Network Working Group J. Yao, Ed.
Internet-Draft W. Mao, Ed.
Updates: RFC4952 CNNIC
(if approved) November 17, 2007
Intended status: Experimental
Expires: May 20, 2008
SMTP extension for internationalized email address
draft-ietf-eai-smtpext-09.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of this specification . . . . . . . . . . . . . . . . 3
1.2. Proposal Context . . . . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Mail Transport-level Protocol . . . . . . . . . . . . . . . . 4
2.1. Framework for the Internationalization Extension . . . . . 4
2.2. The UTF8SMTP Extension . . . . . . . . . . . . . . . . . . 5
2.3. Extended Mailbox Address Syntax . . . . . . . . . . . . . 6
2.4. The ALT-ADDRESS Parameter . . . . . . . . . . . . . . . . 8
2.5. ALT-ADDRESS Parameter Usage and Response Codes . . . . . . 9
2.6. Body Parts and SMTP Extensions . . . . . . . . . . . . . . 10
2.7. Additional ESMTP Changes and Clarifications . . . . . . . 10
2.7.1. The Initial SMTP Exchange . . . . . . . . . . . . . . 11
2.7.2. Mail eXchangers . . . . . . . . . . . . . . . . . . . 11
2.7.3. Trace Information . . . . . . . . . . . . . . . . . . 11
2.7.4. UTF-8 Strings in Replies . . . . . . . . . . . . . . . 12
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
6. Change History . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 15
6.2. draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 15
6.3. draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 15
6.4. draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 15
6.5. draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 16
6.6. draft-ietf-eai-smtpext: Version 05 . . . . . . . . . . . . 16
6.7. draft-ietf-eai-smtpext: Version 06 . . . . . . . . . . . . 16
6.8. draft-ietf-eai-smtpext: Version 07 . . . . . . . . . . . . 16
6.9. draft-ietf-eai-smtpext: Version 08 . . . . . . . . . . . . 16
6.10. draft-ietf-eai-smtpext: Version 09 . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Normative References . . . . . . . . . . . . . . . . . . . 17
7.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Material Updating RFC 4952 . . . . . . . . . . . . . 18
A.1. Conventional Message and Internationalized Message . . . . 19
A.2. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
A.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 19
A.4. Implementation Advice . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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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
extended email transport protocol [RFC2821] provides a negotiation
mechanism 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 [EAI-framework], 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) 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. Proposal Context
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. 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 the change is described in the framework document.
1.3. Terminology
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this specification 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
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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 ABNF [RFC4234] 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.
[[anchor4: 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. 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 "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 [EAI-framework] and
[EAI-downgrading].
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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].
2.2. 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 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
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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 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 a
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.
2.3. 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].
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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
[RFC4234] 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-xtra-char
; Replace character in RFC 2821, section 4.1.2
; atext is defined in RFC 2822
uQuoted-string = DQUOTE *uqcontent DQUOTE
; Replace Quoted-string in RFC 2821, section 4.1.2
; DQUOTE is Double Quote defined in RFC 4234
uqcontent = qcontent / UTF8-xtra-char
; qcontent is defined in RFC 2822, section 3.2.5
uDomain = (sub-udomain 1*("." sub-udomain)) / address-literal
; Replace Domain in RFC 2821, section 4.1.2
; address-literal is defined in RFC2821 section 4.1.2
sub-udomain = uLet-dig [uLdh-str]
; Replace sub-domain in RFC 2821, section 4.1.2
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uLet-dig = Let-dig / UTF8-xtra-char
; Let-dig is defined in RFC 2821, section 4.1.3
uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-xtra-char) uLet-dig
; Replace Ldh-str in RFC 2821, section 4.1.3
UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
; UTF8-2, UTF8-3 and UTF8-4 are two, three, or four
; octet UTF-8 characters, as defined in RFC 3629
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.
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 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).
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 4.1.1.2.
; 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 4.1.1.3.
; 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 2.3 of this document
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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 2.3 of this document
ALT-ADDRESS-parameter="ALT-ADDRESS=" ALT-ADDRESS-value
ALT-ADDRESS-value=xtext
; The value is a mailbox name encoded as xtext.
; xtext is defined in RFC 3461, section 4.2
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.
2.5. 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 one or more ALT-ADDRESSes as discussed in Section 2.2 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". If enhanced mail system
status codes [RFC3463] are used, the response code should be "5.6.x"
[SMTP-codes], 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". If enhanced mail system status codes [RFC3463]
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are used, the response code should be "5.6.z" [SMTP-codes], meaning
that "UTF8SMTP downgrade failed".
[[anchor7: RFC Editor: please insert the proper error codes for
"5.6.x" and "5.6.z" after IANA has made the relevant assignments.]]
2.6. Body Parts and SMTP Extensions
Since there is no ESMTP parameter which tells whether the 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, if the server
advertises BINARYMIME [RFC3030] and the BODY=BINARYMIME is
appropriate, with the BODY=BINARYMIME parameter.
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.
2.7. Additional ESMTP Changes and Clarifications
The information carried in 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
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.
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2.7.1. The Initial SMTP Exchange
When an SMTP or ESMTP 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.
2.7.2. 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 is not a good thing.
2.7.3. 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 2.3 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
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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
[[anchor11: Note: The FOR parameter has been changed to match the
definition in 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 that permitted more than one
address was simply a mistake.]]
Except in the 'uFor' and 'uReverse-path' line 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 is UTF8SMTP when this extension is used. More
information is in the "IANA Considerations" section of this
specification.
2.7.4. UTF-8 Strings in Replies
2.7.4.1. MAIL and RCPT Commands
If the client issues the 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.
2.7.4.2. VRFY and EXPN Commands and the UTF8REPLY Parameter
If the VRFY and EXPN commands are transmitted the 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
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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 2.3 of this document
"EXPN" SP ( uLocal-part / uMailbox ) [ SP "UTF8REPLY" ] CRLF
; uLocal-part and uMailbox are defined in
; Section 2.3 of this document
There is no value associated with the "UTF8REPLY" parameter. If SMTP
reply requires UTF-8, but SMTP client does not use "UTF8REPLY"
parameter in the VERIFY (VRFY) and EXPAND (EXPN) commands, the
response code 252 is used, defined in [RFC2821], meaning "Cannot VRFY
user, but will accept the message and attempt the delivery". Also
response code 550 may be used, meaning "Requested action not taken:
mailbox unavailable". If enhanced mail system status code [RFC3463]
is used, response codes given on below is used. "UTF8REPLY" on the
VERIFY (VRFY) or EXPAND (EXPN) commands enables UTF-8 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 2.3 of this document
; User Name can contain non-ASCII characters.
uMailbox
; uMailbox is defined in section 2.3 of this document
If the SMTP reply requires UTF-8 strings, but UTF-8 is not allowed in
the reply, and enhanced mail system status codes [RFC3463] are used,
the response code should be "5.6.y" or "2.6.y" [SMTP-codes], meaning
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.".
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If the SMTP Client does not support the UTF8SMTP service extension,
but receives a the UTF-8 string in a reply, it may not be able to
properly report the reply to the user or even 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.
3. IANA Considerations
IANA is requested to add "UTF8SMTP" to the SMTP extensions registry
with the entry pointing to this specification for its definition.
IANA is requested to assign the proper error codes for "5.6.x",
"5.6.z", "5.6.y" and "2.6.y", following the guidance in Section 2.5,
and based on [SMTP-codes] and enter them in the appropriate registry.
The "Mail Transmission Types" 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] |
+---------------+----------------------------+----------------------+
4. Security Considerations
See the extended security considerations discussion in the framework
document [EAI-framework].
5. Acknowledgements
Much of the text in the initial version of this specification was
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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.M., and
Soobok Lee. Of course, none of the individuals are necessarily
responsible for the combination of ideas represented here.
6. Change History
[[anchor17: RFC Editor: Please remove this section.]]
6.1. 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.
6.2. 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".
6.3. draft-ietf-eai-smtpext: Version 02
o Upgraded to reflect the recent discussion of the ima@ietf.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.
6.4. 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.
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6.5. draft-ietf-eai-smtpext: Version 04
o Refine some syntax.
o Delete "Message Header Label" section.
o Change "bounce" to "reject".
6.6. 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"
6.7. 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"
6.8. draft-ietf-eai-smtpext: Version 07
o Rename the section 2.5
o Refine sthe section 2.7
6.9. draft-ietf-eai-smtpext: Version 08
o Refine some texts and update some references
6.10. 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.
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7. References
7.1. 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-03.txt (work in progress),
September 2007.
[EAI-framework]
Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
[]
Abel, Y., "Transmission of Email Headers in UTF-8
Encoding", draft-ietf-eai-utf8headers-07.txt (work in
progress), September 2007.
[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.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
7.2. Informative References
[EAI-downgrading]
YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
mechanism for Internationalized eMail Address",
draft-ietf-eai-downgrade-04 (work in progress), 7 2007.
[Klensin-emailaddr]
Klensin, J., "Internationalization of Email Addresses",
draft-klensin-emailaddr-i18n-03 (work in progress),
July 2005.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[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.
[SMTP-codes]
KLensin, J., "An IANA Registry for Extended SMTP Status
Codes", draft-klensin-smtp-code-registry-00 (work in
progress), April 2007.
Appendix A. Material Updating RFC 4952
RFC 4952, the Overview and Framework document covering this set of
extensions for internationalized email [EAI-framework], 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
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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 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.
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Authors' Addresses
Jiankang YAO (editor)
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813007
Email: yaojk@cnnic.cn
Wei MAO (editor)
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813055
Email: maowei_ietf@cnnic.cn
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