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Internet Engineering Task Force (IETF)                       K. FujiwaraRequest for Comments: 6857                                          JPRSCategory: Standards Track                                     March 2013ISSN: 2070-1721Post-Delivery Message Downgrading for Internationalized Email MessagesAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6857.Fujiwara                     Standards Track                    [Page 1]

RFC 6857                  POP or IMAP Downgrade               March 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 2013Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .41.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . .41.2.  Possible Solutions . . . . . . . . . . . . . . . . . . . .41.3.  Approach Taken in This Specification . . . . . . . . . . .52.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .63.  Email Message Header Field Downgrading . . . . . . . . . . . .73.1.  Downgrading Method for Each ABNF Element . . . . . . . . .73.1.1.  Unstructured Downgrading . . . . . . . . . . . . . . .73.1.2.  Word Downgrading . . . . . . . . . . . . . . . . . . .73.1.3.  Comment Downgrading  . . . . . . . . . . . . . . . . .73.1.4.  MIME-Value Downgrading . . . . . . . . . . . . . . . .73.1.5.  Display-Name Downgrading . . . . . . . . . . . . . . .73.1.6.  Domain Downgrading . . . . . . . . . . . . . . . . . .83.1.7.  Group Downgrading  . . . . . . . . . . . . . . . . . .83.1.8.  Mailbox Downgrading  . . . . . . . . . . . . . . . . .83.1.9.  Type-Addr Downgrading  . . . . . . . . . . . . . . . .93.1.10. Encapsulation: A Last Resort . . . . . . . . . . . . .93.2.  Downgrading Method for Each Header Field . . . . . . . . .10       3.2.1.  Address Header Fields That Contain <address>               Elements . . . . . . . . . . . . . . . . . . . . . . .103.2.2.  Non-ASCII Strings in <comment> Elements  . . . . . . .113.2.3.  Message-ID Header Fields . . . . . . . . . . . . . . .113.2.4.  Received Header Field  . . . . . . . . . . . . . . . .113.2.5.  MIME Content Header Fields . . . . . . . . . . . . . .123.2.6.  Non-ASCII Characters in <unstructured> Elements  . . .123.2.7.  Non-ASCII Characters in <phrase> Elements  . . . . . .123.2.8.  Other Header Fields  . . . . . . . . . . . . . . . . .124.  MIME Body Parts and Delivery Status Notifications  . . . . . .124.1.  MIME Body Part Header Field Downgrading  . . . . . . . . .134.2.  Delivery Status Notification Downgrading . . . . . . . . .135.  Security Considerations  . . . . . . . . . . . . . . . . . . .136.  Implementation Note: Encoded-Word Encoding . . . . . . . . . .147.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .157.1.  Obsolescence of Existing Downgraded-* Header Fields  . . .157.2.  Registration of New Downgraded-* Header Fields . . . . . .158.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .169.  References . . . . . . . . . . . . . . . . . . . . . . . . . .169.1.  Normative References . . . . . . . . . . . . . . . . . . .169.2.  Informative References . . . . . . . . . . . . . . . . . .18Appendix A.  Downgrading Example . . . . . . . . . . . . . . . . .19Fujiwara                     Standards Track                    [Page 3]

RFC 6857                  POP or IMAP Downgrade               March 20131.  Introduction1.1.  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 20131.3.  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 (seeSection 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 inSection 3.  It   generates ASCII-only header fields.   Header fields starting with Downgraded- are introduced inSection 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 inRFC 6532.  A delivery status notification may contain   non-ASCII addresses.  MIME header field downgrading is described inSection 4.1.  Delivery status notification downgrading is described   inSection 4.2.  It generates ASCII-only MIME header fields.Fujiwara                     Standards Track                    [Page 5]

RFC 6857                  POP or IMAP Downgrade               March 2013   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 inRFC 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 definedRFC 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   inRFC 6532.   This document refers to the Augmented Backus-Naur Form (ABNF)   [RFC5234] elements that appear inRFC 5322 andRFC 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] andSection 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 20133.  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.Fujiwara                     Standards Track                    [Page 7]

RFC 6857                  POP or IMAP Downgrade               March 20133.1.6.  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 inSection 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 inSection 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 ofSection 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 inSection 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.Fujiwara                     Standards Track                   [Page 11]

RFC 6857                  POP or IMAP Downgrade               March 20133.2.5.  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.Fujiwara                     Standards Track                   [Page 12]

RFC 6857                  POP or IMAP Downgrade               March 20134.1.  MIME Body Part Header Field DowngradingRFC 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 inSection 3.24.2.  Delivery Status Notification Downgrading   If the message contains a delivery status notification (seeSection 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 inRFC 2047 [RFC2047] and with UTF-8 itself as specified inRFC 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 definedFujiwara                     Standards Track                   [Page 13]

RFC 6857                  POP or IMAP Downgrade               March 2013   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.   WhileRFC 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.Fujiwara                     Standards Track                   [Page 14]

RFC 6857                  POP or IMAP Downgrade               March 20137.  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 inRFC 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)Fujiwara                     Standards Track                   [Page 15]

RFC 6857                  POP or IMAP Downgrade               March 2013   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 describedRFC 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.  References9.1.  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 20139.2.  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 2013Appendix 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 MaildropFujiwara                     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 MessageAuthor'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.jpFujiwara                     Standards Track                   [Page 20]