated with each extension . Each service extension registered with the IANA must be defined in aformal standards-track or IESG-approved experimental protocol document. The definition must include: (1) the textual name of the SMTP service extension; (2) the EHLO keyword value associated with the extension; (3) the syntax and possible values of parameters associated with the EHLO keyword value; (4) any additional SMTP verbs associated with the extension (additional verbs will usually be, but are not required to be, the same as the EHLO keyword value); (5) any new parameters the extension associates with the MAIL FROM or RCPT TO verbs; (6) a description of how support for the extension affects the behavior of a server and client SMTP; and, (7) the increment by which the extension is increasing the maximum length of the commands MAIL FROM and/or RCPT TO, over that specified in RFC 821. In addition, any EHLO keyword value starting with an upper or lower case "X" refers to a local SMTP service extension used exclusively through bilateral agreement. Keywords beginning with "X" may not be used in a registered service extension. Conversely, keyword values presented in the EHLO response that do not begin with "X" must correspond to a standard, standards-track, or IESG-approved experimental SMTP service extension registered with IANA. A conforming server MUST NOT offer non-"X"-prefixed keyword values that are not described in a registered extension. Additional verbs and parameter names are bound by the same rules as EHLO keywords; specifically, verbs beginning with "X" are local extensions that may not be registered or standardized. Conversely, verbs not beginning with "X" must always be registered. 2.3 Terminology Most of the terminology in this document is common in the Internet at the time of its writing. However, the following terms and concepts are used in special ways here, or represent differences in terminology between RFC 821 and this document, and should be understood before reading further. These definitions are normative, i.e., they contain specifications to which SMTP implementations are required to conform. 2.3.1 Mail objects SMTP transports a mail object containing an envelope and content. (1) The SMTP envelope is straightforward, and is sent as a series of SMTP protocol units (described in section ##3): it consists of an originator address (to which error reports should be directed); a delivery mode (e.g., deliver to recipient mailboxes); one or more recipient addresses; and optional protocol extension material. (2) The SMTP content is sent in the SMTP DATA protocol unit and has two parts: the headers and the body. The headers form a collection of field/value pairs structured as described in [MSGFMT]; the body, if structured, is defined according to MIME [RFC-MIME]. The content is textual in nature, expressed using the US ASCII repertoire[1]. Although extensions (such as MIME) may relax this restriction for the content body, the content headers are always encoded using the US ASCII repertoire. The algorithm defined in [RFC-INTLHDR] is used to represent header values outside the US ASCII repertoire, while still encoding them using the US ASCII repertoire. 2.3.2. Senders and receivers In RFC 821, the two hosts participating in an SMTP transaction were described as the "SMTP-sender" and "SMTP-receiver". This document has been changed to reflect current industry terminology and hence refers to them as the "SMTP client" (or sometimes just "the client") and "SMTP server" (or just "the server"), respectively. Since a given host may act both as server and client in a relay situation, "receiver" and "sender" terminology is still used where needed for clarity. 2.3.3. Mail agents Additional mail system terminology became common after RFC 821 was published and, where convenient, is used in this specification. In particular, SMTP servers and clients provide a mail transport service and therefore act as Mail Transfer Agents (MTAs). Mail User Agents (MUAs or UAs) are normally thought of as the sources and targets of mail. At the source, an MUA might collect mail to be transmitted from a user and hand it off to an MTA; the final ("delivery") MTA would be thought of as handing the mail off to an MUA (or at least transferring responsibility to it). However, while these terms are used with at least the appearance of great precision in other environments, the implied boundaries between MUAs and MTAs often do not accurately match common, and conforming, practices with Internet mail. Hence, the reader should be cautious about inferring the strong relationships and responsibilities that might be implied if these terms were used elsewhere. 2.3.4 host For the purposes of this specification, a host is a computer system attached to the Internet (or, in some cases, to a private TCP/IP network) and supporting the SMTP protocol. Hosts are known by names (see "domain"); identifying them by numerical address is discouraged. 2.3.5 domain The name of a host (often referred to as a "fully-qualified domain name" or "FQDN"), or some entry in the domain name hierarchy, usually referred to as a "subdomain", that may contain many hosts. A domain, or domain name, may also refer to an alias (label of a CNAME RR) or name the label of Mail eXchanger records to be used to deliver mail. See [RFC-DNS] and section ##5. The domain name, as described in this document and in [RFC-DNS], is the entire, fully-qualified name, and an apparent host name that is not in FQDN form is no more than a local alias. Local aliases MUST NOT appear in any SMTP transaction. In other works, if ab.cd.ef is the fully-qualified name of a host (or label for an MX record), then it is obviously a "domain". However, "cd.ef" may be only a domain name; it is possible for it to not refer to any host. 2.3.6 buffer and state table SMTP sessions are stateful, with both parties carefully maintaining a common view of the current state. In this document we model this state by a virtual "buffer" and a "state table" on the server which may be used by the client to, for example, "clear the buffer" or "reset the state table," causing the information in the buffer to be discarded and the state to be returned to some previous state 2.3.7 lines SMTP commands and, unless altered by a service extension, message data, are transmitted in "lines". Lines consist of zero or more data characters terminated by the ASCII sequence "CR" followed immediately by "LF". Conforming implementations MUST NOT recognize or generate any other character or character sequence as a line terminator. 2.3.8 Gateway, relay, originator, and delivery system This specification makes a distinction among four types of SMTP systems, based on the role those systems play in transmitting electronic mail. An "originating" system (sometimes called an SMTP originator) introduces mail into the Internet or, more generally, into a transport service environment. A "delivery" SMTP system is one that receives mail from a transport service environment and hands it to a mail user agent or deposits it in a maildrop which a mail user agent is expected to subsequently access. A "relay" SMTP system (usually referred to just as a "relay") receives mail from an SMTP client and transmits it, without modification to the message data other than adding trace information, to another SMTP server for further relaying or for delivery. A "gateway" SMTP system (usually referred to just as a "gateway") receives mail from a client system in one transport environment and transmits it to a server system in another transport environment. Differences in protocols or message semantics between the transport environments on either side of a gateway may require that the gateway system perform transformations to the message that are not permitted to SMTP relay systems. 2.3.9 Message content and message body The terms "message content" and "mail data" are used interchangably in this document to describe the material transmitted after the DATA command is accepted and before the end of data indication is transmitted. Message content includes message headers and the possibly-structured message body. The MIME specification [RFC-MIME] provides the Standard mechanisms for structured message bodies See ##2.3.1. 2.3.10 mailbox and address As used in this specification, an "address" is a character string that identifies a user to whom mail will be sent or a location into which mail will be deposited. The term "mailbox" refers to that depository. The two terms are typically used interchangeably unless the distinction between the location in which mail is placed (the mailbox) and a reference to it (the address) is important. An address normally consists of user and domain specifications. The standard mailbox naming convention is defined to be "local-part@domain": contemporary usage permits a much broader set of applications than simple "user names" and, consequently, the local-part is interpreted and assigned semantics only by the host specified in the domain part of the address. 2.3.11. reply An SMTP reply is an acknowledgment (positive or negative) sent from receiver to sender via the transmission channel in response to a command. The general form of a reply is a numeric completion code (indicating failure or success) followed by a text string. The codes are for use by programs and the text is usually intended for human users. 2.4 Syntax Principles 2.4.1 General syntax and transaction model The mail commands and replies have a rigid syntax. Replies also have a numeric code. Complete lists of commands and replies appear in Section ##4 "The SMTP Specification". 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 (this practice impedes interoperability and is discouraged), therefore, SMTP implementations MUST take care to preserve the case of user names as they appear in mailbox arguments. Domain names are not case sensitive. Commands and replies are composed of characters from the ASCII character set [1]. When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero. More specifically, the unextended SMTP service provides seven bit transport only. Originating SMTP clients MUST NOT transmit messages with information in the high-order bit of octets. If such messages are transmitted in violation of this rule, receiving SMTP servers MAY clear the high-order bit or reject the message as invalid. In general, a relay SMTP SHOULD assume that the message content it has received is valid and, assuming that the envelope permits doing so, relay it without inspecting that content. Of course, if the content is mislabelled and the data path cannot accept the actual content, this may result in ultimate delivery of a severely garbled message to the recipient. Delivery SMTP systems MAY reject ("bounce") such messages rather than deliver them. No sending SMTP system is permitted to send envelope commands in any character set other than US-ASCII; receiving systems SHOULD reject such commands, normally using "500 syntax error - invalid character" replies. Eight-bit message content transmission MAY be requested of the server by a client using extended SMTP facilities, notably the "8BITMIME" extension [8BITMIME]. 8BITMIME SHOULD be supported by SMTP servers. However, it MUST not be construed as authorization to transmit unrestricted eight bit material. 8BITMIME MUST NOT be requested by senders for material with the high bit on that is not in MIME format with an appropriate content-transfer encoding and servers MAY reject such messages. The metalinguistic notation used in this document corresponds to the "Augmented BNF" used in other Internet mail system documents. The reader who is not familiar with that syntax should consult [ABNF]. Metalanguage terms used in running text and examples are surrounded by pointed brackets (e.g., ) for clarity. 2.4.2 Command and reply syntax The commands consist of a command code followed by an argument field. Command codes are four alphabetic characters and are case insensitive. 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, case is important in the local-part within the reverse-path and forward-path arguments. In particular, in some hosts the user "smith" is different from the user "Smith". A few SMTP receiver systems, in violation of this specification (and RFC 821) require that a particular case be transmitted by clients. Implementations MAY wish to make provision to accommodate those systems. The argument field consists of a variable length character string ending with the character sequence . The receiver will take no action until this sequence is received. The syntax for each command is shown with the discussion of that command. Common elements and parameters are shown in section ##4.1.2. 3. THE SMTP PROCEDURES: AN OVERVIEW This section contains descriptions of the procedures used in SMTP: session initiation, the mail transaction, forwarding mail, verifying mailbox names and expanding mailing lists, and the opening and closing exchanges. Comments on relaying, a note on mail domains, and a discussion of changing roles are included at the end of this section. Examples of partial command and reply sequences are used throughout; several complete scenarios are presented in Appendix ##F. 3.1 Session initiation An SMTP session is initiated when a client opens a connection to a server and the server responds with an opening message. SMTP server implementations MAY include identification of their software and version information in the connection greeting reply after the 220 code (see section ##7.4), a practice that permits more efficient isolation and repair of any problems. Implementations MAY make provision for SMTP servers to disable the software and version announcement where it causes security concerns. While some systems also identify their contact point for mail problems, this is not a substitute for maintaining the required "postmaster" address (see [MSGFMT]). The SMTP protocol allows a server to formally reject a transaction while still allowing the initial connection as follows: a 554 response MAY be given in the initial connection opening message instead of the 220. A server taking this approach MUST still wait for the client to send a QUIT (see section ##4.1.1.10) before closing the connection and SHOULD respond to any intervening commands with "503 bad sequence of commands". Since an attempt to make an SMTP connection to such a system is probably in error, a server returning a 554 response on connection opening SHOULD provide enough information in the reply text to facilitate debugging of the sending system. Once the server has sent the welcoming message and the client has received it, the client then sends the EHLO command to the server, indicating the client’s identity. In addition to opening the session, use of EHLO indicates that the client is able to process service extensions and requests that the server provide a list of the extensions it supports. Older SMTP systems, unable to support service extensions, MAY use HELO instead of EHLO. Servers SHOULD NOT return the extended EHLO-style response to a HELO command. In the EHLO command the host sending the command identifies itself; the command may be interpreted as saying "Hello, I am " (and, in the case of EHLO, "and I support service extension requests"). ------------------------------------------------------------- | | Example of Connection Opening | | S: 220 UNIX.BBN.COM SMTP Ready - trashmail 1.99.a | C: EHLO ISIF.ISI.EDU | S: 250-UNIX.BBN.COM | S: 250 EXPN | | Example 1 | ------------------------------------------------------------- ------------------------------------------------------------- | | Example of Connection Closing | | S: QUIT | R: 221 UNIX.BBN.COM Service closing transmission channel | | Example 2 | ------------------------------------------------------------- 3.3. Mail Transactions There are three steps to SMTP mail transactions. The transaction starts with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. Then a DATA command initiates transfer of the mail data and is terminated by the "end of mail" data indicator, which also confirms the transaction. The first step in the procedure is the MAIL command. MAIL FROM: [ ] This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. The contains the source mailbox, which can be used to report errors (see section ##4.2 for a discussion of error reporting). If accepted, the SMTP server returns a 250 OK reply. If the mailbox specification is not acceptable for some reason, the server MUST return a reply indicating whether the failure is permanent (i.e., will occur again if the client tries to send the same address again) or temporary (i.e., the address might be accepted if the client tries again later). See section ##4.2.1. Normally, failures produce 550 or 553 replies. Historically, the can contain more than just a mailbox, however, contemporary systems SHOULD NOT use source routing (see Appendix ##C). The optional are associated with negotiated SMTP service extensions (see section ##2.2). The second step in the procedure is the RCPT command. RCPT TO: [ ] This command gives a forward-path (normally a mailbox and domain) identifying one recipient. If accepted, the SMTP server returns a 250 OK reply and stores the forward-path. If the recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible). This step of the procedure can be repeated any number of times. The can contain more than just a mailbox. Historically, the can be a source routing list of hosts and the destination mailbox, however, contemporary SMTP clients SHOULD NOT utilize source routes (see Appendix ##C). Servers MUST be prepared to encounter a list of source routes in the forward path, but SHOULD ignore the routes or MAY decline to support the relaying they imply. Similarly, servers MAY decline to accept mail that is destined for other hosts or systems. These restrictions make a server useless as a relay for clients that do not support full SMTP functionality. Consequently, restricted-capability clients MUST NOT assume that any SMTP server on the Internet can be used as their mail processing (relaying) site. If RCPT TO appears without a previous MAIL FROM, the server MUST return a 503 "Bad sequence of commands" response. The optional are associated with negotiated SMTP service extensions (see section ##2.2). The third step in the procedure is the DATA command (or some alternative specified in a service extension). DATA If accepted, the SMTP server returns a 354 Intermediate reply and considers all succeeding lines up to but not including the end of mail data indicator 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 mail data indicator 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 or full stop). A transparency procedure is used to prevent this from interfering with the user's text (see Section ##4.5.2). The end of mail data indicator also confirms the mail transaction and tells the SMTP server to now process the stored recipients and mail data. If accepted, the SMTP server returns a 250 OK reply. The DATA command can fail in only two ways: o If there was no MAIL FROM, or no RCPT TO, command, or all such commands were rejected, the server MAY return a "command out of sequence" (503) reply. If that reply is received, the client MUST NOT send the message data; more generally, message data MUST NOT be sent unless a 354 reply is received. o If the verb is initially accepted and the 354 reply issued, the DATA command should fail only if the mail transaction was incomplete (for example, no recipients), or if resources were unavailable. However, in practice, some servers do not perform recipient verification until after the message text is received. These servers SHOULD treat a failure for one or more recipients as a "subsequent failure" and return a mail message as discussed in section ##6. Using a "550 mailbox not found" (or equivalent) reply code after the data are accepted makes it difficult or impossible for the client to determine which recipients failed. When RFC 822 format is being used, the mail data include the memo header items such as Date, Subject, To, Cc, From [MSGFMT]. Server SMTP systems SHOULD NOT reject messages based on perceived defects in the RFC 822 or MIME [RFC-MIME] message header or message body. In particular, they MUST NOT reject messages in which the numbers of Resent- fields do not match or Resent-to appears without Resent-from and/or Resent-date. Mail transaction commands MUST be used in the order discussed above. Example 3 (below) illustrates the use of these commands in a mail transaction. ------------------------------------------------------------- | | Example of the SMTP Mail Transaction Procedure | | This SMTP example shows mail sent by Smith at host Alpha.foo, | to Jones, Green, and Brown at host Beta.org. Here we assume | that host Alpha contacts host Beta directly. | | C: MAIL FROM:. | C: Blah blah blah... | C: ...etc. etc. etc. | C: . | S: 250 OK | | The mail has now been accepted for Jones and Brown. Green did | not have a mailbox at domain Beta.org | | Example 3 | ------------------------------------------------------------- 3.4. Forwarding for Address Correction or Updating Forwarding support is most often required to consolidate and simplify addresses within, or relative to, some enterprise and less frequently to establish addresses to link a person’s prior address with a current one.. Silent forwarding of messages (without server notification to the sender), for security or non-disclosure purposes, is common in the contemporary Internet. In both the enterprise and the "new address" cases, information hiding (and sometimes security) considerations argue against exposure of the "final" address through the SMTP protocol as a side-effectof the forwarding activity. This may be especially important when the final address may not even be reachable by the sender. Consequently, the "forwarding" mechanisms described in section 3.2 of RFC 821, and especially the 251 (corrected destination) reply code from RCPT TO are deprecated: Servers SHOULD NOT provide that service or return that code. 3.5. Commands for Debugging Addresses 3.5.1 Overview SMTP provides commands to verify a user name or expand a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. Implementations MUST support VRFY and SHOULD support EXPN (however, see section ##3.5.2 and ##7.3). For the VRFY command, the string is a user name or a user name and domain (see below). The response MAYinclude the full name of the user and MUST include the mailbox of the user, e.g., it MUST be in either User Name or mailbox@domain form. When a name that is the argument to VRFY could identify more than one mailbox, the server MAY either note the ambiguity or identify the alternatives. In other words, either of the following are legitimate response to VRFY: 553 User ambiguous or 553- Ambiguous; Possibilities are 553-Joe Smith 553-Harry Smith 553 Melvin Smith or 553-Ambiguous; Possibilities 553- 553- 553 Under normal circumstances, a client receiving a 553 reply would be expected to expose the result to the user. Use of exactly the forms given, and the "user ambiguous" or "ambiguous" keywords, possibly supplemented by extended reply codes as described in [RFC-REPLY], will facilitate automated translation into other languages as needed. Of course, a client that was highly automated or that was operating in another language than English, might choose to try to translate the response, to return some other indication to the user than the literal text of the reply, or to take some automated action such as consulting a directory service for additional information before reporting to 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. In some hosts the distinction between a mailing list and an alias for a single mailbox is a bit fuzzy, since a common data structure may hold both types of entries, and it is possible to have mailing lists of one mailbox. If a request is made to verify a mailing list, a positive response can be given if a message so addressed would be delivered to everyone on the list, otherwise an error should be reported (e.g., "550 That is a mailing list, not a user"). If a request is made to expand a user name, the server MAY return a positive response consisting of a list containing one name, or an error MAY be reported (e.g., "550 That is a user name, not a mailing list"). In the case of a multiline reply (normal for EXPN) exactly one mailbox is to be specified on each line of the reply. The case of an ambiguous request is discussed above. "User name" is a fuzzy term and has been used deliberately. An implementation of the VRFY or EXPN commands MUST include at least recognition of local mailboxes as "user names". However, since current Internet practice often results in a single host handling mail for multiple domains, hosts, especially hosts that provide this functionality, SHOULD accept the "user@domain" form as a "user name"; hosts MAY also choose to recognize other strings as "user names". The case of verifying a user name is straightforward as shown in example 4. ----------------------------------------------------------------- | | Example of Verifying a User Name | | Either | | C: VRFY Smith | S: 250 Fred Smith | | Or | | C: VRFY Jones | S: 550 String does not match anything. | | Or | | C: VRFY Jones | S: 551 User not local; please try | | Or | | C: VRFY Gourzenkyinplatz | S: 553 User ambiguous. | | Or | | C: VRFY fizzle | S: 252-Cannot VRFY fizzle, but will accept message and | S: 252 attempt delivery | | Example 4 | ----------------------------------------------------------------- The case of expanding a mailbox list requires a multiline reply as shown in example 5. ------------------------------------------------------------- | | Example of Expanding a Mailing List | | Either | | C: EXPN Example-People | S: 250-Jon Postel | S: 250-Sam Q. Smith > | S: 250-Quincy Smith | S: 250 | | Or | | C EXPN Executive-Washroom-List | S: 550 Access Denied to You. | | Example 5 | ------------------------------------------------------------- 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 are a variety of file naming conventions in the Internet. 3.5.2 VRFY normal response. When normal (2yz or 551) responses are returned from a VRFY or EXPN request, the reply should normally include the mailbox name, e.g., "" (where "bar" is a fully qualified domain name) must appear in the syntax. In exceptional circumstances, free-form text MAY be returned. In order to facilitate parsing by both computers and people, addresses SHOULD appear in pointed brackets. EXPN and VRFY MUST return only valid domain addresses that are usable in SMTP RCPT commands. Consequently, if an address implies delivery to a program or other system, the mailbox name used to reach that target MUST be given. Paths (explicit source routes) MUST NOT be returned by VRFY or EXPN. Server implementations MUST support VRFY and SHOULD support EXPN. For security reasons, implementations MAY provide local installations a way to disable either or both of these commands through configuration options or the equivalent. When these commands are supported, they are not required to work across relays when relaying is supported. Since they were both optional in RFC 821, they MUST, if supported, be listed in the response to EHLO if service extensions are supported. 3.5.3 Meaning of VRFY or EXPN success response. A server MUST NOT return a 220 code in response to a VRFY or EXPN command unless it has actually verified the address. In particular, a server MUST NOT return 220 if all it has done is to verify that the syntax given is valid. In that case, 502 (Command not implemented) or 500 (Syntax error, command unrecognized) SHOULD be returned. As stated elsewhere, implementation of VRFY is required and EXPN is strongly recommended. Hence, except as provided in section ##7.3, implementations that return 500 or 502 for VRFY are not in compliance with this specification. There may be circumstances where an address appears to be valid but cannot reasonably be verified in real time, particularly when a server is acting as a mail exchanger for another server or domain. "Apparent validity" in this case would normally involve at least syntax checking and might involve verification that any domains specified were ones to which the host expected to be able to relay mail. In these situations, reply code 252 SHOULD BE returned. These cases parallel the discussion of RCPT verification discussed in section ##2.1 Implementations generally SHOULD be more aggressive about address verification in the case of VRFY than in the case of RCPT, even if it takes a little longer to do so. 3.5.4. Semantics and applications of EXPN. EXPN is often very useful in debugging and understanding problems with mailing lists and multiple-target-address aliases. Some systems have attempted to use source expansion of mailing lists as a means of eliminating duplicates. The propagation of aliasing systems with mail on the Internet--both for hosts (typically with MX and CNAME DNS records) and for mailboxes (various types of local host aliases)--has made it nearly impossible for these strategies to work, and mail systems SHOULD NOT attempt them. 3.6. Domains Only resolvable, fully-qualified, domain names (FQDNs) are permitted when domain names are used in SMTP. In other words, names that can be resolved to MX RRs or A RRs (as discussed in section ##5) are permitted, as are CNAME RRs whose targets can be resolved, in turn, to MX or A RRs. Local nicknames or unqualified names MUST NOT be used. There are two exceptions to this rule: (i) The domain name given in the EHLO command MUST BE either a primary host name (a domain name that resolves to an A RR) or, if the host has no name, an address literal as described in section ##4.1.1.1 and (ii) The reserved mailbox name "postmaster" may be used in a RCPT TO command without domain qualification (see section ##4.1.1.3). 3.7. RELAYING In general, the availability of Mail eXchanger records in the domain name system [RFC-DNS] makes the use of explicit source routes in the Internet mail system unnecessary. Many historical problems with their interpretation have made their use undesirable. SMTP clients SHOULD NOT generate explicit source routes except under unusual circumstances. SMTP servers MAY decline to act as mail relays or to accept addresses that specify source routes. They are also permitted to ignore the route information and simply send to the final destination specified as the last element in the route . There has been an invalid practice of using names that do not appear in the DNS as destination names, with the senders counting on the intermediate hosts specified in source routing to resolve any problems. If source routes are stripped, this practice will cause failures -- one of several reasons why SMTP clients MUST NOT generate invalid source routes or depend on serial resolution of names. When source routes are not used, the process described in RFC 821 for constructing a reverse-path from the forward-path is not applicable and the reverse-path at the time of delivery will simply be the address that appeared in the MAIL command. A relay SMTP server is usually the target of a DNS MX record that designates it, rather than the final delivery system. The relay server may accept or reject the task of relaying the mail in the same way it accepts or rejects mail for a local user. If it accepts the task, it then becomes an SMTP client, establishes a transmission channel to the next SMTP server specified in the DNS (according to the rules in section ##5), and sends it the mail. If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other 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). Formats specified for non-delivery reports by other standards SHOULD be used if possible. This notification message must be from the SMTP server at the relay host or the host that first determines that delivery cannot be accomplished. Of course, SMTP servers MUST NOT send notification messages about problems transporting 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 transmitted the reverse-path MUST beset to null. A MAIL command with a null reverse-path appears as follows: MAIL FROM:<> An undeliverable mail notification message is shown in example 6. This notification is in response to a message originated by JOE at xyz.somecollege.edu and sent to a user on HOSTY.org ------------------------------------------------------------- | | Example Undeliverable Mail Notification Message | | C: MAIL FROM:<> | S: 250 ok | C: RCPT TO:< JOE@xyz.somecollege.edu> | S: 250 ok | C: DATA | S: 354 send the mail data, end with . | C: Date: 23 Oct 81 11:22:33 | C: From: SMTP@HOSTY.org | C: To: JOE@xyz.somecollege.edu | C: Subject: Mail System Problem | C: <> | C: . | S: 250 ok | | Example 6 | As discussed in section ##2.4.1, a relay SMTP has no need to inspect or act upon the headers or body of the message data and MUST NOT do so. 3.8 Mail Gatewaying While the relay function discussed above operates within the Internet SMTP transport service environment, MX records or various forms of explicit routing may require that an intermediate SMTP server perform a translation function between one transport service and another. As discussed in section ##2.3.8, when such a system is at the boundary between two transport service environments, we refer to it as a "gateway" or "gateway SMTP". Gatewaying mail between different mail environments, i.e., different mail formats and protocols, is complex and does not easily yield to standardization. However, some general requirements may be given for a gateway between the Internet and another mail environment. 3.8.1 Header fields MAY be rewritten when necessary as messages are gatewayed across mail environment boundaries. This may involve inspecting the message body or interpreting the local-part of the destination address in spite of the prohibitions in section ##2.4.1 Other mail systems gatewayed to the Internet often use a subset of RFC-822 headers or provide similar functionality with a different syntax, but some of these mail systems do not have an equivalent to the SMTP envelope. Therefore, when a message leaves the Internet environment, it may be necessary to fold the SMTP envelope information into the message header. A possible solution would be to create new header fields to carry the envelope information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would require changes in mail programs in foreign environments. 3.8.2 When forwarding a message into or out of the Internet environment, a gateway MUST prepend a Received: line, but it MUST NOT alter in any way a Received: line that is already in the header. Received: fields of messages originating from other environments may not conform exactly to this specification. However, the most important use of Received: lines is for debugging mail faults, and this debugging can be severely hampered by well-meaning gateways that try to "fix" a Received: line. As another consequence of trace fields arising in non-SMTP environments, receiving systems MUST NOT reject mail based on the format of a trace field and SHOULD be extremely robust in the light of unexpected information or formats in those fields. The gateway SHOULD indicate the environment and protocol in the "via" clauses of Received field(s) that it supplies. 3.8.3 From the Internet side, the gateway SHOULD accept all valid address formats in SMTP commands and in RFC-822 headers, and all valid RFC-822 messages. Gateways are, of course, subject to the same rules for handling source routes as those described for other SMTP systems in section ##3.3. 3.8.4 The gateway MUST ensure that all header fields of a message that it forwards into the Internet meet the requirements for Internet mail. In particular, all addresses in "From:", "To:", "Cc:", etc., fields MUSTbe transformed (if necessary) to satisfy RFC-822 syntax, MUST reference only fully-qualified domain names, and MUSTbe effective and useful for sending replies. 3.8.5 The translation algorithm used to convert mail from the Internet protocols to another environment's protocol SHOULD ensure that error messages from the foreign mail environment are delivered to the return path from the SMTP envelope, not to the sender listed in the "From:" field (or other fields) of the RFC-822 message. 3.8.6 Similarly, when forwarding a message from another environment into the Internet, the gateway SHOULD set the envelope return path in accordance with an error message return address, if supplied by the foreign environment. If the foreign environment has no equivalent concept, the gateway must select and use a best approximation, with the message originator’s address as the default of last resort. 3.9. Terminating Sessions and Connections An SMTP connection is terminated when the client sends a QUIT command. The server responds with a positive reply code, after which it closes the connection. An SMTP server MUST NOT intentionally close the connection except: o After receiving a QUIT command and responding with a 221 reply. o After detecting the need to shutdown the SMTP service and returning a message with a 451 response code. This response code can be issued after the server receives any command or, if necessary, asynchronously from command receipt (on the assumption that the client will receive it after the next command is issued). In particular, a server that closes connections in response to commands that are not understood is in violation of this specification. Servers are expected to be tolerant of unknown commands, issuing a 500 reply and awaiting further instructions from the client. An SMTP server which is forcibly shut down via external means SHOULD attempt to send a line containing 451 response code to the SMTP client before exiting. The SMTP client will normally read the 451 response code after sending its next command. SMTP clients that experience a connection close, reset, or other communications failure due to circumstances not under their control (in violation of the intent of this specification but sometimes unavoidable) should, to maintain the robustness of the mail system, treat the mail transaction as if a 451 response had been received and act accordingly. 3.10 Mailing Lists and Aliases An SMTP-capable host SHOULD support both the alias and the list form of address expansion for multiple delivery. When a message is delivered or forwarded to each address of an expanded list form, the return address in the envelope ("MAIL FROM:") MUST be changed to be the address of a person or other entity who administers the list but the message header MUST be left unchanged; in particular, the "From" field of the message is unaffected. An important mail facility is a mechanism for multi-destination delivery of a single message, by transforming or "expanding" a pseudo-mailbox address into a list of destination mailbox addresses. When a message is sent to such a pseudo-mailbox (sometimes called an "exploder"), copies are forwarded or redistributed to each mailbox in the expanded list. We classify such a pseudo-mailbox as an "alias" or a "list", depending upon the expansion rules. 3.10.1 Alias To expand an alias, the recipient mailer simply replaces the pseudo-mailbox address in the envelope with each of the expanded addresses in turn; the rest of the envelope and the message body are left unchanged. The message is then delivered or forwarded to each expanded address. 3.10.11 List A mailing list may be said to operate by "redistribution" rather than by "forwarding". To expand a list, the recipient mailer replaces the pseudo-mailbox address in the envelope with each of the expanded addresses in turn. The return address in the envelope is changed so that all error messages generated by the final deliveries will be returned to a list administrator, not to the message originator, who generally has no control over the contents of the list and will typically find error messages annoying. 4. THE SMTP SPECIFICATIONS 4.1. SMTP COMMANDS 4.1.1. COMMAND SEMANTICS AND SYNTAX 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 the local part of a mailbox must conform to receiver site conventions and the syntax specified in section ##4.1.2. The SMTP commands are discussed below. The SMTP replies are discussed in 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. 4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO) These commands are used to identify the SMTP client to the SMTP server. The argument field contains the fully-qualified domain name of the SMTP client if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is available), the client should send an address literal (see section ##4.1.3), optionally followed by information that will help to identify the client system. The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command. A client SMTP SHOULD start an SMTP session by issuing the EHLO command. If the SMTP server supports the SMTP service extensions it will give a successful response, a failure response, or an error response. If the SMTP server, in violation of this specification, does not support any SMTP service extensions it will generate an error response. Older client SMTP systems MAY, as discussed above, use HELO (as specified in RFC 821) instead of EHLO and servers MUST support the HELO command and reply properly to it. These commands, and a "250 OK" reply to one of them, confirm that both the SMTP client and the SMTP server are in the initial state, that is, there is no transaction in progress and all state tables and buffers are cleared. Normally, the response to EHLO will be a multiline reply. Each line of the response contains a keyword and, optionally, one or more parameters. The syntax for a positive response, using the ABNF notation and low-level terminals of [ABNF], is: ehlo-ok-rsp ::= "250" domain [ SP greeting ] CR LF / ( "250-" domain [ SP greeting ] CR LF *( "250-" ehlo-line CR LF ) "250" SP ehlo-line CR LF ) greeting ::= 1* ehlo-line ::= ehlo-keyword *( SP ehlo-param ) ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") ; syntax and values depend on ehlo-keyword ehlo-param ::= 1* [[xxx ALPHA ::= DIGIT ::= CR ::= LF ::= SP ::= /xxx]] Although EHLO keywords may be specified in upper, lower, or mixed case, they must always be recognized and processed in a case-insensitive manner. This is simply an extension of practices specified in RFC 821 and section ##2.4.1. 4.1.1.2 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 the sender mailbox or a list of hosts as described in Appendix C. In some types of reporting messages for which a reply is likely to cause a mail loop (for example, mail delivery and nondelivery notifications), the reverse-path may be null (see section ##3.7). 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. If service extensions were negotiated, the MAIL command may also carry parameters associated with a particular service extension. Syntax: "MAIL FROM:" Reverse-path [ SP Mail-parameters ] or "MAIL FROM:<>" 4.1.1.3 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 normally consists of the required destination mailbox(es). Sending systems SHOULD not generate the optimal list of hosts known as a source route. Receiving systems MUST recognize source route syntax but SHOULD strip off the source route specification and utilize the domain name associated with the mailbox as if the source route had not been provided. Similarly, relay hosts SHOULD strip or ignore source routes, and names MUST NOT be copied into the reverse-path. When mail reaches its ultimate destination (the forward-path contains only a destination mailbox), the SMTP server inserts it into the destination mailbox in accordance with its host mail conventions. For example, mail received at relay host A with envelope commands MAIL FROM: will normally be sent directly on to host D.bar.org with envelope commands MAIL FROM: RCPT TO: as provided in Appendix C, HostA MAY also choose to relay the message to HostB, using the envelope commands MAIL FROM: RCPT TO:<@HOSTB.int:USERC@D. BAR.ORG> Of course, since hosts are not required to relay mail at all, HostA may also reject the message entirely when the RCPT TO command is received, using a 550 code (since this is a "policy reason"). If service extensions were negotiated, the RCPT TO command may also carry parameters associated with a particular service extension offered by the server. The client MUST NOT transmit parameters other than those associated with a service extension offered by the server in its EHLO response. Syntax: "RCPT TO:" Forward-path [ SP Rcpt-parameters ] or "RCPT TO:" [ SP Rcpt-parameters ] 4.1.1.4 DATA (DATA) The receiver treats the lines (strings ending in CRLF sequences, see section ##2.3.7) following the command as mail data from the sender. This command causes the mail data to be appended to the mail data buffer. The mail data may contain any of the 128 ASCII character codes, although experience has indicated that use of control characters other than SP, HT, CR, and LF may cause problems and should be avoided when possible. 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. Note that the first of this terminating sequence is also the that ends the final line of the data (message text) or, if there was no data, ends the DATA command itself. An extra MUST NOT be added, as that would cause an empty line to be added to the message. The only exception to this rule would arise if the message body were passed to the originating SMTP-sender with a final "line" that did not end in ; in that case, the originating SMTP system MUST either reject the message as invalid or add in order to have the receiving SMTP server recognize the "end of data" condition. The custom of accepting lines ending only in , as a concession to non-conforming behavior on the part of some UNIX systems, has proven to cause more interoperability problems than it solves, and SMTP server systems MUST NOT do this, even in the name of improved robustness. In particular, the sequence "." (bare line feeds, without carriage returns) MUST NOT be treated as equivalent to . as the end of mail data indication. Receipt of the end of mail data indication requires the server to 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 the receiver must send a failure reply. The SMTP model does not allow for partial failures at this point: either the message is accepted by the server for delivery and a positive response is returned or it is not accepted and a failure reply is returned. Errors that are diagnosed subsequently MUST be reported in a mail message, as discussed in section ##4.4 In sending a positive completion reply to the end of data indication, the receiver takes full responsibility for the message (see section ##6.1). When the SMTP server accepts a message either for relaying or for final delivery, it inserts a trace record (also referred to interchangeably as a "time stamp line" or "Received" line) at the top of the mail data. This trace record indicates the identity of the host that sent the message, 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. Details for formation of these lines, including their syntax, is specified in section ##4.4. 4.1.1.5 RESET (RSET) This command specifies that the current mail transaction will be aborted. Any stored sender, recipients, and mail data MUST be discarded, and all buffers and state tables cleared. The receiver MUST send a "250 OK" reply. A reset command may be issued by the client at any time. It is effectively equivalent to a NOOP if issued immediately after EHLO, or before either of those commands are issued. In other situations, it restores the state to that immediately after the most recent EHLO. An SMTP server MUST NOT close the connection as the result of receiving a RSET; that action is reserved for QUIT (see section ##4.1.1.10, below). Since EHLO imply some additional processing and response by the server, RSET will normally be more efficient than reissuing those commands, even though the formal semantics are the same. There are circumstances, contrary to the intent of this specification, in which an SMTP server may receive an indication that the underlying TCP connection has been closed or reset. To preserve the robustness of the mail system, SMTP servers should be prepared for this condition and should treat it as if a RSET, followed by a QUIT, had been received before the connection disappeared. 4.1.1.6 VERIFY (VRFY) This command asks the receiver to confirm that the argument identifies a user or mailbox. If it is a user name, information is returned as specified in section ##3.5. This command has no effect on the reverse-path buffer, the forward-path buffer, or the mail data buffer. Syntax: "VRFY" SP String 4.1.1.7 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. If the command is successful, a multiline reply is returned containing information as described in section ##3.5. This command has no effect on the reverse-path buffer, the forward-path buffer, or the mail data buffer. Syntax: "EXPN" SP String 4.1.1.8 HELP (HELP) This command causes the server to send helpful information to the client. 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 the reverse-path buffer, the forward-path buffer, or the mail data buffer. SMTP servers SHOULD support HELP without arguments and MAY support it with arguments. Syntax: "HELP" [ SP String ] 4.1.1.9 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 the reverse-path buffer, the forward-path buffer, or the mail data buffer. Syntax: "NOOP" [SP String] 4.1.1.10 QUIT (QUIT) This command specifies that the receiver must send an OK reply, and then close the transmission channel. The receiver MUST NOT intentionally close the transmission channel until it receives and replies to a QUIT command (even if there was an error). The sender MUST NOT intentionally close the transmission channel until it sends a QUIT command and receives the reply (even if there was an error response to a previous command). If the connection is closed prematurely due to violations of the above or system or network failure, the server MUST act as if a RSET command had been received (canceling any pending transaction, but not undoing any previously completed transaction) and the client MUST act as if the command or transaction in progress had received a temporary error (4xx). Syntax: "QUIT" 4.1.2. LOWER-LEVEL SYNTAX The syntax of the argument fields of the above commands (using the syntax specified in [ABNF] where applicable) is given below. Some of the productions given below are used only in conjunction with source routes as described in Appendix C. Reverse-path ::= Path Forward-path ::= Path Path ::= "<" [ A-d-l ":" ] ">" A-d-l ::= At-domain *( "," A-d-l ) At-domain ::= "@" Domain Mail-parameters ::= *( SP Keyword "=" Argument ) Rcpt-parameters ::= *( SP Keyword "=" Argument ) Keyword ::= String <<>>??? Argument ::= String <<>>??? Domain ::= sub-domain 1*("." sub-domain) | address-literal sub-domain ::= let-dig *(ldh-str) address-literal ::= "[" IPv4-address-literal | IPv6-address-literal | General-address-literal "]" IPv4-address-literal ::= snum 3("." snum) IPv6-address-literal ::= "IPv6" SP <> General-address-literal ::= Standardized-tag SP String Standardized-tag ::= String (Specified in a standards-track RFC and registered with IANA) snum = one, two, or three digits representing a decimal integer value in the range 0 through 255 let-dig = Alpha / Digit ldh-str = *( Alpha / Digit / "-" ) let-dig << Placeholder: the following are not right, and the "alpha" one <> Alpha = ASCII character in the range A-Z or a-z. As specified in the domain name system definition [RFC-DNS], case is not significant in domain strings. Digit = 0 - 9 Mailbox ::= Local-part "@" Domain Local-part ::= Dot-string | Quoted-string While the above definition for Local-part is relatively permissive, for maximum interoperability, a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form or where the Local-part is case-sensitive. For any purposes that require generating or comparing Local-parts (e.g., to specific mailbox names), all quoted forms MUST be treated as equivalent and the sending system SHOULD transmit the form that uses the minimum quoting possible. Systems MUST NOT define mailboxes in such a way as to require the use of non-ASCII characters (octets with the high order bit set to one) or ASCII "control characters" (decimal value 0-31 and 127). These characters MUST NOT be used in MAIL FROM or RCPT TO commands or other commands that require mailbox names. <> ::= | <> ::= """ """ <> ::= "\" | "\" | | ::= | "\" ::= | ::= ::= the carriage return character (ASCII code 13) ::= the line feed character (ASCII code 10) ::= the space character (ASCII code 32) ::= 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, but not any or ::= any one of the ten digits 0 through 9 ::= any one of the 128 ASCII characters except , , quote ("), or backslash (\) ::= any one of the 128 ASCII characters (no exceptions) ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "." | "," | ";" | ":" | "@" """ | the control characters (ASCII codes 0 through 31 inclusive and 127) Note that the backslash, "\", is a quote character, which is used to indicate that the next character is to be used literally (instead of its normal interpretation). For example, "Joe\,Smith" indicates a single nine character user field with the comma being the fourth character of the field. Characters outside the set of alphas, digits, and hyphen MUST NOT appear in domain names. In particular, the underscore character is not permitted. 4.1.3. Address literals Sometimes a host is not known to the domain name system and communication (and, in particular, communication to report and repair the error) is blocked. To bypass this barrier a special literal form of the address is allowed as an alternative to a domain name. For IPv4 addresses, this form uses four or more small decimal integers separated by dots and enclosed by brackets, e.g., [123.255.37.2], which indicates an (IPv4) Internet Address in sequence-of-octets form. For IPv6 and other forms of addressing that might eventually be standardized, the form consists of a standardized "tag" that identifies the address syntax, a space, and the address itself, in a format specified elsewhere. (where?)<> 4.1.4. Order of commands There are restrictions on the order in which these commands may be used. A session that will contain mail transactions MUST first be initialized by the use of the EHLO command. An SMTP server SHOULD accept commands for non-mail transactions (e.g., VRFY or EXPN) without this initialization. An EHLO command MAY be issued by a client later in the session. If it is issued after the session begins, the SMTP server MUST clear all buffers and reset the state exactly as if a RSET command had been issued. In other words, the sequence of RSET followed immediately by EHLO is redundant, but not harmful other than in the performance cost of executing unnecessary commands. If the EHLO command is not acceptable to the SMTP server, 501, 500, or 502 failure replies MUST be returned as appropriate. The SMTP server must stay in the same state after transmitting these replies that it was in before the EHLO was received. The SMTP client MUST ensure that the domain parameter to the EHLO command is a valid principal host name (not a CNAME or MX name) for its host. If this is not possible (e.g., when the client's address is dynamically assigned and the client does not have an obvious name), an address literal SHOULD be substituted for the domain name and supplemental information provided that will assist in identifying the client. An SMTP server MAY verify that the domain name parameter in the EHLO command actually corresponds to the IP address of the client. However, the server MUST NOT refuse to accept a message if the verification fails -- the information about verification failure is for logging and tracing only. The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time during a session, or without previously initializing a session. SMTP servers SHOULD process these normally (i.e., not return a 503 code) even if no EHLO command has yet been received; clients SHOULD open a session with EHLO before sending these commands. If these rules are followed, the example in RFC 821 that shows "550 access denied to you" in response to an EXPN command is incorrect unless an EHLO command precedes the EXPN or the denial of access is based on the client's IP address. The MAIL command (or the obsolete SEND, SOML, or SAML commands) begins a mail transaction. Once started, a mail transaction consists of a transaction beginning commands, one or more RCPT commands, and a DATA command, in that order. A mail transaction may be aborted by the RSET (or a new EHLO) command. There may be zero or more transactions in a session. If the transaction beginning command argument is not acceptable, a 501 failure reply MUST be returned and the SMTP server must stay in the same state. If the commands in a transaction are out of order to the degree that they cannot be processed by the server, a 503 failure reply MUST be returned and the SMTP server must stay in the same state. The last command in a session must be the QUIT command. The QUIT command cannot be used at any other time in a session, but SHOULD be used by the client SMTP to request connection closure, even when no session opening command was sent and accepted. 4.1.5 Private-use commands As specified in section 2.2.2, commands starting in "X" may be used by bilateral agreement between the client (sending) and server (receiving) SMTPs. An SMTP server that does not recognize such a command is expected to reply with "500 Command not recognized". An extended SMTP server MAY list the feature names associated with these private commands in the response to the EHLO command. Commands sent or accepted by SMTP systems that do not start with "X" MUST conform to the requirements of section ##2.2.2, above. 4.2. SMTP REPLIES Replies to SMTP commands serve to ensure the synchronization of requests and actions in the process of mail transfer and to guarantee that the SMTP client always knows the state of the SMTP server. Every command must generate exactly one reply. The details of the command-reply sequence are described in Section ##4.3 on Sequencing. An SMTP reply consists of a three digit number (transmitted as three alphanumeric characters) followed by some text. The number is for use by automata to determine what state to enter next; the text is for the human user. The three digits contain enough encoded information that the SMTP client need not examine the text and may either discard it or pass it on to the user, as appropriate. Exceptions are as noted elsewhere in this document. In particular, the 220, 221, 251, 421, and 551 reply codes are associated with message text that must be parsed and interpreted by machines. In the general case, the text may be receiver dependent and context dependent, so there are likely to be varying texts for each reply code. A discussion of the theory of reply codes is given insection ##4.2.1. 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 insection ##4.2.1). Only the EXPN and HELP commands are expected to result in multiline replies in normal circumstances, however, multiline replies are allowed for any command. An SMTP server SHOULD send only the reply codes listed in this document. An SMTP server SHOULD use the text shown in the examples whenever appropriate. A client SMTP MUST determine its actions only by the reply code, not by the text (except for 251 and 551 and, if necessary, 220, 221, and 421 replies); in the general case, any text, including no text at all (although senders SHOULD NOT send bare codes), MUSTbe acceptable. The space (blank) following the reply code is considered part of the text. Whenever possible, a sender-SMTP SHOULD test the first digit (severity indication) of the reply code. The list of codes that appears below must not be construed as permanent. While the addition of new codes should be a rare and significant activity, with supplemental information in the textual part of the response being preferred, new codes may be added as the result of new Standards or Standards-track specifications. Consequently, a sender-SMTP MUST be prepared to handle codes not specified in this document and MUST do so by interpreting the first digit only. 4.2.1. REPLY CODE SEVERITIES AND THEORY 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 SMTP client, or one that receives an unexpected code, will be able to determine its next action (proceed as planned, redo, retrench, etc.) by examining this first digit. An SMTP client that wants to know approximately what kind of error occurred (e.g., mail system error, command syntax error) may examine the second digit. The third digit and any supplemental information that may be present is reserved 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 SMTP client should send another command specifying whether to continue or abort the action. [Note: unextended SMTP does not have any commands that allow this type of reply, and so does not have 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 SMTP client should send another command specifying this information. This reply is used in command sequence groups (i.e., in DATA). 4yz Transient Negative Completion reply The command was not accepted, and the requested action did not occur. However, the error condition is temporary and the action may be requested again. The sender should return to the beginning of the command sequence (if any). It is difficult to assign a meaning to "transient" when two different sites (receiver- and sender- SMTPs) must agree on the interpretation. Each reply in this category might have a different time value, but the SMTP client is encouraged to try again. A rule of thumb to determine whether 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 SMTP client 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 SMTP client to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status). The second digit encodes responses in specific categories: x0z Syntax -- These replies refer to syntax errors, syntactically correct commands that don't fit any functional category, and unimplemented or superfluous commands. x1z Information -- These are replies to requests for information, such as status or help. x2z Connections -- These are replies referring to the transmission channel. x3z Unspecified as yet. x4z Unspecified as yet. x5z Mail system -- These replies indicate the status of the receiver mail system vis-a-vis the requested transfer or other mail system action. The third digit gives a finer gradation of meaning in each category specified by the second digit. The list of replies illustrates this. Each reply text is recommended rather than mandatory, and may even change according to the command with which it is associated. On the other hand, the reply codes must strictly follow the specifications in this section. 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 SMTP client any new information, will return a 250 reply. The reply 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 SMTP client knows when it can stop reading the reply. This requires a special format to indicate a multiple line reply. The format for multiline replies requires that every line, except the last, begin with the reply code, followed immediately by a hyphen, "-" (also known as minus), followed by text. The last line will begin with the reply code, followed immediately by , optionally some text, and . As noted above, clients SHOULD send the if subsequent text is not sent, but servers MUST be prepared for it to be omitted. For example: 123-First line 123-Second line 123-234 text beginning with numbers 123 The last line In many cases the SMTP client then simply needs to search for the reply code followed by at the beginning of a line, and ignore all preceding lines. In a few cases, there is important data for the sender in the reply "text". The sender will be able to identify these cases from the current context. 4.2.2. 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 (see section ##4.2.3) 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 Service ready 221 Service closing transmission channel 421 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 [See section ##3.4] 252 Cannot VRFY user, but will accept message and attempt delivery [See section ##3.5.3] 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, or command rejected for policy reasons] 451 Requested action aborted: error in processing 551 User not local; please try [See section ##3.4] 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 . 554 Transaction failed [Or, in the case of a connection-opening response, "No SMTP service here"] 4.2.3. 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 Service ready 221 Service closing transmission channel 250 Requested mail action okay, completed 251 User not local; will forward to [See section ##3.4] 252 Cannot VRFY user, but will accept message and attempt delivery [See section ##3.5.3] 354 Start mail input; end with . 421 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, or command rejected for policy reasons] 551 User not local; please try [See section ##3.4] 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 [Or, in the case of a connection-opening response, "No SMTP service here"] 4.2.4. Reply code 502 Questions have been raised as to when reply code 502 (Command not implemented) should be returned in preference to other codes. 502 SHOULD be used when the command is actually recognized by the SMTP server, but not implemented. If the command is not recognized, code 500 SHOULD be returned. Extended SMTP systems MUST NOT list capabilities in response to EHLO for which they will return 502 (or 500) replies. 4.2.5 Reply codes after DATA and the subsequent .. When an SMTP server returns a positive completion status (2yz code) after the DATA command is completed with ., it accepts responsibility for: o delivering the message (if the recipient mailbox exists), or o if attempts to deliver the message fail due to transient conditions, retrying delivery some reasonable number of times at intervals as specified in section ##4.2.6. o if attempts to deliver the message fail due to permanent conditions, or if repeated attempts to deliver the message fail due to transient conditions, returning appropriate notification to the sender of the original message (using the address in the SMTP MAIL FROM command). When an SMTP server returns a transient error completion status (4yz) code after the DATA command is completed with ., it MUST NOT make any further attempt to deliver that message. The SMTP client retains responsibility for delivery of that message and may either return it to the user or requeue it for a subsequent attempt (see section ##4.5.4.1). The sending user should be able to interpret the return of a transient or permanent failure status as a non-delivery indication. 4.3. SEQUENCING OF COMMANDS AND REPLIES 4.3.1 Sequencing overview The communication between the sender and receiver is an alternating dialogue, controlled by the sender. As such, the sender issues a command and the receiver responds with a reply. Unless other arrangements are negotiated through service extensions, the sender must wait for this response before sending further commands. One important reply is the connection greeting. Normally, a receiver will send a 220 "Service ready" reply when the connection is completed. The sender should wait for this greeting message before sending any commands. Note: all the greeting-type replies have the official name (i.e., the fully-qualified primary domain name) of the server host as the first word following the reply code. Sometimes the host will have no meaningful name. See ##4.1.3 for a discussion of alternatives in these situations. For example, 220 ISIF.USC.EDU Service ready or 220 LOSER.BOGUS.COM Trashmail v 6.1.2 Service ready The table below lists alternative success and failure replies for each command. These SHOULD 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. 4.3.2 Command-Reply Sequences Each command is listed with its usual 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. Since some servers may generate other replies under special circumstances, and to allow for future extension, SMTP clients SHOULD, when possible, interpret only the first digit of the reply and MUST be prepared to deal with unrecognized reply codes by interpreting the first digit only. SMTP servers MUST NOT transmit reply codes to an SMTP client that are other than three digits or that do not start in a digit between 2 and 5 inclusive. CONNECTION ESTABLISHMENT S: 220 F: 421, 554 EHLO (or HELO) S: 250 E: 500*, 501, 504, 421, 550 MAIL S: 250 F: 552, 451, 452 E: 500*, 501, 421, 550, 553 RCPT S: 250, 251 (but see section ##3.4 for discussion of 251) F: 550, 551, 552, 553, 450, 451, 452 E: 500*, 501, 503, 421, 550 DATA I: 354 -> data -> S: 250 F: 552, 554, 451, 452 F: 451, 554 E: 500*, 501, 503, 421 RSET S: 250 E: 500*, 501, 504, 421 SEND S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 SOML S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 SAML S: 250 F: 552, 451, 452 E: 500, 501, 502, 421 VRFY S: 250, 251, 252 F: 550, 551, 553 E: 500*, 501, 502, 504, 421 EXPN S: 250, 252 F: 550 E: 500, 501, 502, 504, 421 HELP S: 211, 214 E: 500, 501, 502, 504, 421 NOOP S: 250 E: 500*, 421 QUIT S: 221 E: 500* TURN S: 250 F: 502 E: 500, 503 * Since support of this command is required, returning this reply code as part of an "unrecognized command" status places an implementation out of conformance with this specification. 4.4 Trace information When an SMTP server receives a message for delivery or further processing, it MUST insert trace ("time stamp" or "Received") information at the beginning of the message content, as discussed under the DATA command in section ##4.1.1.4. This line must be structured as follows: * The FROM field SHOULD contain both (1) the name of the source host as presented in the EHLO command and (2) an address literal containing the IP address of the source, determined from the TCP connection. * The ID field MAY contain an "@" as suggested in RFC-822, but this is not required. * The FOR field MAY contain a list of entries when multiple RCPT commands have been given. An Internet mail program MUST NOT change a Received: line that was previously added to the message header. SMTP servers MUST prepend Received lines to messages; they MUST NOT change the order of existing lines or insert Received lines in any other location. As the Internet grows, comparability of Received fields is important for detecting problems, especially slow relays. SMTP servers that create Received fields SHOULD use explicit offsets in the dates (e.g., -0800), rather than time zone names of any type. Local time (with an offset) is preferred to UT when feasible. If a time zone name is used, it should be included in a comment. When the delivery SMTP server makes the "final delivery" of a message, it inserts a return-path line at the beginning of the mail data. This use of return-path is required; mail systems MUST support it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message has left the SMTP world. Normally, this would mean it had been delivered to the destination user or an associated mail drop, but in some cases it may be further processed and transmitted by another mail system. It is possible for the mailbox in the return path to be different from the actual sender's mailbox, for example, if error responses are to be delivered a special error handling mailbox rather than to the message sender. When mailing lists are involved, this arrangement is common and useful as a means of directing errors to the list maintainer rather than the message originator. The text above implies that the final mail data will begin with a return path line, followed by one or more time stamp lines. These lines will be followed by the mail data headers and body [RFC822]. It is sometimes difficult for an SMTP server to determine whether or not it is making final delivery since forwarding or other operations may occur after the message is accepted for delivery. Consequently, any further (forwarding, gateway, or relay) systems MAY remove the return path and rebuild the MAIL FROM command as needed to ensure that exactly one such line appears in a delivered message. A message-originating SMTP system SHOULD NOT send a message that already contains a Return-path header. SMTP servers performing a relay function MUST NOT inspect the message data, and especially not to the extent needed to determine if Return-path headers are present. SMTP servers making final delivery MAY remove Return-path headers before adding their own. The primary purpose of the Return-path is to designate the address to which messages indicating non-delivery or other mail system failures are to be sent. For this to be unambigious, exactly one return path should be present when the message is delivered. Systems using RFC 822 syntax with non-SMTP transports SHOULD designate an unambiguous address, associated with the transport envelope, to which error reports (e.g., non-delivery messages) should be sent. Historical note: Text in RFC 822 that appears to contradict the use of Return-path (or the envelope MAIL FROM address) as the destination for error messages is not applicable on the Internet. The MAIL FROM address (as copied into the Return-path) MUST be used as the target of any mail containing delivery error messages. << Probably should take this out if [MSGFMT] adequately clarifies that point --Ed.>> In particular, (i) a gateway from SMTP->elsewhere SHOULD insert a return-path header, unless it is known that the "elsewhere" transport also uses Internet domain addresses and maintains the envelope sender address separately. (ii) a gateway from elsewhere->SMTP SHOULD delete any return-path header present in the message, and either copy that information to the SMTP envelope or combine it with information present in the envelope of the other transport system to construct the MAIL FROM part of the SMTP envelope. The server must give special treatment to cases in which the processing following the end of mail data indication is only partially successful. This could happen if, after accepting several recipients and the mail data, the SMTP server finds that the mail data could be successfully delivered to some, but not all, of the recipients. In such cases, the response to the DATA command must be an OK reply. However, the SMTP server must compose and send an "undeliverable mail" notification message to the originator of the message. A single notification listing all of the failed recipients or separate notification messages must be sent for each failed recipient. For economy of processing by the sender, the former is preferred when possible. All undeliverable mail notification messages are sent using the MAIL command (even if they result from processing the obsolete SEND, SOML, or SAML commands) and use a null return path as discussed in section ##3.7. <>> The time stamp line and the return path line are formally defined as follows: ::= "Return-Path:" ::= "Received:" ::= ";" ::= "FROM" ::= "BY" ::= [] [] [] [] ::= "VIA" ::= "WITH" ::= "ID" ::= "FOR" << FOR and need to be nailed down.>> ::= The standard names for links are registered with the Internet Assigned Numbers Authority (IANA). ::= The standard names for protocols are registered with the Internet Assigned Numbers Authority (IANA). ::=