Network Working Group J. Yao
Internet-Draft W. Mao
Obsoletes: RFC5336 (if approved) CNNIC
Updates: RFC5321 and 5322 June 2, 2011
(if approved)
Intended status: Standards Track
Expires: December 4, 2011
SMTP Extension for Internationalized Email Address
draft-ietf-eai-rfc5336bis-10.txt
Abstract
This document specifies an SMTP extension for transport and delivery
of email messages with internationalized email addresses or header
information.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 4, 2011.
Copyright Notice
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Role of This Specification . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Updates to Other Specifications . . . . . . . . . . . . . 5
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
3. Mail Transport-Level Protocol . . . . . . . . . . . . . . . . 5
3.1. Framework for the Internationalization Extension . . . . . 5
3.2. The UTF8SMTPbis Extension . . . . . . . . . . . . . . . . 6
3.3. Extended Mailbox Address Syntax . . . . . . . . . . . . . 7
3.4. MAIL Command Parameter Usage . . . . . . . . . . . . . . . 9
3.5. Non-ASCII addresses and Reply-codes . . . . . . . . . . . 9
3.6. Body Parts and SMTP Extensions . . . . . . . . . . . . . . 10
3.7. Additional ESMTP Changes and Clarifications . . . . . . . 10
3.7.1. The Initial SMTP Exchange . . . . . . . . . . . . . . 10
3.7.2. Mail eXchangers . . . . . . . . . . . . . . . . . . . 11
3.7.3. Trace Information . . . . . . . . . . . . . . . . . . 11
3.7.4. UTF-8 Strings in Replies . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
7. Change History . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. draft-yao-eai-rfc5336bis: Version 00 . . . . . . . . . . . 16
7.2. draft-ietf-eai-rfc5336bis: Version 00 . . . . . . . . . . 16
7.3. draft-ietf-eai-rfc5336bis: Version 01 . . . . . . . . . . 17
7.4. draft-ietf-eai-rfc5336bis: Version 02 . . . . . . . . . . 17
7.5. draft-ietf-eai-rfc5336bis: Version 03 . . . . . . . . . . 17
7.6. draft-ietf-eai-rfc5336bis: Version 04 . . . . . . . . . . 17
7.7. draft-ietf-eai-rfc5336bis: Version 05 . . . . . . . . . . 17
7.8. draft-ietf-eai-rfc5336bis: Version 06 . . . . . . . . . . 17
7.9. draft-ietf-eai-rfc5336bis: Version 07 . . . . . . . . . . 17
7.10. draft-ietf-eai-rfc5336bis: Version 08 . . . . . . . . . . 17
7.11. draft-ietf-eai-rfc5336bis: Version 09 . . . . . . . . . . 17
7.12. draft-ietf-eai-rfc5336bis: Version 10 . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
The Simple Mail Transfer Protocol [RFC5321] provides a negotiation
mechanism about service extension by which SMTP clients can discover
SMTP server capabilities and make decisions for further processing.
This document uses this mechanism and specifies an SMTP extension to
permit internationalized email addresses (see section 1.2) in the
SMTP envelope, and Unicode characters encoded in UTF-8 [RFC3629] in
the headers. An extended overview of the extension model for
internationalized email addresses and the email header appears in
[RFC4952bis], referred to as "the framework document" or just as
"framework" elsewhere in this specification.
[[anchor1: Note in Draft and to RFC Editor: The keyword represented
in this document by "UTF8SMTPbis" (and in the XML source
byUTF8SMTPbis) is a placeholder. The actual keyword will be assigned
when the standards track SMTP extension in this series [RFC5336bis-
SMTP] is approved for publication and should be substituted here.
This paragraph should be treated as normative reference to that SMTP
extension draft, creating a reference hold until it is approved by
the IESG. This paragraph should be removed before RFC publication.]]
1.1. Role of This Specification
The framework document [RFC4952bis] 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 [RFC5321] [RFC5322] 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 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 "UTF-8 string" or "UTF-8 character" are used to refer to
Unicode characters encoded in UTF-8. All other specialized terms
used in this specification are defined in the framework document or
in the base Internet email specifications. In particular, the terms
"ASCII address", "internationalized email address", "non-ASCII
address", "UTF8SMTPbis", "internationalized message", and "message"
are used in this document according to the definitions in the
framework document.
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Non-ASCII characters or strings referred in this document MUST be
expressed in UTF-8, a standard Unicode Encoding Form.
This specification uses Augmented BNF (ABNF) rules [RFC5234]. Some
basic rules in this document can be found from [RFC5234] or [RFC5321]
or [RFC5322] under the same names.
1.3. Updates to Other Specifications
This specification modifies RFC 5321 by permitting internationalized
email address in the envelope. It also updates some syntax rules
defined in RFC 5321. It modifies RFC 5322 by permitting data formats
defined in [RFC5335bis]. It does not modify the 8BITMIME
specification [RFC6152] in any way, but it does require that the
8BITMIME extension be announced by the EAI-aware SMTP server and used
with "BODY=8BMITMIME" by the EAI-aware SMTP client.
2. Overview of Operation
This specification describes an optional extension to the email
transport mechanism that permits non-ASCII characters in both the
envelope and header fields of messages, which are encoded in UTF-8.
The extension is identified with the token "UTF8SMTPbis".
The EAI UTF-8 header specification [RFC5335bis] provides the details
of email header features enabled by this extension
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
"UTF8SMTPbis".
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 this keyword.
The EAI-aware SMTP client MUST ignore any parameters if they
appear for this keyword; that is, the EAI-aware SMTP client MUST
behave as if the parameters do not appear. If an SMTP server
includes UTF8SMTPbis in its EHLO response, it MUST be fully
compliant with this version of this specification.
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4. One OPTIONAL parameter "UTF8SMTPbis" is added to the MAIL
command. The parameter has no value. If this parameter is set
in the MAIL command, it indicates that the SMTP client is EAI-
aware and asserts that the envelope includes the non-ASCII
address or the message being sent is internationalized message
or the message being sent needs the UTF8SMTPbis support.
5. The maximum length of a MAIL command line is increased by 12
characters by the possible addition of the UTF8SMTPbis
parameter. [[anchor6: RFC Editor: the number '12' will be
replaced by the new number (1 space + length of the new keyword
supposed to replace "UTF8SMTPbis").]]
6. One OPTIONAL parameter "UTF8SMTPbis" is added to the VRFY and
EXPN commands. The parameter UTF8SMTPbis 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.
7. No additional SMTP verbs are defined by this extension.
8. Servers offering this extension MUST provide support for, and
announce, the 8BITMIME extension [RFC6152].
9. 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).
10. The mail message body is extended as specified in [RFC5335bis].
11. The UTF8SMTPbis extension is valid on the submission port
[RFC4409], and can be used with LMTP [RFC2033].
3.2. The UTF8SMTPbis Extension
An SMTP server that announces this UTF8SMTPbis extension MUST be
prepared to accept a UTF-8 string [RFC3629] in any position in which
RFC 5321 specifies that a <mailbox> can appear. Although the
characters in the <local-part> are permitted to contain non-ASCII
characters, actual parsing of the <local-part>, and the delimiters
used, are unchanged from the base email specification [RFC5321]. Any
domain names to be looked up in the DNS MUST allow for [RFC5890]
behavior. When doing lookups, the EAI-aware SMTP client or server
MUST either use a Unicode aware DNS library, or transform it to
A-label defined in [RFC5890].
An SMTP client that receives the UTF8SMTPbis 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 [RFC5335bis] (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 as A-labels or U-labels
[RFC5890]. The presence of the UTF8SMTPbis extension does not change
RFC 5321 server relaying behaviors.
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If the UTF8SMTPbis SMTP extension is not offered by the SMTP server,
the EAI-aware SMTP client MUST NOT transmit an internationalized
email address and MUST NOT transmit a mail message containing
internationalized mail headers as described in [RFC5335bis] at any
level within its MIME structure [RFC2045] and [RFC2047]. (For this
paragraph, the internationalized domain name in the form of A-labels
as specified in IDNA definitions [RFC5890] is not considered to be
"internationalized".) Instead, if an EAI-aware SMTP client (EAI-
aware SMTP sender) attempts to transfer an internationalized message
and encounters an SMTP server that does not support the extension, it
MUST make one of the following three choices and the priority order
is 1, 2 and 3.
1. It MAY 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 5321 [RFC5321], RFC 3464 [RFC3464], and the EAI
delivery status notification (DSN) specification [RFC5337bis].
2. If and only if the EAI-aware SMTP client (sender) is a Message
Submission Agent ("MSA") [RFC4409] [RFC5598], MSA may choose its
own way to deal with this scenario according to the provisions of
[RFC4409] or its future versions. But the detailed specification
of this process and its results is outside the scope of this
document.
3. It MAY find an alternate route to the destination that permits
UTF8SMTPbis. That route MAY be discovered by trying alternate
Mail eXchanger (MX) hosts (using preference rules as specified in
RFC 5321) or using other means available to the EAI-aware SMTP
client.
This document applies only when an EAI-aware SMTP client is trying to
send an internationalized message to an EAI-aware SMTP server. For
all other cases, and for addresses and messages that do not require
an UTF8SMTPbis extension, EAI-aware SMTP clients and servers do not
change the behavior specified in [RFC5321].
An EAI-aware MUA/MSA sending to a legacy SMTP server [RFC5321] and
[RFC5322] MAY convert an ASCII@U-label [RFC5890] address into the
format of ASCII@A-label [RFC5890] if the email address is in the
format of ASCII@U-label.
3.3. Extended Mailbox Address Syntax
RFC 5321, section 4.1.2, defines the syntax of a <Mailbox> entirely
in terms of ASCII characters. This document will make <Mailbox> to
support non-ASCII characters.
The key changes made by this specification include:
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o In order to update the <Mailbox> to support the internationalized
email address, the <Mailbox> ABNF rule will be importted from RFC
5321 directly, and other related rules are importted from RFC
5321, RFC 5234, RFC 5890 or RFC 5335bis, or are extended in this
document.
o Extend the definition of <sub-domain> to permit both the RFC 5321
definition and a UTF-8 string in a DNS label that is conforming
with IDNA definitions [RFC5890].
o Extend the definition of <atext> to permit both the RFC 5321
definition and a UTF-8 string. That string MUST NOT contain any
of the ASCII graphics or controls characters.
The following ABNF rules will be importted from RFC 5321, section
4.1.2 directly:
o <Mailbox>
o <Local-part>
o <Dot-string>
o <Quoted-string>
o <QcontentSMTP>
o <Domain>
o <Atom>
The following ABNF rule will be importted from RFC 5335bis, section
4.1 directly:
o <UTF8-non-ascii>
The following ABNF rule will be importted from RFC 5234, appendix B.1
directly:
o <DQUOTE>
The following ABNF rule will be importted from RFC 5890, section
2.3.2.1 directly:
o <U-label>
The following rules are extended in ABNF [RFC5234] as follows.
sub-domain =/ U-label
; extend the defintion of sub-domain in RFC5321, section 4.1.2
atext =/ UTF8-non-ascii
; extend the defintion of atext in RFC5321, section 4.1.2
quoted-pairSMTP =/ %d92 UTF8-non-ascii
; extend the defintion of quoted-pairSMTP in RFC5321, section 4.1.2
qtextSMTP =/ UTF8-non-ascii
; extend the defintion of qtextSMTP in RFC5321, section 4.1.2
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3.4. MAIL Command Parameter Usage
If the envelope or message being sent requires the capabilities of
the UTF8SMTPbis extension, the EAI-aware SMTP client MUST supply the
UTF8SMTPbis parameter with the MAIL command. If this parameter is
provided, it MUST have no value. If the EAI-aware SMTP client is
aware that neither the envelope nor the message being sent requires
any of the UTF8SMTPbis extension capabilities, it SHOULD NOT supply
the UTF8SMTPbis parameter with the MAIL command.
Because there is no guarantee that a next-hop SMTP server will
support the UTF8SMTPbis extension, use of the UTF8SMTPbis extension
always carries a risk of transmission failure. In fact, during the
early stages of deployment for the UTF8SMTPbis extension, the risk
will be quite high. Hence there is a distinct near-term advantage
for ASCII-only messages to be sent without using this extension. The
long-term advantage of casting ASCII [ASCII] characters(0x7f and
below) as UTF-8 form is that it permits pure-Unicode environments.
This specification does not require that the EAI-aware SMTP client
inspect the message or otherwise go to extraordinary lengths to
assure itself whether the UTF8SMTPbis extension is REQUIRED for the
particular message.
3.5. Non-ASCII addresses and Reply-codes
An EAI-aware SMTP client MUST only send an internationalized message
to an SMTP server that supports UTF8SMTPbis. If the SMTP server does
not support this option, then the EAI-aware SMTP client has three
choices according to section 3.2 of this specification.
The three-digit Reply-codes used in this section are based on their
meanings as defined in RFC 5321.
When messages are rejected because the RCPT command requires an ASCII
address, the reply-code 553 is returned with the meaning "mailbox
name not allowed". When messages are rejected because the MAIL
command requires an ASCII address, the reply-code 550 is returned
with the meaning "mailbox unavailable". When the EAI-aware SMTP
server supports enhanced mail system status codes [RFC3463], reply-
code "X.6.7" [RFC5248] is used, meaning that "non-ASCII addresses not
permitted for that sender/recipient".
When messages are rejected for other reasons, the server follows the
model of the base email specifications in RFC 5321; this extension
does not change those circumstances or reply messages.
If the reply-code is issued after the final "." of the DATA command,
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the reply-code "554" is used with the meaning "Transaction failed".
When the EAI-aware SMTP server supports enhanced mail system status
codes [RFC3463], reply-code "X.6.9" [RFC5248] is used, meaning that
"UTF-8 header message can not be transmitted to one or more
recipients, so the message MUST be rejected".
3.6. Body Parts and SMTP Extensions
The MAIL command parameter UTF8SMTPbis asserts that a message is an
internationalized message or the message being sent needs the
UTF8SMTPbis support. The message being sent via the MAIL command
with the UTF8SMTPbis parameter has still a chance of that the message
transmitted is not an internationalized message. An EAI-aware SMTP
client or server that requires accurate knowledge of whether a
message is internationalized needs to parse all message header fields
and MIME header fields [RFC2045] and [RFC2047] in the message body.
However, this specification does not require that the EAI-aware SMTP
client or server inspects the message.
While this specification requires that EAI-aware SMTP servers support
the 8BITMIME extension [RFC6152] 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 email addresses
obviously does not require non-ASCII body parts in the MIME message
in RFC 2045 and RFC 2047. The UTF8SMTPbis extension MAY be used with
the BODY=8BITMIME parameter [RFC6152] if that is appropriate given
the body content or, with the BODY=BINARYMIME parameter, if the SMTP
server advertises BINARYMIME [RFC3030] and that is appropriate.
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 5321 specifies a
mailbox, this SMTP extension requires UTF-8 form to be used for the
entire string; when RFC 5321 specifies a domain name, the name SHOULD
be in the form of A-label if this domain name is an internationalized
domain name[RFC5890].
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 SMTP server sends a "greeting"
response consisting of the 220 reply-code and some information. The
SMTP client then sends the EHLO command. Since the SMTP client
cannot know whether the SMTP server supports UTF8SMTPbis until after
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it receives the response from EHLO, the EAI-aware SMTP client MUST
send only ASCII (LDH label or A-label [RFC5890] ) domains in the EHLO
command and that, if the EAI-aware SMTP server provides domain names
in the EHLO response, they MUST be in the form of LDH labels or
A-labels.
3.7.2. Mail eXchangers
If multiple DNS MX records are used to specify multiple servers for a
domain in section 5 of [RFC5321], it is strongly advised that all or
none of them SHOULD support the UTF8SMTPbis extension. Otherwise,
surprising rejections can happen during temporary or permanent
failures, which users might perceive as serious reliability issues.
3.7.3. Trace Information
The trace information <Return-path-line>, <Time-stamp-line> and their
related rules have been defined in in section 4.4 of RFC 5321
[RFC5321]. This document has updated <Mailbox> and <Domain> to
support non-ASCII characters. So Return-path-line may include the
'Reverse-path' clause where internationalized domain name with the
U-label form may be used. Time-stamp-line may include the 'For'
clause where the internationalized domain name with the U-label form
may be used.
Except in the 'For' clause and 'Reverse-path' clause where
internationalized domain name with the U-label form MAY be used,
internationalized domain names in Received fields MUST be transmitted
in the form of A-labels. The protocol value of the 'WITH' clause
when this extension is used is one of the UTF8SMTPbis values
specified in the "IANA Considerations" section of this document.
3.7.4. UTF-8 Strings in Replies
3.7.4.1. MAIL Command
If an SMTP client follows this specification and sends any MAIL
commands containing the UTF8SMTPbis parameter, the EAI-aware SMTP
server is permitted to use UTF-8 characters in the email address
associated with 251 and 551 reply-codes, and the SMTP client MUST be
able to accept and process them. If a given MAIL command does not
include the UTF8SMTPbis parameter, the EAI-aware SMTP 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 non-ASCII addresses.
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3.7.4.2. VRFY and EXPN Commands and the UTF8SMTPbis Parameter
If the VRFY and EXPN commands are transmitted with the parameter
"UTF8SMTPbis", it indicates the SMTP client can accept UTF-8 strings
in replies to those commands. This parameter for the VRFY and EXPN
commands SHOULD only be used after the SMTP client sees the EHLO
response with the UTF8SMTPbis keyword. This allows the EAI-aware
SMTP server to use UTF-8 strings in mailbox names and full names that
occur in replies without concern that the SMTP 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 EAI-aware 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 string is not needed in them.
Some replies, notably those resulting from successful execution of
the VRFY and EXPN commands, do include the mailbox.
VERIFY (VRFY) and EXPAND (EXPN) command syntaxes are changed to:
vrfy = "VRFY" SP String
[ SP "UTF8SMTPbis" ] CRLF
; String may include UTF-8 characters
expn = "EXPN" SP String
[ SP "UTF8SMTPbis" ] CRLF
; String may include UTF-8 characters
The "UTF8SMTPbis" parameter does not use a value. If the reply to a
VERIFY (VRFY) or EXPAND (EXPN) command requires UTF-8 string, but the
SMTP client did not use the "UTF8SMTPbis" parameter, then the EAI-
aware SMTP server MUST use either the reply-code 252 or 550. Reply-
code 252, defined in [RFC5321], means "Cannot VRFY user, but will
accept the message and attempt the delivery". Reply-code 550, also
defined in [RFC5321], means "Requested action not taken: mailbox
unavailable". When the EAI-aware SMTP server supports enhanced mail
system status codes [RFC3463], the enhanced reply-code as specified
below is used. Using the "UTF8SMTPbis" 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:
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User Name <Mailbox>
; Mailbox is defined in section 3.3 of this document.
; User Name can contain non-ASCII characters.
Mailbox
; Mailbox is defined in section 3.3 of this document.
If the SMTP reply requires UTF-8 strings, but UTF-8 string is not
allowed in the reply, and the EAI-aware SMTP server supports enhanced
mail system status codes [RFC3463], the enhanced reply-code is
"X.6.8" [RFC5248], 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 SMTP client".
If the SMTP client does not support the UTF8SMTPbis 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 string MUST NOT appear in the reply.
Although UTF-8 form 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 string for any other purposes. EAI-
aware SMTP servers MUST NOT include non-ASCII characters in replies
except in the limited cases specifically permitted in this section.
4. IANA Considerations
IANA is requested to add a new value "UTF8SMTPbis" to the SMTP
Service Extension subregistry of the Mail Parameters registry,
according to the following data:
+-------------+---------------------------------+-----------+
| Keywords | Description | Reference |
+-------------+---------------------------------+-----------+
| UTF8SMTPbis | Internationalized email address | [RFCXXXX] |
+-------------+---------------------------------+-----------+
This document updates the values to the SMTP Enhanced Status Code
subregistry of the Mail Parameters registry, following the guidance
in Sections 3.5 and 3.7.4.2 of this document, and being based on
[RFC5248]. The registration data is as follows:
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Code: X.6.7
Sample Text: non-ASCII addresses not permitted
for that sender/recipient
Associated basic status code: 550, 553
Description: This indicates the reception of a MAIL or RCPT
command that non-ASCII addresses are not permitted
Defined: RFC XXXX (Standard track)
Submitter: Jiankang YAO
Change controller: ima@ietf.org
Code: X.6.8
Sample Text: UTF-8 string reply is required,
but not permitted by the SMTP client
Associated basic status code: 252, 550, 553
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 SMTP client.
Defined: RFC XXXX (Standard track)
Submitter: Jiankang YAO
Change controller: ima@ietf.org
Code: X.6.9
Sample Text: UTF-8 header message can not be transferred
to one or more recipient so the message
must be rejected
Associated basic status code: 550
Description: This indicates that transaction failed
after the final "." of the DATA command.
Defined: RFC XXXX (Standard track)
Submitter: Jiankang YAO
Change controller: ima@ietf.org
Code: X.6.10
Description: This is a duplicate of X.6.8 and
is thus deprecated.
The following entries SHOULD be updated or added in the "Mail
Transmission Types" registry under the Mail Parameters registry.
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+--------------+-------------------------------+--------------------+
| WITH | Description | Reference |
| protocol | | |
| types | | |
+--------------+-------------------------------+--------------------+
| UTF8SMTP | ESMTP with UTF8SMTP | [RFCXXXX] |
| UTF8SMTPA | ESMTP with UTF8SMTP and SMTP | [RFC4954] |
| | AUTH | [RFCXXXX] |
| UTF8SMTPS | ESMTP with UTF8SMTP and | [RFC3207] |
| | STARTTLS | [RFCXXXX] |
| UTF8SMTPSA | ESMTP with UTF8SMTP and both | [RFC3207] |
| | STARTTLS and SMTP AUTH | [RFC4954] |
| | | [RFCXXXX] |
| UTF8LMTP | LMTP with UTF8SMTP | [RFCXXXX] |
| UTF8LMTPA | LMTP with UTF8SMTP and SMTP | [RFC4954] |
| | AUTH | [RFCXXXX] |
| UTF8LMTPS | LMTP with UTF8SMTP and | [RFC3207] |
| | STARTTLS | [RFCXXXX] |
| UTF8LMTPSA | LMTP with UTF8SMTP and both | [RFC3207] |
| | STARTTLS and LMTP AUTH | [RFC4954] |
| | | [RFCXXXX] |
+--------------+-------------------------------+--------------------+
5. Security Considerations
The extended security considerations discussion in the framework
document [RFC4952bis] will be applied here.
More security considerations are discussed below:
Beyond the use inside the email global system (in SMTP envelopes and
message headers), internationalized email addresses will also show up
inside other cases, in particular:
o the logging systems of SMTP transactions and other logs to monitor
the email systems;
o the trouble ticket systems used by Security Teams to manage
security incidents, when an email address is involved;
In order to avoid problems that could cause loss of data, this will
likely require extending these systems to support full UTF-8, or to
require to provide an adequate mechanisms for mapping non-ASCII
strings to ASCII.
Another security aspect to be considered is related to the ability by
security team members to quickly understand, read and identify email
addresses from the logs, when they are tracking an incident.
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Mechanims to automatically and quickly provide the origin or
ownership of an internationalized email address SHALL be implemented
for use also by log readers which cannot read easily non-ASCII
information.
The SMTP commands VRFY and EXPN are sometimes used in SMTP
transactions where there is no message to transfer (by tools used to
take automated actions in case potential spam messages are
identified). RFC 5321 section 3.5 and 7.3 give some detailed
description of use and possible behaviours. Implementation of
internationalized addrsses can affect also logs and actions by these
tools.
6. Acknowledgements
This document revised the [RFC5336]document based on the EAI WG's
discussion result. Many EAI WG members did some tests and
implementations to move this document to the Standard Track document.
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, Miguel Garcia, Magnus Westerlund,
and Lars Eggert. Of course, none of the individuals are necessarily
responsible for the combination of ideas represented here.
Thanks a lot to Dave Crocker for his comments and helping of ABNF
refinement.
7. Change History
[[anchor14: RFC Editor: Please remove this section.]]
7.1. draft-yao-eai-rfc5336bis: Version 00
Applied errata suggested by Alfred Hoenes.
7.2. draft-ietf-eai-rfc5336bis: Version 00
Applied the changes suggested by the EAI new charter.
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7.3. draft-ietf-eai-rfc5336bis: Version 01
Applied the changes suggested by 78 IETF EAI meeting.
7.4. draft-ietf-eai-rfc5336bis: Version 02
remove the appendix since rfc4952bis has added this material
improve the text
remove the text about no body parameter
7.5. draft-ietf-eai-rfc5336bis: Version 03
improve the text
7.6. draft-ietf-eai-rfc5336bis: Version 04
update the abstract
improve the text
7.7. draft-ietf-eai-rfc5336bis: Version 05
improve the text based on AD and Co-chairs
7.8. draft-ietf-eai-rfc5336bis: Version 06
update the iana consideration
7.9. draft-ietf-eai-rfc5336bis: Version 07
improve the iana consideration
7.10. draft-ietf-eai-rfc5336bis: Version 08
improve the texts
add the mail parameter
add the new section about mail command parameter usage
update the security consideration
7.11. draft-ietf-eai-rfc5336bis: Version 09
improve the texts
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7.12. draft-ietf-eai-rfc5336bis: Version 10
refine the ABNF definitions
improve the texts
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.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.
[RFC4952bis]
Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", I-D rfc4952bis, September 2010.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5248] Hansen , T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", RFC 5248, June 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
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October 2008.
[RFC5335bis]
Abel, Y. and S. Steel, "Internationalized Email Headers",
I-D rfc5335bis, March 2011.
[RFC5337bis]
Hansen, T., Ed., Newman, C., and A. Melnikov, Ed.,
"Internationalized Delivery Status and Disposition
Notifications", I-D 5337bis, October 2010.
[RFC5890] Klensin, J., "Internationalizing Domain Names in
Applications (IDNA definitions)", RFC 5890, June 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
Service Extension for 8-bit MIME Transport", STD 71,
RFC 6152, March 2011.
8.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 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.
[RFC5336] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email Addresses", RFC 5336, September 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
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Authors' Addresses
Jiankang YAO
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813007
Email: yaojk@cnnic.cn
Wei MAO
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58812230
Email: maowei_ietf@cnnic.cn
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