Network Working Group J. Yao, Ed. Internet-Draft W. Mao, Ed. Updates: RFC4952, 2821 and 2822 CNNIC (if approved)
AprilJuly 8, 2008 Intended status: Experimental Expires: October 10, 2008January 9, 2009 SMTP extension for internationalized email address draft-ietf-eai-smtpext-12.txtdraft-ietf-eai-smtpext-13.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 10, 2008.January 9, 2009. Abstract This document specifies an SMTP extension for transport and delivery of email messages with internationalized email addresses or header information. Communication with systems that do not implement this specification is specified in another document. This document updates some syntaxes and rules defined in RFC 2821 and RFC 2822, and has some material updating RFC 4952. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Role of this specification . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4 3. Mail Transport-level Protocol . . . . . . . . . . . . . . . . 4 3.1. Framework for the Internationalization Extension . . . . . 4 3.2. The UTF8SMTP Extension . . . . . . . . . . . . . . . . . . 5 3.3. Extended Mailbox Address Syntax . . . . . . . . . . . . . 7 3.4. The ALT-ADDRESS Parameter . . . . . . . . . . . . . . . . 8 3.5. ALT-ADDRESS Parameter Usage and Response Codes . . . . . . 910 3.6. Body Parts and SMTP Extensions . . . . . . . . . . . . . . 10 3.7. Additional ESMTP Changes and Clarifications . . . . . . . 11 3.7.1. The Initial SMTP Exchange . . . . . . . . . . . . . . 11 3.7.2. Mail eXchangers . . . . . . . . . . . . . . . . . . . 1112 3.7.3. Trace Information . . . . . . . . . . . . . . . . . . 1112 3.7.4. UTF-8 Strings in Replies . . . . . . . . . . . . . . . 1213 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1415 5. Security Considerations . . . . . . . . . . . . . . . . . . . 1617 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 1617 7. Change History . . . . . . . . . . . . . . . . . . . . . . . . 17 7.1. draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 17 7.2. draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 17 7.3. draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 1718 7.4. draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 1718 7.5. draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 1718 7.6. draft-ietf-eai-smtpext: Version 05 . . . . . . . . . . . . 1718 7.7. draft-ietf-eai-smtpext: Version 06 . . . . . . . . . . . . 18 7.8. draft-ietf-eai-smtpext: Version 07 . . . . . . . . . . . . 18 7.9. draft-ietf-eai-smtpext: Version 08 . . . . . . . . . . . . 1819 7.10. draft-ietf-eai-smtpext: Version 09 . . . . . . . . . . . . 1819 7.11. draft-ietf-eai-smtpext: Version 10 . . . . . . . . . . . . 1819 7.12. draft-ietf-eai-smtpext: Version 11 . . . . . . . . . . . . 1819 7.13. draft-ietf-eai-smtpext: Version 12 . . . . . . . . . . . . 19 7.14. draft-ietf-eai-smtpext: Version 13 . . . . . . . . . . . . 19 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 1920 8.1. Normative References . . . . . . . . . . . . . . . . . . . 1920 8.2. Informative References . . . . . . . . . . . . . . . . . . 2021 Appendix A. Material Updating RFC 4952 . . . . . . . . . . . . . 21 A.1. Conventional Message and Internationalized Message . . . . 2122 A.2. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 2122 A.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 2122 A.4. Implementation Advice . . . . . . . . . . . . . . . . . . 22 A.5. Applicability of SMTP Extension to Additional Uses . . . . 2223 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 2223 Intellectual Property and Copyright Statements . . . . . . . . . . 24 1. Introduction An internationalized email address includes two parts, the local part and the domain part. The ways email addresses are used by protocols are different from the ways domain names are used. The most critical difference is that emails are delivered through a chain of clients and servers while domain names are resolved by name servers looking up those names in their own tables. In addition to this, the simple mail transfer protocol [RFC2821] provides a negotiation mechanism about service extension with which clients can discover server capabilities and make decisions for further processing. An extended overview of the extension model for internationalized addresses and headers appears in [RFC4952], referred to as "the framework document" or just as "Framework" elsewhere in this specification. This document specifies an SMTP extension to permit internationalized email addresses in envelopes, and UNICODE characters (encoded in UTF-8) [RFC3629] in headers. 1.1. Role of this specification The framework document specifies the requirements for, and describes components of, full internationalization of electronic mail. A thorough understanding of the information in that document and in the base Internet email specifications [RFC2821] [RFC2822] is necessary to understand and implement this specification. This document specifies an element of the email internationalization work, specifically the definition of an SMTP extension [RFC2821] for internationalized email address transport delivery. 1.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. The terms "conventional message" and "internationalized message" are defined in an appendix to this specification. The terms "UTF-8 string" or "UTF-8 character" are used informally to refer to Unicode characters encoded in UTF-8 [RFC3629]. All other specialized terms used in this specification are defined in the framework document or in the base Internet email specifications [RFC2821] [RFC2822]. In particular, the terms "ASCII address", "internationalized email address", "non-ASCII address", "i18mail address", "UTF8SMTP", "message" and "mailing list" are used in this document according to the definitions in the framework one. This specification defines only those Augmented BNF for Syntax Specifications (ABNF) [RFC5234] syntax rules that are different from those of the base email specifications [RFC2821][RFC2822] and, where the earlier rules are upgraded or extended, gives them new names. When the new rule is a small modification to the older one, it is typically given a name starting with "u". Rules that are undefined here may be found in the base email specifications under the same names. [[anchor3: NOTE TO RFC EDITOR: Please remove the following text before publication.]] This specification is being discussed on the EAI mailing list. See https://www1.ietf.org/mailman/listinfo/ima for information about subscribing. The list's archive is at http://www1.ietf.org/mail-archive/web/ima/index.html. 2. Overview of Operation This specification describes an optional extension to the email transport mechanism that permits non-ASCII [ASCII] characters in both the envelope and header fields of messages, which are encoded with UTF-8 [RFC3629] characters. The extension is identified with the token "UTF8SMTP". In order to provide information that may be needed in downgrading, an optional alternate ASCII address may be needed if an SMTP client attempts to transfer an internationalized message and encounters a server that does not support this extension. The EAI-utf8header specification [EAI-utf8header] provides the details of how and where non-ASCII characters are permitted in the header fields of messages. The context for this specification is described in the framework document. 3. Mail Transport-level Protocol 3.1. Framework for the Internationalization Extension The following service extension is defined: 1. The name of the SMTP service extension is "Email Address Internationalization". 2. The EHLO keyword value associated with this extension is "UTF8SMTP". 3. No parameter values are defined for this EHLO keyword value. In order to permit future (although unanticipated) extensions, the EHLO response MUST NOT contain any parameters for that keyword. Clients MUST ignore any parameters, that is, clients MUST behave as if the parameters do not appear. If a server includes UTF8SMTP in its EHLO response, it MUST be fully compliant with this version of this specification. 4. One optional parameter, ALT-ADDRESS, is added to the MAIL and RCPT commands of SMTP. ALT-ADDRESS specifies an all-ASCII address which can be used as a substitute for the corresponding primary (i18mail) address when downgrading. More discussion of the use of this parameter appears in [RFC4952] and [EAI-downgrading]. 5. One optional parameter "UTF8REPLY" is added to the VRFY and EXPN commands. The parameter UTF8REPLY has no value. The parameter indicates that the SMTP client can accept Unicode characters in UTF-8 encoding in replies from the VRFY and EXPN commands. 6. No additional SMTP verbs are defined by this extension. 7. Servers offering this extension MUST provide support for, and announce, the 8BITMIME extension [RFC1652]. 8. The reverse-path and forward-path of the SMTP MAIL and RCPT commands are extended to allow Unicode characters encoded in UTF-8 in mailbox names (addresses). 9. The mail message body is extended as specified in [EAI-utf8header]. 10. The maximum length of MAIL and RCPT command lines is increased by 460 characters by the possible addition of the ALT-ADDRESS keyword and value. 11. The UTF8SMTP extension is valid on the submission port [RFC4409]. 3.2. The UTF8SMTP Extension An SMTP Server that announces this extension MUST be prepared to accept a UTF-8 string [RFC3629] in any position in which RFC 2821 specifies that a mailbox can appear. That string MUST be parsed only as specified in RFC 2821, i.e., by separating the mailbox into source route, local part and domain part, using only the characters colon (U+003A), comma (U+002C), and at-sign (U+0040) as specified there. Once isolated by this parsing process, the local part MUST be treated as opaque unless the SMTP Server is the final delivery MTA. Any domain names that are to be looked up in the DNS MUST first be processed into the form specified in IDNA [RFC3490] by means of the ToASCII() operation unless they are already in that form. Any domain names that are to be compared to local strings SHOULD be checked for validity and then MUST be compared as specified in section 3.4 of IDNA. An SMTP Client that receives the UTF8SMTP extension keyword in response to the "EHLO" command MAY transmit mailbox names within SMTP commands as internationalized strings in UTF-8 form. It MAY send a UTF-8 header [EAI-utf8header] (which may also include mailbox names in UTF-8). It MAY transmit the domain parts of mailbox names within SMTP commands or the message header in either the form of ACE (ASCII Compatible Encoding) labels as specified in IDNA [RFC3490] or as UTF-8 strings. All labels in domain parts of mailbox names which are IDNs (either UTF-8 or ACE strings) MUST be valid. If the original client submits a message to a Message Submission Server ("MSA") [RFC4409], it is the responsibility of the MSA that all domain labels are valid; otherwise it is the original client's responsibility. The presence of the UTF8SMTP extension does not change the requirement of RFC 2821 that servers relaying mail MUST NOT attempt to parse, evaluate, or transform the local part in any way. If the UTF8SMTP SMTP extension is not offered by the Server, the SMTP client MUST NOT transmit an internationalized address (For this paragraph, the internationalized domain name in the form of ACE labels as specified in IDNA [RFC3490] is not considered as "internationalized".) and MUST NOT transmit a mail message containing internationalized mail headers as described in [EAI-utf8header] at any level within its MIME structure. Instead, if an SMTP client (SMTP sender) attempts to transfer an internationalized message and encounters a server that does not support the extension, it MUST make one of the following four choices: 1. If and only if the SMTP client (sender) is a Message Submission Server ("MSA") [RFC4409], it MAY, consistent with the general provisions for changes by such servers, rewrite the envelope, headers, or message material to make them entirely ASCII and consistent with the provisions of RFC 2821 [RFC2821] and RFC 2822 [RFC2822]. 2. Either reject the message during the SMTP transaction or accept the message and then generate and transmit a notification of non- deliverability. Such notification MUST be done as specified in RFC 2821 [RFC2821], RFC 3464 [RFC3464], and the EAI DSN specification [EAI-dsn]. 3. Find an alternate route to the destination that permits UTF8SMTP. That route may be discovered by trying alternate MX hosts (using preference rules as specified in RFC 2821) or using other means available to the SMTP-sender. 4. If and only if ASCII addresses are available for all addresses that appear in the return path and the specific forward paths being attempted, downgrade the message to an all-ASCII form as specified in [EAI-downgrading]. An ASCII address is considered to be "available" for a particular address if the original address in the envelope is in ASCII or if an ALT-ADDRESS parameter is specified for a UTF8SMTP address. The difference between "choice 1" and "choice 4" is that "choice 1" is constrained by Message Submission [RFC4409], while "choice 4" is constrained by [EAI-downgrading]. 3.3. Extended Mailbox Address Syntax RFC 2821, section 4.1.2, defines the syntax of a mailbox entirely in terms of ASCII characters, using the production for a mailbox and those on which it depends. The key changes made by this specification are, informally, to o Change the definition of "sub-domain" to permit either the definition above or a UTF-8 string representing a DNS label that is conformant with IDNA [RFC3490]. o Change the definition of "Atom" to permit either the definition above or a UTF-8 string. That string MUST NOT contain any of the ASCII characters (either graphics or controls) that are not permitted in "atext"; it is otherwise unrestricted. According to the description above, the syntax of an internationalized email mailbox name (address) is defined in ABNF [RFC5234] as uMailbox = uLocal-part "@" uDomain ; Replace Mailbox in RFC 2821, section 4.1.2 uLocal-part = uDot-string / uQuoted-string ; MAY be case-sensitive ; Replace Local-part in RFC 2821, section 4.1.2 uDot-string = uAtom *("." uAtom) ; Replace Dot-string in RFC 2821, section 4.1.2 uAtom = 1*ucharacter ; Replace Atom in RFC 2821, section 4.1.2 ucharacter = atext / UTF8-non-ascii ;atext is defined in= <See section 3.2.4 of RFC 28222822> uQuoted-string = DQUOTE *uqcontent DQUOTE ; Replace Quoted-string in RFC 2821, section 4.1.2 ;DQUOTE is Double Quote defined in= <See appendix B.1 of RFC 52345234> uqcontent = qcontent / UTF8-non-ascii ;qcontent is defined in RFC 2822,= <See section 3.2.5 of RFC 2822> uDomain = (sub-udomain 1*("." sub-udomain)) / address-literal ; Replace Domain in RFC 2821, section 4.1.2 ;address-literal is defined in RFC2821= <See section 4.1.2 of RFC 2822> sub-udomain = uLet-dig [uLdh-str] ; Replace sub-domain in RFC 2821, section 4.1.2 uLet-dig = Let-dig / UTF8-non-ascii ;Let-dig is defined in RFC 2821,= <See section 4.1.3 of RFC 2821> uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-non-ascii) uLet-dig ; Replace Ldh-str in RFC 2821, section 4.1.3 UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4 ; UTF8-2,UTF8-2 = <See section 4 of RFC 3629> UTF8-3 and= <See section 4 of RFC 3629> UTF8-4 are two, three, or four ; octet UTF-8 characters, as defined in= <See section 4 of RFC 36293629> The value of "uDomain" SHOULD be verified by applying the tests specified as part of IDNA [RFC3490]. If that verification fails, the email address with that uDomain MUST NOT be regarded as a valid email address. 3.4. The ALT-ADDRESS Parameter If the UTF8SMTP extension is offered, the syntax of the SMTP MAIL and RCPT commands is extended to support the optional esmtp-keyword "ALT- ADDRESS". That keyword specifies an alternate all-ASCII address which may be used when downgrading. If the ALT-ADDRESS esmtp-keyword is used, it MUST have an associated esmtp-value (ALT-ADDRESS-esmtp- value, which is defined below). While it may be tempting to consider ALT-ADDRESS as a general-purpose second-chance address, such behavior is not defined here. Instead, in this specification ALT-ADDRESS only has meaning when the associated primary address is non-ASCII and the message is downgraded. This restriction allows for future extension of the specification even though no such extensions are currently anticipated. Based on the definition of mail-parameters in [RFC2821], the ALT- ADDRESS parameter usage in the commands of "MAIL" and "RCPT" is defined as follows. The following definitions are given in the same format as used in RFC 2821. "MAIL FROM:" ("<>" / uReverse-path) [ SP Mail-parameters ] CRLF ; Update the MAIL command in RFC 2821, section 184.108.40.206. ; A new parameter defined by the ABNF non-terminal ; <ALT-ADDRESS-parameter> is added. It complies ; with the syntax specified for <esmtp-param> in RFC 2821. "RCPT TO:" ("<Postmaster@" uDomain ">" / "<Postmaster>" / uForward-path) [ SP Rcpt-parameters ] CRLF ; Update RCPT command in RFC 2821, section 220.127.116.11. ; A new parameter defined by the ABNF non-terminal ; <ALT-ADDRESS-parameter> is added. It complies ; with the syntax specified for <esmtp-param>. ; uDomain is defined in section 3.3 of this document uReverse-path = uPath ; Replace Reverse-path in RFC 2821, section 4.1.2 uForward-path = uPath ; Replace Forward-path in RFC 2821, section 4.1.2 uPath = "<" [ A-d-l ":" ] uMailbox ">" ; Replace Path in RFC 2821, section 4.1.2 ; A-d-l is defined in RFC 2821, section 4.1.2 ;uMailbox is defined in section 3.3 of this document A-d-l = <See section 4.1.2 of RFC 2821> ALT-ADDRESS-parameter="ALT-ADDRESS=" ALT-ADDRESS-value ALT-ADDRESS-value=xtextALT-ADDRESS-value= xtext ; The value is a mailbox name encoded as xtext. ; xtext is defined in RFC 3461,xtext= <See section 4.2 of RFC 3461> The ALT-ADDRESS-parameter MUST NOT appear more than once in any MAIL or RCPT command. ALT-ADDRESS-esmtp-value MUST be an all-ASCII email address before xtext encoding. 3.5. ALT-ADDRESS Parameter Usage and Response Codes An "internationalized message" as defined in the appendix of this specification MUST NOT be sent to an SMTP server that does not support UTF8SMTP. Such a message MAY be rejected by a server if it lacks ALT-ADDRESSes as discussed in Section 3.2 of this specification. The three-digit reply codes used in this section are consistent with their meanings as defined in RFC 2821. When messages are rejected because the RCPT command requires an ALT- ADDRESS, the response code 553 is used with the meaning "mailbox name not allowed". When messages are rejected for other reasons, such as the MAIL command requiring an ALT-ADDRESS, the response code 550 is used with the meaning "mailbox unavailable". When the server supports enhanced mail system status codes [RFC3463], response code "5.6.x" [SMTP-codes] is used, meaning that "The ALT-ADDRESS is required but not specified". If the response code is issued after the final "." of the DATA command, the response code "554" is used with the meaning "Transaction failed". When the server supports enhanced mail system status codes [RFC3463], response code "5.6.z" [SMTP-codes] is used, meaning that "UTF8SMTP downgrade failed". [[anchor6: RFC Editor: please insert the proper error codes for "5.6.x" and "5.6.z" after IANA has made the relevant assignments.]] 3.6. Body Parts and SMTP Extensions There is no ESMTP parameter to assert that a message is an internationalized message. An SMTP server that requires accurate knowledge of whether a message is internationalized is required to parse all message header fields and MIME header fields in the message body. While this specification requires that servers support the 8BITMIME extension [RFC1652] to ensure that servers have adequate handling capability for 8-bit data and to avoid a number of complex encoding problems, the use of internationalized addresses obviously does not require non-ASCII body parts in the MIME message. The UTF8SMTP extension MAY be used with the BODY=8BITMIME parameter if that is appropriate given the body content or, with the BODY=BINARYMIME parameter, if the server advertises BINARYMIME [RFC3030] and that is appropriate. Assuming that the server advertises UTF8SMTP and 8BITMIME, and receives at least one non-ASCII address, with or without ALT-ADDRESS, the precise interpretation of 'No BODY parameter', "BODY=8BITMIME", and "BODY=BINARYMIME" in the MAIL command is: 1. If there is no BODY parameter, the header contains UTF-8 characters, but all the body parts are in ASCII (possibly as the result of a Content-transfer-encoding). 2. If a BODY=8BITMIME parameter is present, the header contains UTF-8 characters and some or all of the body parts contain 8-bit line-oriented data. 3. If a BODY=BINARYMIME parameter is present, the header contains UTF-8 characters and some or all body parts contain binary data without restriction as to line lengths or delimiters. 3.7. Additional ESMTP Changes and Clarifications The information carried in the mail transport process involves addresses ("mailboxes") and domain names in various contexts in addition to the MAIL and RCPT commands and extended alternatives to them. In general, the rule is that, when RFC 2821 specifies a mailbox, this specification expects UTF-8 to be used for the entire string; when RFC 2821 specifies a domain name, the name SHOULD be in the form of ACE labels if its raw form is non-ASCII. The following subsections list and discuss all of the relevant cases. 3.7.1. The Initial SMTP Exchange When an SMTP connection is opened, the server normally sends a "greeting" response consisting of the '220' reply code and some information. The client then sends the EHLO command. Since the client cannot know whether the server supports UTF8SMTP until after it receives the response from EHLO, any domain names that appear in this dialogue, or in responses to EHLO, MUST be in the hostname form, i.e., internationalized ones MUST be in the form of ACE labels. 3.7.2. Mail eXchangers Organizations often authorize multiple servers to accept mail addressed to them. For example, the organization may itself operate more than one server, and may also or instead have an agreement with other organizations to accept mail as a backup. Authorized servers are generally listed in MX records as described in RFC2821. When more than one server accepts mail for the domain-part of a mailbox, it is strongly advised that either all or none of them support the UTF8SMTP extension. Otherwise, surprising downgrades can happen during temporary failures, which users might perceive as a serious reliability issue. 3.7.3. 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. "Time stamp" or "Received" appears in the form of "Received:" lines. The most important use of Received: lines is for debugging mail faults. 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. 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 the trace information, this memo updates the time stamp line and the return path line [RFC2821] formally defined as follows: uReturn-path-line = "Return-Path:" FWS uReverse-path <CRLF> ; Replaces Return-path-line in section 4.4 of RFC2821 ; uReverse-path is defined in Section 3.3 of this document uTime-stamp-line = "Received:" FWS uStamp <CRLF> ; Replaces Time-stamp-line in section 4.4 of RFC2821 uStamp = From-domain By-domain uOpt-info ";" FWS date-time ; Replaces Stamp in section 4.4 of RFC2821 uOpt-info = [Via] [With] [ID] [uFor] ; Replaces Opt-info in section 4.4 of RFC2821 ; The protocol value for With will allow a UTF8SMTP value uFor = "FOR" ( FWS (uPath / uMailbox) ) CFWS ; Replaces For in section 4.4 of RFC2821 ; uPath and uMailbox are defined in Sections 2.4 and ; 2.3, respectively, of this document [[anchor10:Note: The FOR parameter has been changed to match the definition in RFC2821bis,[RFC2821bis], permitting only one address in the For clause. The group working on that document reached mailing list consensus that the syntax in RFC 2821[RFC2821] that permitted more than one address was simply a mistake.]]mistake. Except in the 'uFor' clause and 'uReverse-path' value where non-ASCII domain names may be used, internationalized domain names in Received fields MUST be transmitted in the form of ACE labels. The protocol value of the WITH clause when this extension is used is one of the UTF8SMTP values specified in the "IANA Considerations" section of this document. 3.7.4. UTF-8 Strings in Replies 18.104.22.168. MAIL and RCPT Commands If the client issues a RCPT command containing non-ASCII characters, the SMTP server is permitted to use UTF-8 characters in the email address associated with 251 and 551 response codes. If an SMTP client follows this specification and sends any RCPT commands containing non-ASCII addresses, it MUST be able to accept and process 251 or 551 replies containing UTF-8 email addresses. If a given RCPT command does not include a non-ASCII envelope address, the server MUST NOT return a 251 or 551 response containing a non- ASCII mailbox. Instead, it MUST transform such responses into 250 or 550 responses that do not contain addresses. 22.214.171.124. VRFY and EXPN Commands and the UTF8REPLY Parameter If the VRFY and EXPN commands are transmitted with an optional parameter "UTF8REPLY", it indicates the client can accept UTF-8 strings in replies from those commands. This allows the server to use UTF-8 strings in mailbox names and full names which occur in replies without concern that the client might be confused by them. An SMTP client that conforms to this specification MUST accept and correctly process replies from the VRFY and EXPN commands that contain UTF-8 strings. However the SMTP server MUST NOT use UTF-8 strings in replies if the SMTP client does not specifically allow such replies by transmitting this parameter. Most replies do not require that a mailbox name be included in the returned text and therefore UTF-8 is not needed in them. Some replies, notably those resulting from successful execution of the VRFY and EXPN commands, do include the mailbox, making the provisions of this section important. VERIFY (VRFY) and EXPAND (EXPN) command syntaxes are changed to: "VRFY" SP (uLocal-part / uMailbox) [SP "UTF8REPLY"] CRLF ; uLocal-part and uMailbox are defined in ; Section 3.3 of this document "EXPN" SP ( uLocal-part / uMailbox ) [ SP "UTF8REPLY" ] CRLF ; uLocal-part and uMailbox are defined in ; Section 3.3 of this document The "UTF8REPLY" parameter does not use a value. If the reply to a VERIFY (VRFY) or EXPAND (EXPN) command requires UTF-8, but the SMTP client does not use the "UTF8REPLY" parameter, then the server MUST use either the reply code 252 or 550. Response code 252, defined in [RFC2821], means "Cannot VRFY user, but will accept the message and attempt the delivery". Response code 550, also defined in [RFC2821], means "Requested action not taken: mailbox unavailable". When the server supports enhanced mail system status codes [RFC3463], the enhanced response code as specified below is used. Using the "UTF8REPLY" parameter with a VERIFY (VRFY) or EXPAND (EXPN) command enables UTF-8 replies for that command only. If a normal success response (i.e., 250) is returned, the response MAY include the full name of the user and MUST include the mailbox of the user. It MUST be in either of the following forms: User Name <uMailbox> ; uMailbox is defined in section 3.3 of this document ; User Name can contain non-ASCII characters. uMailbox ; uMailbox is defined in section 3.3 of this document If the SMTP reply requires UTF-8 strings, but UTF-8 is not allowed in the reply, and the server supports enhanced mail system status codes [RFC3463], the enhanced response code is either "5.6.y" or "2.6.y" [SMTP-codes], meaning "A reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the client". If the SMTP Client does not support the UTF8SMTP extension, but receives a UTF-8 string in a reply, it may not be able to properly report the reply to the user, and some clients might crash. Internationalized messages in replies are only allowed in the commands under the situations described above. Under any other circumstances, UTF-8 text MUST NOT appear in the reply. Although UTF-8 is needed to represent email addresses in responses under the rules specified in this section, this extension does not permit the use of UTF-8 for any other purposes. SMTP servers MUST NOT include non-ASCII characters in replies except in the limited cases specifically permitted in this section. [[anchor12:[[anchor11: RFC Editor: please insert the proper error codes for "5.6.y" and "2.6.y" after IANA has made the relevant assignments.]] 4. IANA Considerations IANA is requested to add a new value "UTF8SMTP" to the SMTP Service Extension subregistry of the Mail Parameters registry, according to the following data: +----------+---------------------------------+-----------+ | Keywords | Description | Reference | +----------+---------------------------------+-----------+ | UTF8SMTP | Internationalized email address | [RFCXXXX] | +----------+---------------------------------+-----------+ This document adds new values to the SMTP Enhanced Status Code subregistry of the Mail Parameters registry, following the guidance in Section 3.5 and Section 126.96.36.199 of this document, and being based on [SMTP-codes]. The registration data is as follows: Code: 5.6.x Sample Text: The ALT-ADDRESS is required but not specified Associated basic status code: 553, 550 Description: This indicates the reception of a MAIL or RCPT commands that required an ALT-ADDRESS parameter but such parameter was not present. Defined: RFC XXXX. (Experimental track) Submitter: Jiankang YAO Change controller: IESG. Code: 5.6.y Sample Text: UTF-8 string reply is required, but not permitted by the client Associated basic status code: 553, 550 Description: This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the client Defined: RFC XXXX. (Experimental track) Submitter: Jiankang YAO Change controller: IESG. Code: 5.6.z Sample Text: UTF8SMTP downgrade failed Associated basic status code: 550 Description: This indicates that transaction failed after the final "." of the DATA command Defined: RFC XXXX. (Experimental track) Submitter: Jiankang YAO Change controller: IESG. Code: 2.6.y Sample Text: UTF-8 string reply is required, but not permitted by the client Associated basic status code: 252 Description: This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the client Defined: RFC XXXX. (Experimental track) Submitter: Jiankang YAO Change controller: IESG. The "Mail Transmission Types" registry under Mail Parameters registry is requested to be updated to include the following new entries: +---------------+----------------------------+----------------------+ | WITH protocol | Description | Reference | | types | | | +---------------+----------------------------+----------------------+ | UTF8SMTP | UTF8SMTP with Service | [RFCXXXX] | | | Extensions | | | UTF8SMTPA | UTF8SMTP with SMTP AUTH | [RFC4954] [RFCXXXX] | | UTF8SMTPS | UTF8SMTP with STARTTLS | [RFC3207] [RFCXXXX] | | UTF8SMTPSA | UTF8SMTP with both | [RFC3207] [RFC4954] | | | STARTTLS and SMTP AUTH | [RFCXXXX] | +---------------+----------------------------+----------------------+ 5. Security Considerations See the extended security considerations discussion in the framework document [RFC4952]. 6. Acknowledgements Much of the text in the initial version of this specification was derived or copied from [Klensin-emailaddr] with the permission of the author. Significant comments and suggestions were received from Xiaodong LEE, Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other members of the JET team and were incorporated into the specification. Additional important comments and suggestions, and often specific text, were contributed by many members of the WG and design team. Those contributions include material from John C Klensin, Charles Lindsey, Dave Crocker, Harald Tveit Alvestrand, Marcos Sanz, Chris Newman, Martin Duerst, Edmon Chung, Tony Finch, Kari Hurtta, Randall Gellens, Frank Ellermann, Alexey Melnikov, Pete Resnick, S. Moonesamy, Soobok Lee, Shawn Steele, Alfred Hoenes andMiguel Garcia.Garcia, Magnus Westerlund and Lars Eggert. Of course, none of the individuals are necessarily responsible for the combination of ideas represented here. 7. Change History [[anchor16:[[anchor15: RFC Editor: Please remove this section.]] 7.1. draft-ietf-eai-smtpext: Version 00 This version supercedes draft-yao-ima-smtpext-03.txt. It refines the ABNF definition of the internationalized email address. It represents as the EAI working group document. 7.2. draft-ietf-eai-smtpext: Version 01 o Upgraded to reflect discussions during IETF 66. o Remove the atomic parameter. o Add the new section of "the Suggestion of the value of the ALT- ADDRESS parameter". 7.3. draft-ietf-eai-smtpext: Version 02 o Upgraded to reflect the recent discussion of the firstname.lastname@example.org mailing list. o Add the section of "Body Parts and SMTP Extensions". o Add the new section of "Change History". o Add the subsection about SMTP extensions for DSN. 7.4. draft-ietf-eai-smtpext: Version 03 o Update the syntax related to mailbox. o Update the trace field section. o Add the new section about message retry. o Update the subsection about SMTP extensions for DSN. 7.5. draft-ietf-eai-smtpext: Version 04 o Refine some syntax. o Delete "Message Header Label" section. o Change "bounce" to "reject". 7.6. draft-ietf-eai-smtpext: Version 05 o Refine the abstract. o Delete "The Suggestion of the Value of the ALT-ADDRESS parameter" section. o Move original section 2.7.4 and 2.7.5 to section 3 with the name "Issues with other parts of the email system". o Add the new section "LMTP". o Refine some text according to suggestions from the EAI mailing list discussion o Remove the section "Mailing List Question" 7.7. draft-ietf-eai-smtpext: Version 06 o Delete the section about message retry. o Add the new subsection about Mail eXchangers o Add the new section about "UTF-8 Reply" o Refine some response code for the section "Using the ALT-ADDRESS parameter" 7.8. draft-ietf-eai-smtpext: Version 07 o Rename the section 2.5 o Refine the section 2.7 7.9. draft-ietf-eai-smtpext: Version 08 o Refine some texts and update some references 7.10. draft-ietf-eai-smtpext: Version 09 o Add the appendix o Move section 3.1, 3.2 and section 5 to Appendix o Remove section 3.3 and section 4 o Add the new term definitions of conventional message and international message in the appendix o Refine some texts according to suggestions from the EAI mailing list discussion during WG Last call o Use the same reference for ASCII as RFC 2821. o General editorial revision and cleanup, including extensive modifications to the XML to produce a version that has better odds of getting through the various checkers and validators. 7.11. draft-ietf-eai-smtpext: Version 10 o Refine the text o Add some text about "ALT-ADDRESS" in the section 2.4 o Add the appendix A.5 7.12. draft-ietf-eai-smtpext: Version 11 o Refine the text o Reference updating 7.13. draft-ietf-eai-smtpext: Version 12 o Remove the section 1.2 about Proposal Context and merge the text into new section 2 o Add the new section 2 about overview of operation o Update the IANA consideration o Refine the text 7.14. draft-ietf-eai-smtpext: Version 13 o Update the Abstract o Refine the syntax about the equivalent of import clause 8. References 8.1. Normative References [ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968. [EAI-dsn] Newman, C. and A. Melnikov, "SMTP extensions for DSNs", draft-ietf-eai-dsn-06.txt (work in progress), January 2008. [EAI-utf8header] Abel, Y., "Transmission of Email Headers in UTF-8 Encoding", draft-ietf-eai-utf8headers-09.txtdraft-ietf-eai-utf8headers-12.txt (work in progress), JanuaryJuly 2008. [RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July 1994. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 2003. [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003. [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, January 2003. [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3629, November 2003. [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 4952, July 2007. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [SMTP-codes] Hansen , T. and J. Klensin, "A Registry for SMTP Enhanced Mail System Status Codes", draft-hansen-4468upd-mailesc-registry-03 (work in progress), January 2008. 8.2. Informative References [EAI-downgrading] YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading mechanism for Internationalized eMail Address", draft-ietf-eai-downgrade-06draft-ietf-eai-downgrade-07 (work in progress), 13 2008. [Klensin-emailaddr] Klensin, J., "Internationalization of Email Addresses", draft-klensin-emailaddr-i18n-03 (work in progress), July 2005. [RFC0974] Partridge, C., "Mail routing and the domain system", RFC 974, January 1986. [RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033, October 1996. [RFC2821bis] Klensin, J., "Simple Mail Transfer Protocol", draft-klensin-rfc2821bis-10 (work in progress), 4 2008. [RFC3030] Vaudreuil, G., "SMTP Service Extensions for Transmission of Large and Binary MIME Messages", RFC 3030, December 2000. [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, February 2002. [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension for Authentication", RFC 4954, July 2007. Appendix A. Material Updating RFC 4952 RFC 4952, the Overview and Framework document covering this set of extensions for internationalized email [RFC4952], was completed before this specification, which specifies a particular part of the protocol set. This appendix, which is normative, contains material that would have been incorporated into RFC 4952 had it been delayed until the work described in the rest of this specification was completed and that should be included in any update to RFC 4952. A.1. Conventional Message and Internationalized Message o A conventional message is one that does not use any extension defined in this document or in the UTF8header specification [EAI-utf8header], and which is strictly conformant to RFC 2822 [RFC2822]. o An internationalized message is a message utilizing one or more of the extensions defined in this specification or in the UTF8header specification [EAI-utf8header], so that it is no longer conformant to the RFC 2822 specification of a message. A.2. LMTP LMTP [RFC2033] may be used as the final delivery agent. In such cases, LMTP may be arranged to deliver the mail to the mail store. The mail store may not have UTF8SMTP capability. LMTP need to be updated to deal with these situations. A.3. SMTP Service Extension for DSNs The existing draft standard Delivery status notifications (DSNs) [RFC3461] is limited to ASCII text in the machine readable portions of the protocol. "International Delivery and Disposition Notifications" [EAI-dsn] adds a new address type for international email addresses so an original recipient address with non-ASCII characters can be correctly preserved even after downgrading. If an SMTP server advertises both the UTF8SMTP and the DSN extension, that server MUST implement EAI-dsn [EAI-dsn] including support for the ORCPT parameter. A.4. Implementation Advice In the absence of this extension, SMTP clients and servers are constrained to using only those addresses permitted by RFC 2821. The local parts of those addresses MAY be made up of any ASCII characters, although some of them MUST be quoted as specified there. It is notable in an internationalization context that there is a long history on some systems of using overstruck ASCII characters (a character, a backspace, and another character) within a quoted string to approximate non-ASCII characters. This form of internationalization SHOULD be phased out as this extension becomes widely deployed but backward-compatibility considerations require that it continue to be supported. A.5. Applicability of SMTP Extension to Additional Uses Among other protocol changes, the SMTP extension allows an optional alternate address to be supplied with the MAIL and RCPT commands. For the purposes of this set of specifications, this alternate address only has meaning when the primary address contains UTF-8 characters and the message is downgraded. While it may be tempting to consider the alternate address as a general-purpose second-chance address, to be used whenever the primary address is rejected, such behavior is not defined here. This restriction allows for future extensions to be developed which create such a general-purpose second-chance address, although no specific work on such an extension is currently anticipated. Note that any such extension needs to consider the question of what the [RFC0974] sequencing rules mean when different possible servers support different sets of ESMTP options (or, in this case, addresses). The answer to this question may also imply updates to [RFC2821]. Authors' Addresses Jiankang YAO (editor) CNNIC No.4 South 4th Street, Zhongguancun Beijing Phone: +86 10 58813007 Email: email@example.com Wei MAO (editor) CNNIC No.4 South 4th Street, Zhongguancun Beijing Phone: +86 10 5881305558812230 Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at email@example.com.