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rfc:rfc6857

Internet Engineering Task Force (IETF) K. Fujiwara Request for Comments: 6857 JPRS Category: Standards Track March 2013 ISSN: 2070-1721

Post-Delivery Message Downgrading for Internationalized Email Messages

Abstract

 The Email Address Internationalization (SMTPUTF8) extension to SMTP
 allows Unicode characters encoded in UTF-8 and outside the ASCII
 repertoire in mail header fields.  Upgraded POP and IMAP servers
 support internationalized messages.  If a POP or IMAP client does not
 support Email Address Internationalization, a POP or IMAP server
 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.  As part of the conversion process,
 message elements that require internationalized treatment are recoded
 or removed, and receivers are able to recognize 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.

Fujiwara Standards Track [Page 1] RFC 6857 POP or IMAP Downgrade March 2013

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
 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.

Fujiwara Standards Track [Page 2] RFC 6857 POP or IMAP Downgrade March 2013

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . .  4
   1.2.  Possible Solutions . . . . . . . . . . . . . . . . . . . .  4
   1.3.  Approach Taken in This Specification . . . . . . . . . . .  5
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
 3.  Email Message Header Field Downgrading . . . . . . . . . . . .  7
   3.1.  Downgrading Method for Each ABNF Element . . . . . . . . .  7
     3.1.1.  Unstructured Downgrading . . . . . . . . . . . . . . .  7
     3.1.2.  Word Downgrading . . . . . . . . . . . . . . . . . . .  7
     3.1.3.  Comment Downgrading  . . . . . . . . . . . . . . . . .  7
     3.1.4.  MIME-Value Downgrading . . . . . . . . . . . . . . . .  7
     3.1.5.  Display-Name Downgrading . . . . . . . . . . . . . . .  7
     3.1.6.  Domain Downgrading . . . . . . . . . . . . . . . . . .  8
     3.1.7.  Group Downgrading  . . . . . . . . . . . . . . . . . .  8
     3.1.8.  Mailbox Downgrading  . . . . . . . . . . . . . . . . .  8
     3.1.9.  Type-Addr Downgrading  . . . . . . . . . . . . . . . .  9
     3.1.10. Encapsulation: A Last Resort . . . . . . . . . . . . .  9
   3.2.  Downgrading Method for Each Header Field . . . . . . . . . 10
     3.2.1.  Address Header Fields That Contain <address>
             Elements . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.2.  Non-ASCII Strings in <comment> Elements  . . . . . . . 11
     3.2.3.  Message-ID Header Fields . . . . . . . . . . . . . . . 11
     3.2.4.  Received Header Field  . . . . . . . . . . . . . . . . 11
     3.2.5.  MIME Content Header Fields . . . . . . . . . . . . . . 12
     3.2.6.  Non-ASCII Characters in <unstructured> Elements  . . . 12
     3.2.7.  Non-ASCII Characters in <phrase> Elements  . . . . . . 12
     3.2.8.  Other Header Fields  . . . . . . . . . . . . . . . . . 12
 4.  MIME Body Parts and Delivery Status Notifications  . . . . . . 12
   4.1.  MIME Body Part Header Field Downgrading  . . . . . . . . . 13
   4.2.  Delivery Status Notification Downgrading . . . . . . . . . 13
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
 6.  Implementation Note: Encoded-Word Encoding . . . . . . . . . . 14
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   7.1.  Obsolescence of Existing Downgraded-* Header Fields  . . . 15
   7.2.  Registration of New Downgraded-* Header Fields . . . . . . 15
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
 Appendix A.  Downgrading Example . . . . . . . . . . . . . . . . . 19

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1. Introduction

1.1. Problem Statement

 Traditional (legacy) mail systems, which are defined by the Internet
 Message Format [RFC5322] and other specifications, allow only ASCII
 characters in mail header field values.  The SMTPUTF8 extension
 [RFC6530] [RFC6531] [RFC6532] allows Unicode characters encoded in
 UTF-8 [RFC3629] in these mail header fields.  "Raw non-ASCII strings"
 refers to strings of those characters in which at least one of them
 is not part of the ASCII repertoire.
 If a header field contains non-ASCII strings, a POP or IMAP server
 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, a POP or IMAP server cannot even safely discard the message.

1.2. Possible Solutions

 There are four plausible approaches to the problem.  The preferred
 approach depends 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.  The
 four approaches are as follows:
 1.  If the delivery Mail Transport Agent (MTA) has sufficient
     knowledge about the POP or IMAP server and the clients being
     used, the message may be rejected as undeliverable.
 2.  A new, surrogate, message may be created by downgrading the
     original one in 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.  These surrogate messages are referred to as "downgraded"
     in this specification and as "surrogate messages" elsewhere.
 3.  Some intermediate downgrading may be applied that balances
     additional information loss against lower complexity and greater
     ease of implementation.
 4.  The POP or IMAP server may fabricate a message that is intended
     to notify the client that an internationalized message is waiting
     but cannot be delivered until an upgraded client is available.

Fujiwara Standards Track [Page 4] RFC 6857 POP or IMAP Downgrade March 2013

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 one of the available "least bad"
 options.  While this document specifies a well-designed mechanism, it
 is only an interim solution while clients are being upgraded
 [RFC6855] [RFC6856].
 This message downgrading mechanism converts mail header fields to an
 all-ASCII representation.  The POP or IMAP server can use the
 downgrading mechanism and then deliver the internationalized message
 in a traditional form, which allows receivers to know whether a
 message is internationalized or unknown or broken.
 The Internationalized Mail Header specification [RFC6532] allows
 UTF-8 characters (see Section 2) to be used in mail header fields and
 MIME header fields.  The Internationalized Mail Transport
 specification [RFC6531] allows UTF-8 characters to be used in some
 trace header fields.  The message downgrading mechanism specified
 here describes the method by which internationalized messages
 [RFC6530] [RFC6532] are converted to traditional email messages
 [RFC5322].
 This document provides a precise definition of the minimum-
 information-loss message downgrading process.
 Downgrading consists of the following two parts:
 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.
 Header fields starting with Downgraded- are introduced in
 Section 3.1.10.  They preserve the information that appeared in the
 original header fields.
 The definition of MIME header fields in internationalized messages is
 described in RFC 6532.  A delivery status notification may contain
 non-ASCII addresses.  MIME header field downgrading is described in
 Section 4.1.  Delivery status notification downgrading is described
 in Section 4.2.  It generates ASCII-only MIME header fields.

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 Displaying downgraded messages that originally contained
 internationalized header fields is out of scope of this document.  A
 POP or IMAP client that does not support UTF8 extensions as defined
 for POP3 "UTF8 command" and IMAP "ENABLE UTF8=ACCEPT command" does
 not recognize the internationalized message format [RFC6532].

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].
 Many of the specialized terms used in this specification are defined
 in other documents.  They include "Overview and Framework for
 Internationalized Email" [RFC6530], the Internet Message Format
 specification [RFC5322], and some of the basic MIME documents
 [RFC2045] [RFC2183].  This specification makes extensive use of the
 MIME Message Header Extensions [RFC2047] and extended MIME parameter
 encodings [RFC2231].  For convenience, both are described as
 "encoded-words" or "encoded-word encoding".  All of the encoded-words
 generated according to this specification use UTF-8 as their charset.
 The terms "U-label", "A-label", and "IDNA" are used as defined in the
 IDNA Definitions document [RFC5890].  The terms "ASCII address",
 "non-ASCII address", "SMTPUTF8", "message", and "internationalized
 message" are used as defined RFC 6530.  The term "non-ASCII string"
 is used with the definition provided in the Internationalized Email
 Headers document [RFC6532].  The term "UTF-8 character" is used
 informally in this document to denote a Unicode character, encoded in
 UTF-8, outside the ASCII repertoire.  Such characters are more
 formally described using the ABNF element <UTF8-non-ascii>, defined
 in RFC 6532.
 This document refers to the Augmented Backus-Naur Form (ABNF)
 [RFC5234] elements that appear in RFC 5322 and RFC 2045.  RFC 5322
 describes the ABNF elements <CFWS>, <comment>, <display-name>,
 <group>, <id-left>, <id-right>, <mailbox>, <quoted-string>,
 <unstructured>, and <word>.  RFC 2045 describes the ABNF element
 <value>.  Section 3.3 of the Internationalized Mail Transport
 specification [RFC6531] and Section 3.2 of the Internationalized
 Email Headers document [RFC6532] updated <domain> to allow non-ASCII
 characters.
 Some additional terms are defined locally in-line below.

Fujiwara Standards Track [Page 6] RFC 6857 POP or IMAP Downgrade March 2013

3. Email Message Header Field Downgrading

 This section defines the method for converting each header field that
 may contain non-ASCII strings into ASCII.  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 characters other
 than those in the ASCII repertoire remain in the header field.

3.1.1. Unstructured Downgrading

 If the header field has an <unstructured> field that contains
 non-ASCII strings, apply encoded-word encoding.

3.1.2. Word Downgrading

 If the header field has any <word> fields that contain non-ASCII
 strings, apply encoded-word encoding.

3.1.3. Comment Downgrading

 If the header field has any <comment> fields that contain non-ASCII
 strings, apply encoded-word encoding.

3.1.4. MIME-Value Downgrading

 If the header field has any <value> elements [RFC2045] that contain
 non-ASCII strings, remove any <CFWS> that appear outside DQUOTE
 [RFC5234] that appear in those elements, then encode the <value>
 elements as extended MIME parameter encodings [RFC2231] and leave the
 language information empty.

3.1.5. Display-Name Downgrading

 If the header field has any <address> (<mailbox> or <group>)
 elements, and they have <display-name> elements that contain
 non-ASCII strings, encode the <display-name> elements as encoded-
 words.  Display-Name downgrading uses the same algorithm as Word
 downgrading.

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3.1.6. Domain Downgrading

 If the header field has any <domain> elements that contain U-labels,
 rewrite the non-ASCII domain name into an ASCII domain name using
 A-labels [RFC5891].

3.1.7. Group Downgrading

 <group> is defined in Section 3.4 of the Internet Message Format
 specification [RFC5322].  The <group> element may contain <mailbox>
 elements that contain non-ASCII addresses.
 If a <group> element contains <mailbox> elements and one of those
 <mailbox> elements contains a non-ASCII <local-part>, rewrite the
 <group> element as
 display-name " " ENCODED_WORD " :;"
 where the <ENCODED_WORD> is the original <group-list> encoded as
 encoded-words.
 Otherwise, the <group> element contains an ASCII-only <local-part>.
 If the <group> element contains non-ASCII <mailbox> elements, they
 contain non-ASCII domain names.  Rewrite the non-ASCII domain names
 into ASCII domain names using A-labels [RFC5891].  Generated
 <mailbox> elements contain ASCII addresses only.

3.1.8. Mailbox Downgrading

 If the <local-part> of the <mailbox> element contains no characters
 other than those in the ASCII repertoire, the <domain> element may
 contain non-ASCII characters.  Rewrite the non-ASCII domain names
 into ASCII domain names using A-labels [RFC5891].
 Otherwise, the <local-part> may contain non-ASCII characters.  The
 <local-part> that contains characters outside the ASCII repertoire
 has no equivalent format for ASCII addresses.  The <addr-spec>
 element that contains non-ASCII strings may appear in two forms as:
 "<" addr-spec ">"
 or
 addr-spec
 Rewrite both as:
 ENCODED-WORD " :;"

Fujiwara Standards Track [Page 8] RFC 6857 POP or IMAP Downgrade March 2013

 where the <ENCODED-WORD> is the original <addr-spec> encoded as
 encoded-words.

3.1.9. Type-Addr Downgrading

 If the header field contains <utf-8-type-addr> and the
 <utf-8-type-addr> contains raw non-ASCII strings (<UTF8-non-ascii>),
 it is in utf-8-address form [RFC6533].  Convert it to
 utf-8-addr-xtext form [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.  This mechanism is known as
 Encapsulation downgrading in this specification because it preserves
 the original information in a different form.  The syntax of these
 new header fields is:
 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.

Fujiwara Standards Track [Page 9] RFC 6857 POP or IMAP Downgrade March 2013

 2.  Treat the initial new header field value as if it were
     unstructured, and then apply the encoded-word encoding as
     necessary so that the resulting new header field value is
     completely in ASCII.
 3.  Remove the original header field.

3.2. Downgrading Method for Each Header Field

 The Mail and MIME Header Fields document [RFC4021] establishes a
 registry of header fields.  This section describes the downgrading
 method for each header field listed in that registry as of the date
 of publication of this specification.
 If the entire mail header field contains no characters other than
 those in the ASCII repertoire, email header field downgrading is not
 required.  Each header field's downgrading method is described below.

3.2.1. Address Header Fields That Contain <address> 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, complete the following two steps:
 1.  If the header field contains <group> elements that contain
     non-ASCII addresses, perform Group downgrading on those elements.
 2.  If the header field contains <mailbox> elements that contain
     non-ASCII addresses, perform Mailbox downgrading on those
     elements.

Fujiwara Standards Track [Page 10] RFC 6857 POP or IMAP Downgrade March 2013

 This procedure may generate empty <group> elements in the "From:" and
 "Sender:" header fields.  The Group Syntax document [RFC6854] updates
 the Internet Message Format specification [RFC5322] to allow (empty)
 <group> elements in the "From:" and "Sender:" header fields.

3.2.2. Non-ASCII Strings in <comment> Elements

 Date:
 Resent-Date:
 MIME-Version:
 Content-ID:
 Content-Transfer-Encoding:
 Content-Language:
 Accept-Language:
 Auto-Submitted:
 Except in comments, these header fields do not contain characters
 other than those in the ASCII repertoire.  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:
 If there are non-ASCII strings in <id-left> or <id-right> elements,
 perform Encapsulation.  Otherwise, the header field contains UTF-8
 characters in comments and Comment downgrading should be performed.

3.2.4. Received Header Field

 Received:
 If <domain> elements or <mailbox> 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 <local-part>, remove the FOR
 clause.  If the "ID" clause contains a non-ASCII value, remove the ID
 clause.

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3.2.5. MIME Content Header Fields

 Content-Type:
 Content-Disposition:
 If there are non-ASCII strings in <value> or <CFWS> elements, perform
 MIME-Value and Comment downgrading.

3.2.6. Non-ASCII Characters in <unstructured> Elements

 Subject:
 Comments:
 Content-Description:
 If non-ASCII strings are present in <unstructured> elements, perform
 Unstructured downgrading.

3.2.7. Non-ASCII Characters in <phrase> Elements

 Keywords:
 If non-ASCII strings are present in <phrase> elements, 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) might also contain
 non-ASCII strings.  Any header field that does not have a conversion
 method defined above will be in this category and treated as follows.
 If there are non-ASCII strings present in the header fields, 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
 when there is no other option.
 Mailing list header fields (those that start in "List-") are part of
 this category.

4. MIME Body Parts and Delivery Status Notifications

 Both the MIME body part header fields [RFC2045] [RFC6532] and the
 contents of a delivery status notification [RFC6533] may contain
 non-ASCII characters.

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4.1. MIME Body Part Header Field Downgrading

 RFC 6532 specifies an extension that permits MIME header fields,
 including body part header fields, to contain non-ASCII strings.
 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.  The downgrading methods used for the MIME body
 part header fields Content-ID, Content-Type, Content-Disposition, and
 Content-Description are the same as those used for the header fields
 of the same name described in Section 3.2

4.2. Delivery Status Notification Downgrading

 If the message contains a delivery status notification (see Section 6
 of the SMTP DSN Extension [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 and
 IMAP servers to deliver internationalized messages to traditional POP
 and IMAP clients and to permit the clients to display those messages.
 Users that 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.
 After downgrading as specified in this document, the header fields of
 a message will contain ASCII characters only, some of them in
 encoded-word form.  Nothing in this document or other SMTPUTF8
 specifications [RFC6530] [RFC6531] alters the basic properties of
 MIME that allow characters outside the ASCII repertoire in encodings
 as specified for them.  Thus, this document inherits the security
 considerations associated with MIME-encoded header fields as
 specified in RFC 2047 [RFC2047] and with UTF-8 itself as specified in
 RFC 3629 [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

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 header fields for which traditional MUAs have no special processing
 procedures.
 The techniques described here may 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.  In addition,
 MUAs may be able to use the presence of an Authentication-Results
 header field [RFC5451] to assess whether the digital signatures were
 valid before the header fields were downgraded.
 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 evaluate Downgraded-* header fields.
 See the Security Considerations sections in the Group Syntax document
 [RFC6854] and the Internationalized Email Framework [RFC6530] for
 more discussion.

6. Implementation Note: Encoded-Word Encoding

 While the specification of encoded-words includes specific rules for
 dealing with whitespace in adjacent encoded words [RFC2047], 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 RFC 5322 states that implementations SHOULD limit lines to 78
 characters or less, implementations MAY choose to allow overly long
 encoded words to work around faulty implementations of encoded-words.
 Implementations that choose to do so SHOULD have an optional
 mechanism to limit line length to 78 characters.

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7. IANA Considerations

 The experimental specification from which this document was partially
 derived [RFC5504] specifies that no new header fields beginning with
 Downgraded- are to be registered.  That restriction is now 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 RFC 5504 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 the Header Field Registration document
 [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)
 Header field name:  Downgraded-Original-Recipient
 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-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 RFC 5504.  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.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.

Fujiwara Standards Track [Page 16] RFC 6857 POP or IMAP Downgrade March 2013

 [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, March 2013.
 [RFC6855]  Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
            Support for UTF-8", RFC 6855, March 2013.
 [RFC6856]  Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "Post
            Office Protocol Version 3 (POP3) Support for UTF-8",
            RFC 6856, March 2013.

Fujiwara Standards Track [Page 17] RFC 6857 POP or IMAP Downgrade March 2013

9.2. Informative References

 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
            Message Authentication Status", RFC 5451, April 2009.
 [RFC5504]  Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
            Email Address Internationalization", RFC 5504, March 2009.

Fujiwara Standards Track [Page 18] RFC 6857 POP or IMAP Downgrade March 2013

Appendix A. 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".  Their 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.
 Return-Path: <NON-ASCII-LOCAL@example.com>
 Received: from ... by ... for <NON-ASCII-REMOTE1@example.net>
 Received: from ... by ... for <NON-ASCII-REMOTE1@example.net>
 From: DISPLAY-LOCAL <NON-ASCII-LOCAL@example.com>
 To: DISPLAY-REMOTE1 <NON-ASCII-REMOTE1@example.net>,
     DISPLAY-REMOTE2 <NON-ASCII-REMOTE2@example.com>
 Cc: DISPLAY-REMOTE3 <NON-ASCII-REMOTE3@example.org>
 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

Fujiwara Standards Track [Page 19] RFC 6857 POP or IMAP Downgrade March 2013

 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 as encoded-words.
 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
                     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

Fujiwara Standards Track [Page 20]

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