Network Working Group J. Yao, Ed. Internet-Draft W. Mao, Ed.
Expires: January 27, 2007Intended status: Informational CNNIC JulyExpires: April 26, 2007 October 23, 2006 SMTP extension for internationalized email address draft-ietf-eai-smtpext-01.txtdraft-ietf-eai-smtpext-02.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 January 27,April 26, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Internationalized email address includes two parts, the local part and the domain part. The wayways email addresses are used by protocols are different from the wayways domain names are used. The most critical difference is that emails are delivered through a chain of peering clients and servers while domain names are resolved by name servers by looking up their own tables. In addition to this, email transport protocols SMTP and ESMTP provide a negotiation mechanism through which clients can make decisions for further processing. So internationalized email address is different from the internationalized domain name (IDN). It can be solved by exploiting the negotiation mechanism while IDN can not use the negotiation mechanism. So internationalized email address SHOULD be solved in the mail transport-level using the negotiation mechanism, which is an architecturally desirable approach. This document specifies the use of SMTP extension for internationalized email address delivery. It also mentions the backward compatible mechanism for downgrade procedure, as specified in an associated specification. The protocol proposed here is MTA-level solution which is feasible, architecturally more elegant, and not as difficult to deploy in relevant communities. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 34 1.1. Role of this specification . . . . . . . . . . . . . . . . 34 1.2. Proposal Context . . . . . . . . . . . . . . . . . . . . . 34 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 34 2. Mail Transport-level Protocol . . . . . . . . . . . . . . . . 45 2.1. Framework for the Internationalization Extension . . . . . 45 2.2. The Address Internationalization Service Extension . . . . 45 2.3. Extended Mailbox Address Syntax . . . . . . . . . . . . . 56 2.4. The ALT-ADDRESS parameter . . . . . . . . . . . . . . . . 67 2.5. The Suggestion of the Value of the ALT-ADDRESS parameter . . . . . . . . . . . . . . . . . . . . . . . . 78 2.6. Body Parts and SMTP Extensions . . . . . . . . . . . . . . 9 2.7. Additional ESMTP Changes and Clarifications . . . . . . . 8 188.8.131.52 2.7.1. The Initial SMTP Exchange . . . . . . . . . . . . . . 8 184.108.40.206 2.7.2. Trace Fields . . . . . . . . . . . . . . . . . . . . . 8 220.127.116.11 2.7.3. Mailing List Question . . . . . . . . . . . . . . . . 9 18.104.22.168 2.7.4. Message Header Label . . . . . . . . . . . . . . . . . 9 22.214.171.124 2.7.5. POP and IMAP . . . . . . . . . . . . . . . . . . . . . 910 2.7.6. SMTP Service Extension for DSNs . . . . . . . . . . . 11 3. Potential problems . . . . . . . . . . . . . . . . . . . . . . 911 3.1. Impact to IRI . . . . . . . . . . . . . . . . . . . . . . 911 3.2. Impact to RFC 2476 and many email related RFC . . . . . . 911 4. Implementation Advice . . . . . . . . . . . . . . . . . . . . 1011 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1011 6. Security considerations . . . . . . . . . . . . . . . . . . . 1012 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 1012 8. Change History . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 12 8.2. draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 12 8.3. draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1.13 9.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.2.13 9.2. Informative References . . . . . . . . . . . . . . . . . . 1215 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 1415 Intellectual Property and Copyright Statements . . . . . . . . . . 1516 1. Introduction 1.1. Role of this specification An overview document [EAI-overview] specifies the requirements for, and components of, full internationalization of electronic mail. This document specifies an element of that work, specifically the definition of an SMTP extension [RFC1869] for the internationalized email address transport delivery. 1.2. Proposal Context In order to use internationalized email addresses, we need to internationalize both the domain part and the local part of the email address. Domain part of the email address has been internationalized through IDNA [RFC3490]. But the local part of the email address still remains as non-internationalized. The syntax of Internet email addresses is restricted to a subset of 7-bit ASCII for the domain-part, with a less-restricted subset for the local-part. These restrictions are specified in RFC 2821 [RFC2821]. To be able to deliver internationalized email through SMTP servers, we need to upgrade SMTP server to be able to carry the internationalized email address. Since older SMTP servers and the mail-reading clients and other systems that are downstream from them MAY not be prepared to handle these extended addresses, an SMTP extension is specified to identify and protect the addressing mechanism. This specification describes a change to the email transport mechanism that permits non-ASCII address in both the envelope and header fields of messages. The context for the change is described in [EAI-overview] and the details of the header changes are described in [EAI-utf8header]. 1.3. Terminology The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119 [RFC2119]. All specialized terms used in this specification are defined in the EAI overview [EAI-overview] or in [RFC2821] and [RFC2822]. The terms "ASCII address", "internationalized email address", "non-ASCII address", "i18mail address", "UTF8SMTP", "message" and "mailing list" are used with the definitions from the EAI overview document. This document 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. Mail Transport-level Protocol 2.1. Framework for the Internationalization Extension The following service extension is defined: 1. The name of the SMTP service extension is "Internationalized Email and Extensions";"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. If a parameter appears, the SMTP client that is conformant to this version of this specification MUST treat the ESMTP response as if the "UTF8SMTP" keyword did not appear. 4. An optional parametersparameter is added to the SMTP MAIL and RCPT commands. The parameter is named as ALT-ADDRESS. The "ALT- ADDRESS" requires an all-ASCII address as a substitute for the i18mail addresses that we call the primary address; you can learn more in [EAI-overview] or [EAI-downgrading]. The value of "ALT- ADDRESS" is set by the sender when MUA and the submit ionSubmission server have a communication. 5. No additional SMTP verbs are defined by this extension. 6. Servers offering this extension MUST provide support for, and announce, the 8BITMIME extension [RFC1652]. 2.2. The Address Internationalization Service 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" MAY 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 as 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 a mailbox name as an internationalized string in UTF-8 form and MAY send an internationalized mail header [EAI-utf8header]. It MAY transmit the domain part of that string in either punycode (derived from the IDNA process) or UTF-8 form. If it sends the domain in UTF-8 form, the original SMTP client SHOULD first verify that the string is valid for a domain name according to IDNA rules. As required by RFC 2821, it MUST not attempt to parse, evaluate, or transform the local part in any way if the UTF8SMTP SMTP extension is offered by the server. If the UTF8SMTP SMTP extension is not offered by the Server, the SMTP Client MUST NOT transmit an internationalized address and MUST NOT transmit a mail body which contains internationalized mail headers [EAI-utf8header]. Instead, it MUST either return the message to the user as undeliverable or replace it with the alternate ASCII address. If it is replaced, the replacement MUST be the ASCII-only address specified with the ALT-ADDRESS parameter.[EAI-downgrading]. 2.3. Extended Mailbox Address Syntax RFC 2821, section 4.1.2, defines the syntax of a mailbox as Mailbox = Local-part "@" Domain Local-part = Dot-string / Quoted-string ; MAY be case-sensitive Dot-string = Atom *("." Atom) Atom = 1*atext Quoted-string = DQUOTE *qcontent DQUOTE Domain = (sub-domain 1*("." sub-domain)) / address-literal sub-domain = Let-dig [Ldh-str] 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]. That label MUST NOT contain the characters "@" or ".", even though those characters can normally be inserted into a DNS label. 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, define the syntax of an internationalized email mailbox with ABNF [RFC4234] as Mailbox = Local-part "@" Domain Local-part = Dot-string / Quoted-string ; MAY be case-sensitive Dot-string = Atom *("." Atom) Atom = 1*Ucharacter Ucharacter = atext / UTF8-2 / UTF8-3 / UTF8-4 Quoted-string = DQUOTE *qcontent DQUOTE Domain = (sub-domain 1*("." sub-domain)) / address-literal sub-domain = ULet-dig [ULdh-str] ULet-dig = Let-dig / Non-ASCII ULdh-str = *( ALPHA / DIGIT / "-" / Non-ASCII) ULet-dig Non-ASCII = UTF8-2 / UTF8-3 / UTF8-4 Where "atext", "qcontent" and "DQUOTE" are defined in [RFC2822], "Let-dig", "Ldh-str" and "address-literal" are defined in [RFC2821] and UTF8-2, UTF8-3 and UTF8-4 are defined in [RFC3629]. The value of "domain" SHOULD be verified with [RFC3490]; If failed, the email address with that domain can not be regarded as the valid email address. 2.4. 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 "ALT-ADDRESS" parameter. The "ALT-ADDRESS" requires an all-ASCII address. The ALT-ADDRESS parameter usage in the commands of "mail from" and "rcpt to" is defined according to the definition of mail-parameters in [RFC2821] below. MAIL FROM:<reverse-path> [SP[ SP <mail-parameters> ] <CRLF> RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF> Mail-parameters = esmtp-param *(SP esmtp-param) Rcpt-parameters = esmtp-param *(SP esmtp-param) esmtp-param = esmtp-keyword ["=" esmtp-value] esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") esmtp-value = 1*(%d33-60 / %d62-127) ; any CHAR excluding "=", SP, and control characters Reverse-path = Path Forward-path = Path Path = "<" [ A-d-l ":" ] Mailbox ">" A-d-l = At-domain *( "," A-d-l ) ; Note that this form, the so-called "source route", ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be ; ignored. At-domain = "@" domain where the value of esmtp-keyword is "ALT-ADDRESS" and the value of esmtp-value is all-ASCII email address, and where the domain and Mailbox are defined at section 2.3 of this document. The use of the ALT-ADDRESS is specified below: If some involved SMTP servers can not support UTF8SMTP capability and if the sender has already set the ALT-ADDRESS value, the client SMTP server will use this address as the email address when the SMTP server does the subsequent operations. If the ALT-ADDRESS value is not set by the sender, the email must be bounced to the original sender. If the email is bounced due to the incapability of supporting UTF8SMTP, the relative server should issue the response error code "5.3.3" defined in [RFC3463] which means that System is not capable of selected features, permanent failure. 2.5. The Suggestion of the Value of the ALT-ADDRESS parameter The "ALT-ADDRESS" requires an all-ASCII address. There are two alternative ways to set ALT-ADDRESS value: one is set by the sender using the all-ASCII address, the other is set using the transformed email address. Some may prefer transformed the non-ASCII address to the ASCII Compatible Encoding(ACE) address as the value of the ALT-ADDRESS. The big problem with applying an ACE to all local-parts is that the sending or converting system doesn't know if there are some specific data or instructions embedded in the address that the ACE process would hide. Some SMTP servers may depend on these specific data or instructions to do some operations while the local parts applied with ACE will lose or hide these data or instructions. SMTP [RFC2821] prohibits SMTP relays from converting local parts because the level of SMTP relays' knowledge on the structure of local parts is assumed to be zero. However, we can raise the knowledge level by supplying additional information. Many human users' email addresses do not have any embedded structure processed by the final delivery MTA. In that case, the sender can specify that these email addresses are safe to be converted in the predefined way. The final delivery SMTP server can revert the addresses even though they are as in all ASCII form. Unless the MUA or the submission server clearly knows that the non- ASCIInon-ASCII address can be safely transformed into the all-ASCII address, the non-ASCII address should not be transformed because transformed email address may cause some potential problems. This document suggests that the ALT-ADDRESS is set directly by the sender; In default, the all-ASCII address canshould not be transformed.gotten from the transformation of the non-ASCII address. 2.6. Additional ESMTP Changes and Clarifications The mail transport process involves addresses ("mailboxes")Body Parts and domain names in contexts in addition toSMTP Extensions While this specification requires that servers support the MAIL and RCPT commands8BITMIME extension [RFC1652] to ensure that servers have adequate handling capability for 8-bit data and extended alternativesto them. In general, the rule is that, when RFC 2821 specifiesavoid a mailbox, this document expects UTF-8 to be used fornumber of complex encoding problems, the entire string; when RFC 2821 specifies a domainuse 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, if the server advertises it and it is appropriate, with the BODY=BINARYMIME parameter specified in [RFC3030]. Assuming that the server advertises UTF8SMTP and 8BITMIME, and at least one non-ASCII address, with or without ALT-ADDRESS, the precise interpretation of these parameters on the MAIL command is: 1. Headers are in UTF-8, body parts are in ASCII. 2. Headers are in UTF-8, some or all body parts contain 8-bit line- oriented data. 3. Headers are in UTF-8, some or all body parts contain binary data without restriction as to line lengths or delimiters. 2.7. Additional ESMTP Changes and Clarifications The mail transport process involves addresses ("mailboxes") and domain names in 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 document expects UTF-8 to be used for the entire string; when RFC 2821 specifies a domain name, the name SHOULD be in punycode form if its raw form is non-ASCII. The following subsections list and discuss all of the relevant cases. Support and use of this extension requires support for 8BITMIME. It means that 8BITMIME MUST be advertised by the UTF8SMTP capability SMTP server. 126.96.36.199.7.1. The Initial SMTP Exchange When an SMTP or ESMTP connection is opened, the server sends a "banner" 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 hostname form, i.e., internationalized ones MUST be in punycode form. 188.8.131.52.7.2. Trace Fields Internationalized domain names in Received fields MUST be transmitted in the punycode form. Addresses in "for" clauses need further examination and might be treated differently depending on [EAI- utf8header].[EAI-utf8header]. The reasoning in the introductory portion of [EAI- overview][EAI-overview] strongly suggests that these addresses be in UTF-8 form, rather than some specialized encoding. 184.108.40.206.7.3. Mailing List Question How a mixture of traditional and internationalized addresses on a mailing list will impact message flows, error reports, and delivery notifications in all plausible combinations of UTF8SMTP capability and un-capability servers is discussed and specified in the [EAI- mailing[EAI-mailing list]. 220.127.116.11.7.4. Message Header Label The message header label MAY be used to identify and distinguish the i18mail message from the normal message when SMTP messages are transmitted on wire. This issue is discussed and specified in [EAI- utf8header]. 2.6.5.[EAI-utf8header]. 2.7.5. POP and IMAP While SMTP mainly takes care of the transportation of messages and the header fields on wire, POP essentially handles the retrieval of mail objects from the server by a client. In order to use internationalized user names based on i18mail for the retrieval of messages from a mail server using the POP protocol, a new capability SHOULD be introduced following the POP3 extension mechanism [RFC2449]. This is discussed and specified in the [EAI-pop]. IMAP [RFC3501] uses the traditional user name which is based on ASCII. IMAP SHOULD be updated to support the internationalized user names based on i18mail for the retrieval of messages from a mail server. This is discussed and specified in the [EAI-imap]. 2.7.6. SMTP Service Extension for DSNs How to facilitate the use of SMTP Service Extension for DSNs [RFC3461] in the work of EAI will be addressed in the [EAI-dsn]. 3. Potential problems 3.1. Impact to IRI The mailto: schema in IRI [RFC3987] MAY need to be modified when EAI is standardized. 3.2. Impact to RFC 2476 and many email related RFC The EAI protocolprotocols will impact on many email related RFC documents such as Message Submission [RFC2476] and SMTP Service Extension for DSNs [RFC3461].[RFC2476]. These protocolprotocols SHOULD be considered when implementing the EAI protocol. 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 certain 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. 5. IANA Considerations IANA is requested to add "UTF8SMTP" to the SMTP extensions registry with the entry pointing to this specification for its definition. 6. Security considerations See the extended security considerations discussion in [EAI-overview] 7. Acknowledgements Much of the text in the initial version of this document 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 document. Special thanks to those contributors for this version of document, those includes (but not limited to) John C Klensin, Charles Lindsey, Dave Crocker, Harald Tveit Alvestrand, Marcos Sanz, Chris Newman, Martin Duerst, Edmon Chung, Tony Finch.Finch, Kari Hurtta. 8. References 8.1. Normative References [ASCII] American National Standards Institute (formerly United StatesChange History [[anchor20: REMOVE THIS: This section is used for tracking the update of America Standards Institute), "USA Codethis document. It may be useful to retain parts of it to facilitate establishing dates and documents for Information Interchange", ANSI X3.4-1968, 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, butthe 1968history of this work.]] 8.1. draft-ietf-eai-smtpext: Version 00 This version remains definitive forsupercedes draft-yao-ima-smtpext-03.txt. It refines the Internet.ABNF definiton of the internationalized email address. It represents as the EAI working group document. 8.2. 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". 8.3. 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. 9. References 9.1. Normative References [ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet. [EAI-downgrading] YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading mechanism for Internationalized eMail Address (IMA)", draft-ietf-eai-downgrade-00draft-ietf-eai-downgrade-02 (work in progress), October 2005.August 2006. [EAI-dsn] Newman, C., "SMTP extensions for DSNs", 12 2006, <http:// www.ietf.org/internet-drafts/draft-ietf-eai-dsn-00.txt>. [EAI-imap] Resnick, P. and C. Newman, "Considerations for IMAP in Conjunction with Email Address Internationalization", draft-ietf-eai-imap-utf8-00 (work in progress), May 2006. [EAI-mailing list] Chung, E., "Mailing Lists and Internationalized Email Addresses", June 2006. Forthcoming [EAI-overview] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", draft-ietf-eai-framework-01.txtdraft-ietf-eai-framework-02.txt (work in progress), JuneOctober 2006. [EAI-pop] Newman, C., "POP3 Support for UTF-8", June 2006, <http:// www.ietf.org/internet-drafts/draft-ietf-eai-pop-00.txt>. [EAI-utf8header] Yeh, J., "Transmission of Email Headers in UTF-8 Encoding", draft-ietf-eai-utf8headers-00.txtdraft-ietf-eai-utf8headers-01.txt (work in progress), JuneAugust 2006. [RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July 1994. [RFC1869] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extensions", STD 10, RFC 1869, November 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension Mechanism", RFC 2449, November 1998. [RFC2476] Gellens, R. and J. Klensin, "Message Submission", RFC 2476, December 1998. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3030] Vaudreuil, G., "SMTP Service Extensions for Transmission of Large and Binary MIME Messages", RFC 3030, December 2000. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [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. [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3629, November 2003. [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. 18.104.22.168. Informative References [Klensin-emailaddr] Klensin, J., "Internationalization of Email Addresses", draft-klensin-emailaddr-i18n-03 (work in progress), July 2005. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. 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 58813055 Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The Internet Society (2006). 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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.Acknowledgment Funding for the RFC Editor function is currentlyprovided by the Internet Society.IETF Administrative Support Activity (IASA).