Internet Engineering Task Force (IETF) K. Fujiwara
Request for Comments: 6857 JPRS
Category: Standards Track January 2013
ISSN: 2070-1721
Post-Delivery Message Downgrading for Internationalized Messages
Abstract
The Email Address Internationalization (SMTPUTF8) extension to SMTP
allows UTF-8 characters in mail header fields. Upgraded POP and IMAP
servers support internationalized messages. If a POP/IMAP client
does not support Email Address Internationalization, POP/IMAP servers
cannot deliver internationalized messages to the client and cannot
remove the message. To avoid that situation, this document describes
a mechanism for converting internationalized messages into the
traditional message format. In the process, message elements
requiring internationalized treatment are recoded or removed and
receivers are able to know that they received messages containing
such elements, even if they cannot process the internationalized
elements.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6857.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
1.2. Possible Solutions . . . . . . . . . . . . . . . . . . . . 3
1.3. Approach Taken in This Specification . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Email Header Fields Downgrading . . . . . . . . . . . . . . . 5
3.1. Downgrading Method for Each ABNF Element . . . . . . . . . 5
3.1.1. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 5
3.1.2. WORD Downgrading . . . . . . . . . . . . . . . . . . . 5
3.1.3. COMMENT Downgrading . . . . . . . . . . . . . . . . . 5
3.1.4. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 6
3.1.5. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 6
3.1.6. DOMAIN Downgrading . . . . . . . . . . . . . . . . . . 6
3.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 6
3.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 7
3.1.9. TYPE-ADDR Downgrading . . . . . . . . . . . . . . . . 7
3.1.10. ENCAPSULATION: A Last Resort . . . . . . . . . . . . . 7
3.2. Downgrading Method for Each Header Field . . . . . . . . . 9
3.2.1. Address Header Fields That Contain
Elements . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Non-ASCII Strings in Elements . . . . . . . 10
3.2.3. Message-ID Header Fields . . . . . . . . . . . . . . . 10
3.2.4. Received Header Field . . . . . . . . . . . . . . . . 10
3.2.5. MIME Content Header Fields . . . . . . . . . . . . . . 11
3.2.6. Non-ASCII Characters in Elements . . . 11
3.2.7. Non-ASCII Characters in Elements . . . . . . 11
3.2.8. Other Header Fields . . . . . . . . . . . . . . . . . 11
4. MIME Downgrading . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. MIME Body Part Header Field Downgrading . . . . . . . . . 12
4.2. Delivery Status Notification Downgrading . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13
6.1. The Encoding of RFC 2047 . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. Obsolescence of Existing Downgraded-* Header Fields . . . 14
7.2. Registration of New Downgraded-* Header Fields . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17
A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
1.1. Problem Statement
Traditional (legacy) mail systems, which are defined by [RFC5322] and
other specifications, allow only ASCII characters in mail header
field values. The SMTPUTF8 extension ([RFC6530], [RFC6531], and
[RFC6532]) allow raw UTF-8 in these mail header fields.
If a header field contains non-ASCII strings, POP/IMAP servers cannot
deliver internationalized messages to legacy clients that do not send
UTF8 commands or have UTF8 capability. Also, because they have no
obvious or standardized way to explain what is going on to clients,
they cannot even safely discard the message.
1.2. Possible Solutions
There are four plausible approaches to the problem, with the
preferred one depending on the particular circumstances and
relationship among the delivery SMTP server, the mail store, the POP
or IMAP server, and the users and their Mail User Agent (MUA)
clients:
1. If the delivery Mail Transport Agent (MTA) has sufficient
knowledge about the POP and/or IMAP servers and clients being
used, the message may be rejected as undeliverable.
2. The message may be downgraded by the POP or IMAP server in a way
that preserves maximum information at the expense of some
complexity and that does not create security or operational
problems in the mail system.
3. Some intermediate downgrading may be applied that balances more
information loss against lower complexity and greater ease of
implementation.
4. The POP or IMAP server may fabricate a message whose intent is to
notify the client that an internationalized message is waiting
but cannot be delivered until an upgraded client is available.
1.3. Approach Taken in This Specification
This specification describes the second of these options. It is
worth noting that, at least in the general case, none of these
options preserves sufficient information to guarantee that it is
possible to reply to an incoming message without loss of information,
so the choice may be considered to be among the "least bad" options.
While this document specifies a well-designed mechanism, it is only
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an interim solution while clients are being upgraded [RFC6856]
[RFC6855].
This message downgrading mechanism converts mail header fields to an
all-ASCII representation. The POP/IMAP servers can use the
downgrading mechanism and deliver the internationalized message in a
traditional form. Receivers can know they received some
internationalized messages or some unknown or broken messages.
[RFC6532] allows UTF-8 characters to be used in mail header fields
and MIME header fields. [RFC6531] allows UTF-8 characters to be used
in some trace header fields. The message downgrading mechanism
specified here describes the conversion method from the
internationalized messages that are defined in [RFC6530] and
[RFC6532] to the traditional email messages defined in [RFC5322].
This document provides a precise definition of the minimum-
information-loss message downgrading process.
Downgrading consists of the following three parts:
o New header field definitions
o Email header field downgrading
o MIME header field downgrading
Email header field downgrading is described in Section 3. It
generates ASCII-only header fields.
In Section 3.1.10 of this document, header fields starting with
Downgraded- are introduced. They preserve the information that
appeared in the original header fields.
The definition of MIME header fields in internationalized messages is
described in [RFC6532]. MIME header field downgrading is described
in Section 4.1. It generates ASCII-only MIME header fields.
Displaying downgraded messages that originally contained
internationalized header fields is out of scope of this document. A
POP/IMAP client that does not support UTF8 extensions as defined for
POP3 [UTF8 command] and IMAP [ENABLE UTF8=ACCEPT command] does not
know the internationalized message format described in [RFC6532].
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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].
All specialized terms used in this specification are defined in the
"Overview and Framework for Internationalized Email" [RFC6530], in
the mail message specifications [RFC5322], or in the MIME documents
[RFC2045] [RFC2047] [RFC2183] [RFC2231]. The terms "U-label",
"A-label", and "IDNA" are used as defined in [RFC5890]. The terms
"ASCII address", "non-ASCII address", "SMTPUTF8", "message",
"internationalized message" are used as defined in [RFC6530]. The
term "non-ASCII string" is used as defined in [RFC6532].
3. Email Header Fields Downgrading
This section defines the method to convert to ASCII for each header
field that may contain non-ASCII strings. Section 3.1 describes the
methods for rewriting each ABNF element. Section 3.2 describes the
methods for rewriting each header field.
3.1. Downgrading Method for Each ABNF Element
Header field downgrading is defined below for each ABNF element.
Conversion of the header field terminates when no non-ASCII strings
remain in the header field.
[RFC5322] describes the ABNF elements , ,
, , , and . [RFC2045]
describes the ABNF element . is updated to allow non-
ASCII characters in Section 3.3 of [RFC6531] and Section 3.2 of
[RFC6532].
3.1.1. UNSTRUCTURED Downgrading
If the header field has an field that contains non-
ASCII strings, apply the encoding of [RFC2047] with charset UTF-8.
3.1.2. WORD Downgrading
If the header field has any fields that contain non-ASCII
strings, apply the encoding of [RFC2047] with charset UTF-8.
3.1.3. COMMENT Downgrading
If the header field has any fields that contain non-ASCII
strings, apply the encoding of [RFC2047] with charset UTF-8.
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3.1.4. MIME-VALUE Downgrading
If the header field has any elements defined by [RFC2045] and
those elements contain non-ASCII strings, encode the elements
according to [RFC2231] with charset UTF-8 and leave the language
information empty. If the element is and it
contains outside the DQUOTE, remove the before this
conversion.
3.1.5. DISPLAY-NAME Downgrading
If the header field has any ( or )
elements, and they have elements that contain non-
ASCII strings, encode the elements according to
[RFC2047] with charset UTF-8. DISPLAY-NAME downgrading uses the same
algorithm as WORD downgrading.
3.1.6. DOMAIN Downgrading
If the header field has any elements that contain U-labels,
rewrite the non-ASCII domain name into an ASCII domain name using
A-labels as specified in IDNA [RFC5891].
3.1.7. GROUP Downgrading
is defined in Section 3.4 of [RFC5322]. The element
may contain elements that contain non-ASCII addresses.
If a element contains elements and one of those
elements contains a non-ASCII , rewrite the
element as
display-name " " ENCODED_WORD " :;"
where the is the original encoded
according to [RFC2047].
Otherwise, the element does not contain a non-ASCII . If the element contains non-ASCII elements,
they contain non-ASCII domain names. Rewrite the non-ASCII domain
names into ASCII domain names using A-labels as specified in IDNA
[RFC5891]. Generated elements contain ASCII addresses
only.
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3.1.8. MAILBOX Downgrading
If the of the element does not contain non-
ASCII characters, the element may contain non-ASCII
characters. Rewrite the non-ASCII domain name into ASCII domain name
using A-labels as specified in IDNA [RFC5891].
Otherwise, the may contain non-ASCII characters. The
non-ASCII has no equivalent format for ASCII addresses.
The element that contains non-ASCII strings may appear in
two forms as:
"<" addr-spec ">"
or
addr-spec
Rewrite both as:
ENCODED-WORD " :;"
where the is the original encoded
according to [RFC2047].
3.1.9. TYPE-ADDR Downgrading
If the header field contains and the contains raw non-ASCII strings, it is in utf-8-address form.
Convert it to utf-8-addr-xtext form. Both utf-8-address and utf-8-
addr-xtext are described in [RFC6533]. COMMENT downgrading is also
performed in this case. If the address type is unrecognized and the
header field contains non-ASCII strings, then fall back to using
ENCAPSULATION on the entire header field as specified in
Section 3.1.10.
3.1.10. ENCAPSULATION: A Last Resort
As a last resort, when header fields cannot be converted as discussed
in the previous subsection, the fields are deleted and replaced by
specialized new header fields. Those fields are defined to preserve,
in encoded form, as much information as possible from the header
field values of the incoming message. The syntax of these new header
fields is:
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fields =/ downgraded
downgraded = "Downgraded-Message-Id:" unstructured CRLF /
"Downgraded-Resent-Message-Id:" unstructured CRLF /
"Downgraded-In-Reply-To:" unstructured CRLF /
"Downgraded-References:" unstructured CRLF /
"Downgraded-Original-Recipient:" unstructured CRLF /
"Downgraded-Final-Recipient:" unstructured CRLF
Applying this procedure to the "Received:" header field is
prohibited. ENCAPSULATION Downgrading is allowed for "Message-ID:",
"In-Reply-To:", "References:", "Original-Recipient:", and "Final-
Recipient:" header fields.
To preserve a header field in a Downgraded- header field:
1. Generate a new header field.
* The field name is a concatenation of Downgraded- and the
original field name.
* The initial new field value is the original header field
value.
2. Treat the initial new header field value as if it were
unstructured, and then apply the encoding of [RFC2047] with
charset UTF-8 as necessary so that the resulting new header field
value is completely in ASCII.
3. Remove the original header field.
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3.2. Downgrading Method for Each Header Field
[RFC4021] establishes a registry of header fields. This section
describes the downgrading method for each header field.
If the entire mail header field contains no non-ASCII strings, email
header field downgrading is not required. Each header field's
downgrading method is described below.
3.2.1. Address Header Fields That Contain Elements
From:
Sender:
To:
Cc:
Bcc:
Reply-To:
Resent-From:
Resent-Sender:
Resent-To:
Resent-Cc:
Resent-Bcc:
Resent-Reply-To:
Return-Path:
Disposition-Notification-To:
If the header field contains non-ASCII characters, first perform
COMMENT downgrading and DISPLAY-NAME downgrading as described in the
corresponding subsections of Section 3.1. If the header field still
contains non-ASCII characters after that, complete the following two
steps:
1. If the header field contains elements that contain non-
ASCII addresses, perform GROUP downgrading on those elements.
2. If the header field contains elements that contain non-
ASCII addresses, perform MAILBOX downgrading on those elements.
This procedure may generate empty elements in the "From:",
and "Sender:" header fields. [RFC6854] updates [RFC5322] to allow
(empty) elements in "From:" and "Sender:".
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3.2.2. Non-ASCII Strings in Elements
Date:
Resent-Date:
MIME-Version:
Content-ID:
Content-Transfer-Encoding:
Content-Language:
Accept-Language:
Auto-Submitted:
These header fields do not contain non-ASCII strings except in
comments. If the header field contains UTF-8 characters in comments,
perform COMMENT downgrading.
3.2.3. Message-ID Header Fields
Message-ID:
Resent-Message-ID:
In-Reply-To:
References:
Perform ENCAPSULATION as specified in Section 3.1.10.
3.2.4. Received Header Field
Received:
If elements or elements contain U-labels, perform
DOMAIN downgrading as specified in Section 3.1.6. Comments may
contain non-ASCII strings; if so, perform COMMENT downgrading.
After the DOMAIN downgrading and the COMMENT downgrading, if the FOR
clause contains a non-ASCII , remove the "FOR" clause.
If the ID clause contains a non-ASCII values, remove the "ID" clause.
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3.2.5. MIME Content Header Fields
Content-Type:
Content-Disposition:
Perform MIME-VALUE downgrading and COMMENT downgrading.
3.2.6. Non-ASCII Characters in Elements
Subject:
Comments:
Content-Description:
Perform UNSTRUCTURED downgrading.
3.2.7. Non-ASCII Characters in Elements
Keywords:
Perform WORD downgrading.
3.2.8. Other Header Fields
Other header fields that are not covered in this document (such as
implementation-specific or user-defined fields) also might contain
non-ASCII strings. Any header field that does not have a conversion
method defined above will be in this category and is treated as
follows.
Perform UNSTRUCTURED downgrading.
If the software understands the header field's structure and a
downgrading algorithm other than UNSTRUCTURED is applicable, that
software SHOULD use that algorithm; UNSTRUCTURED downgrading is used
as a last resort.
Mailing list header fields (those that start in "List-") are part of
this category.
4. MIME Downgrading
Both MIME body part header fields and contents of a delivery status
notification may contain non-ASCII characters.
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4.1. MIME Body Part Header Field Downgrading
MIME body part header fields may contain non-ASCII strings [RFC6532].
This section defines the conversion method to ASCII-only header
fields for each MIME header field that contains non-ASCII strings.
Parse the message body's MIME structure at all levels and check each
MIME header field to see whether it contains non-ASCII strings. If
the header field contains non-ASCII strings in the header field
value, the header field is a target of the MIME body part header
field's downgrading. Each MIME header field's downgrading method is
described below. COMMENT downgrading, MIME-VALUE downgrading, and
UNSTRUCTURED downgrading are described in Section 3.
Content-ID:
The "Content-ID:" header field does not contain non-ASCII strings
except in comments. If the header field contains UTF-8 characters
in comments, perform COMMENT downgrading.
Content-Type:
Content-Disposition:
Perform MIME-VALUE downgrading and COMMENT downgrading.
Content-Description:
Perform UNSTRUCTURED downgrading.
4.2. Delivery Status Notification Downgrading
If the message contains a delivery status notification defined in
Section 6 of [RFC3461], perform the following tests and conversions.
If there are "Original-Recipient:" and "Final-Recipient:" header
fields, and the header fields contain non-ASCII strings, perform
TYPE-ADDR downgrading.
5. Security Considerations
The purpose of post-delivery message downgrading is to allow POP/IMAP
servers to deliver internationalized messages to traditional POP/IMAP
clients and permit the clients to display those messages. Users who
receive such messages can know that they were internationalized. It
does not permit receivers to read the messages in their original form
and, in general, will not permit generating replies, at least without
significant user intervention.
A downgraded message's header fields contain ASCII characters only.
However, they still contain MIME-encapsulated header fields that
contain non-ASCII strings. Furthermore, the body part may contain
UTF-8 characters. Implementations parsing Internet messages need to
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accept UTF-8 body parts and UTF-8 header fields that are MIME-
encoded. Thus, this document inherits the security considerations of
MIME-encoded header fields ([RFC2047] and [RFC3629]).
Rewriting header fields increases the opportunities for undetected
spoofing by malicious senders. However, the rewritten header field
values are preserved in equivalent MIME form or in newly defined
header fields for which traditional MUAs have no special processing
procedures.
The techniques described here invalidate methods that depend on
digital signatures over any part of the message, which includes the
top-level header fields and body part header fields. Depending on
the specific message being downgraded, at least the following
techniques are likely to break: DomainKeys Identified Mail (DKIM) and
possibly S/MIME and Pretty Good Privacy (PGP). The downgrade
mechanism SHOULD NOT remove signatures even if the signatures will
fail validation after downgrading. As much of the information as
possible from the original message SHOULD be preserved.
While information in any email header field should usually be treated
with some suspicion, current email systems commonly employ various
mechanisms and protocols to make the information more trustworthy.
Information in the new Downgraded-* header fields is not inspected by
traditional MUAs and may be even less trustworthy than the
traditional header fields. Note that the Downgraded-* header fields
could have been inserted with malicious intent (and with content
unrelated to the traditional header fields); however, traditional
MUAs do not parse Downgraded-* header fields.
In addition, if an Authentication-Results header field [RFC5451] is
present, traditional MUAs may consider the digital signatures to be
valid.
See the Security Considerations sections in [RFC6854] and [RFC6530]
for more discussion.
6. Implementation Notes
6.1. The Encoding of RFC 2047
While [RFC2047] has a specific algorithm to deal with whitespace in
adjacent encoded words, there are a number of deployed
implementations that fail to implement the algorithm correctly. As a
result, whitespace behavior is somewhat unpredictable, in practice,
when multiple encoded words are used. While [RFC5322] states that
implementations SHOULD limit lines to not more than 78 characters,
implementations MAY choose to allow overly long encoded words in
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order to work around faulty implementations of [RFC2047].
Implementations that choose to do so SHOULD have an optional
mechanism to limit line length to 78 characters.
7. IANA Considerations
[RFC5504] specified that no new header fields be registered that
begin with Downgraded-. That restriction has now been lifted, and
this document makes a new set of registrations, replacing the
experimental fields with standard ones.
7.1. Obsolescence of Existing Downgraded-* Header Fields
The Downgraded-* header fields that were registered as experimental
fields in [RFC5504] are no longer in use. IANA has changed the
status from "experimental" to "obsoleted" for every name in the
"Permanent Message Header Field Names" registry that began with
Downgraded-.
7.2. Registration of New Downgraded-* Header Fields
The following header fields have been registered in the "Permanent
Message Header Field Names" registry, in accordance with the
procedures set out in [RFC3864].
Header field name: "Downgraded-Message-Id:"
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
Header field name: "Downgraded-In-Reply-To:"
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
Header field name: "Downgraded-References:"
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
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Header field name: "Downgraded-Original-Recipient:"
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
Header field name: "Downgraded-Final-Recipient:"
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
8. Acknowledgements
This document draws heavily from the experimental in-transit message
downgrading procedure described in [RFC5504]. The contributions of
the coauthor of that earlier document, Y. Yoneya, are gratefully
acknowledged. Significant comments and suggestions were received
from John Klensin, Barry Leiba, Randall Gellens, Pete Resnick, Martin
J. Durst, and other WG participants.
9. References
9.1. Normative 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, February 2012.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[RFC6533] Hansen, T., Newman, C., and A. Melnikov,
"Internationalized Delivery Status and Disposition
Notifications", RFC 6533, February 2012.
[RFC6854] Leiba, B., "Update to Internet Message Format to Allow
Group Syntax in the "From:" and "Sender:" Header Fields",
RFC 6854, January 2013.
[RFC6856] Randy, R., Newman, C., Yao, J., and K. Fujiwara, "Post
Office Protocol Version 3 (POP3) Support for UTF-8",
RFC 6856, January 2013.
[RFC6855] Resnick, P., Newman, C., and S. Shen, "IMAP Support for
UTF-8", RFC 6855, January 2013.
9.2. Informative References
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
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RFC 6857 POP/IMAP Downgrade January 2013
[RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
Email Address Internationalization", RFC 5504, March 2009.
Appendix A. Examples
A.1. Downgrading Example
This appendix shows a message downgrading example. Consider a
received mail message where:
o The sender address is a non-ASCII address,
"NON-ASCII-LOCAL@example.com". Its display-name is "DISPLAY-
LOCAL".
o The "To:" header field contains two non-ASCII addresses,
"NON-ASCII-REMOTE1@example.net" and
"NON-ASCII-REMOTE2@example.com" Its display-names are "DISPLAY-
REMOTE1" and "DISPLAY-REMOTE2".
o The "Cc:" header field contains a non-ASCII address,
"NON-ASCII-REMOTE3@example.org". Its display-name is "DISPLAY-
REMOTE3".
o Four display-names contain non-ASCII characters.
o The "Subject:" header field is "NON-ASCII-SUBJECT", which contains
non-ASCII strings.
o The "Message-Id:" header field contains "NON-ASCII-MESSAGE_ID",
which contains non-ASCII strings.
o There is an unknown header field "X-Unknown-Header:", which
contains non-ASCII strings.
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Return-Path:
Received: from ... by ... for
Received: from ... by ... for
From: DISPLAY-LOCAL
To: DISPLAY-REMOTE1 ,
DISPLAY-REMOTE2
Cc: DISPLAY-REMOTE3
Subject: NON-ASCII-SUBJECT
Date: Mon, 30 Jul 2012 01:23:45 -0000
Message-Id: NON-ASCII-MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
X-Unknown-Header: NON-ASCII-CHARACTERS
MAIL_BODY
Figure 1: Received Message in a Maildrop
The downgraded message is shown in Figure 2. "Return-Path:",
"From:", "To:", and "Cc:" header fields are rewritten. "Subject:"
and "X-Unknown-Header:" header fields are encoded using [RFC2047].
The "Message-Id:" header field is encapsulated as a "Downgraded-
Message-Id:" header field.
Return-Path: =?UTF-8?Q?NON-ASCII-LOCAL@example.com?= :;
Received: from ... by ...
Received: from ... by ...
From: =?UTF-8?Q?DISPLAY-LOCAL?=
=?UTF-8?Q?NON-ASCII-LOCAL@example.com?= :;
To: =?UTF-8?Q?DISPLAY-REMOTE1?=
=?UTF-8?Q?NON-ASCII-REMOTE1@example.net?= :;,
=?UTF-8?Q?DISPLAY-REMOTE2?=
=?UTF-8?Q?NON-ASCII-REMOTE2@example.com?= :;,
Cc: =?UTF-8?Q?DISPLAY-REMOTE3?=
=?UTF-8?Q?NON-ASCII-REMOTE3@example.org?= :;
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Date: Mon, 30 Jul 2012 01:23:45 -0000
Downgraded-Message-Id: =?UTF-8?Q?MESSAGE_ID?=
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
X-Unknown-Header: =?UTF-8?Q?NON-ASCII-CHARACTERS?=
MAIL_BODY
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Figure 2: Downgraded Message
Author's Address
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
EMail: fujiwara@jprs.co.jp
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