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 Fujiwara Standards Track [Page 1] RFC 6857 POP/IMAP Downgrade January 2013 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 Fujiwara Standards Track [Page 2] RFC 6857 POP/IMAP Downgrade January 2013 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 Fujiwara Standards Track [Page 3] RFC 6857 POP/IMAP Downgrade January 2013 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]. Fujiwara Standards Track [Page 4] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 5] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 6] RFC 6857 POP/IMAP Downgrade January 2013 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: Fujiwara Standards Track [Page 7] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 8] RFC 6857 POP/IMAP Downgrade January 2013 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:". Fujiwara Standards Track [Page 9] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 10] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 11] RFC 6857 POP/IMAP Downgrade January 2013 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 Fujiwara Standards Track [Page 12] RFC 6857 POP/IMAP Downgrade January 2013 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 Fujiwara Standards Track [Page 13] RFC 6857 POP/IMAP Downgrade January 2013 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) Fujiwara Standards Track [Page 14] RFC 6857 POP/IMAP Downgrade January 2013 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. Fujiwara Standards Track [Page 15] RFC 6857 POP/IMAP Downgrade January 2013 [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. Fujiwara Standards Track [Page 16] 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. Fujiwara Standards Track [Page 17] RFC 6857 POP/IMAP Downgrade January 2013 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 Fujiwara Standards Track [Page 18] RFC 6857 POP/IMAP Downgrade January 2013 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 19]