7Internet Engineering Task Force (IETF) A. Gulbrandsen
8Request for Comments: 6858 March 2013
10Category: Standards Track
14 Simplified POP and IMAP Downgrading for Internationalized Email
18 This document specifies a method for IMAP and POP servers to serve
19 internationalized messages to conventional clients. The
20 specification is simple, easy to implement, and provides only
25 This is an Internet Standards Track document.
27 This document is a product of the Internet Engineering Task Force
28 (IETF). It represents the consensus of the IETF community. It has
29 received public review and has been approved for publication by the
30 Internet Engineering Steering Group (IESG). Further information on
31 Internet Standards is available in Section 2 of RFC 5741.
33 Information about the current status of this document, any errata,
34 and how to provide feedback on it may be obtained at
35 http://www.rfc-editor.org/info/rfc6858.
39 Copyright (c) 2013 IETF Trust and the persons identified as the
40 document authors. All rights reserved.
42 This document is subject to BCP 78 and the IETF Trust's Legal
43 Provisions Relating to IETF Documents
44 (http://trustee.ietf.org/license-info) in effect on the date of
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46 carefully, as they describe your rights and restrictions with respect
47 to this document. Code Components extracted from this document must
48 include Simplified BSD License text as described in Section 4.e of
49 the Trust Legal Provisions and are provided without warranty as
50 described in the Simplified BSD License.
58Gulbrandsen Standards Track [Page 1]
60RFC 6858 POP and IMAP for Internationalized Email March 2013
65 A conventional IMAP or POP client may open a mailbox containing
66 internationalized messages or may even attempt to read
67 internationalized messages, for instance, when a user has both
68 internationalized and conventional Mail User Agents (MUAs).
70 Some operations cannot be performed by conventional clients. Most
71 importantly, an internationalized message usually contains at least
72 one internationalized address, so address-based operations are rarely
73 possible. This includes displaying the addresses, replying to
74 messages, and the processing of most types of address-based signature
77 However, the sender's name, message subject, body of text, and
78 attachments can easily be displayed, so a helpful IMAP or POP server
79 may prefer to display as much of the message as possible, rather than
80 hide the message entirely.
82 This document specifies a way to present such messages to the client.
83 It values simplicity of implementation over fidelity of
84 representation, since implementing a high-fidelity downgrade
85 algorithm such as the one specified in a companion document [RFC6857]
86 is likely more work than implementing proper UTF-8 support for POP
87 [RFC6856] and/or IMAP [RFC6855].
89 The server is assumed to be internationalized internally and to store
90 messages that are internationalized messages natively. When it needs
91 to present an internationalized message to a conventional client, the
92 server synthesizes a conventional message containing most of the
93 information and presents the "surrogate message".
95 This specification modifies the base IMAP specification [RFC3501] by
96 relaxing a requirement that sizes be exact and adding a reporting
97 requirement as discussed in Section 3 below.
992. Information Preserved and Lost
101 The surrogate message is intended to convey the most important
102 information to the user. Where information is lost, the user should
103 consider the message incomplete rather than modified.
105 The surrogate message is not intended to convey any information to
106 the client software that would require or enable it to apply special
107 handling to the message. Client authors who wish to handle
108 internationalized messages are encouraged to implement POP [RFC6856]
109 and/or IMAP [RFC6855] support for UTF-8.
114Gulbrandsen Standards Track [Page 2]
116RFC 6858 POP and IMAP for Internationalized Email March 2013
119 Uppercase letters in examples represent non-ASCII characters.
120 example.com is a plain domain; EXAMPLE.com represents a non-ASCII
121 domain in the .com top-level domain.
125 Each internationalized email address in the header fields listed
126 below is replaced with an invalid email address whose display-name
127 tells the user what happened.
129 The format of the display-name is explicitly unspecified. Anything
130 that tells the user what happened is good. Anything that produces an
131 email address that might belong to someone else is bad.
133 Given an internationalized address "Fred Foo <fred@EXAMPLE.com>", an
134 implementation may choose to render it as one of these examples:
136 "fred@EXAMPLE.com" <invalid@internationalized-address.invalid>
137 Fred Foo <invalid@internationalized.invalid>
138 internationalized-address:;
141 The .invalid top-level domain is reserved as a Top Level DNS Name
142 [RFC2606]; therefore, the first two examples are syntactically valid,
143 but they will never belong to anyone. Note that the display-name
144 often needs encoding (see the Message Header Extensions document
147 The affected header fields are "Bcc:", "Cc:", "From:", "Reply-To:",
148 "Resent-Bcc:", "Resent-Cc:", "Resent-From:", "Resent-Sender:",
149 "Resent-To:", "Return-Path:", "Sender:", and "To:". Any addresses
150 present in other header fields, such as "Received:", are not regarded
151 as addresses by this specification.
155 Any MIME parameter [RFC2045] (whether in the message header or a body
156 part header) that cannot be presented to the client exactly as it
157 appears in the incoming message is silently excised.
159 Given a field such as
161 Content-Disposition: attachment; filename=FOO
163 the field is presented as
165 Content-Disposition: attachment
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172RFC 6858 POP and IMAP for Internationalized Email March 2013
177 If the Subject field cannot be presented to the client exactly as it
178 appears in the incoming message, the server presents a representation
179 encoded as specified in RFC 2047.
1812.4. Remaining Header Fields
183 Any header field that cannot be presented to the client, even with
184 the modifications listed in Sections 2.1-2.3, is silently excised.
1863. IMAP-Specific Details
188 IMAP allows clients to retrieve the message size without downloading
189 the message, using RFC822.SIZE, BODY.SIZE[] and so on. The IMAP
190 specification [RFC3501] requires that the returned size be exact.
192 This specification relaxes that requirement. When a conventional
193 client requests size information for a message, the IMAP server is
194 permitted to return size information for the internationalized
195 message, even though the size of the surrogate message differs.
197 When an IMAP server performs downgrading as part of generating FETCH
198 responses, it reports which messages were synthesized using a
199 response code and attendant UID (Unique Identifier) set. This can be
200 helpful to humans debugging the server and/or client.
202 C: a UID FETCH 1:* BODY.PEEK[HEADER.FIELDS(To From Cc)]
203 S: 1 FETCH (UID 65 [...]
204 S: 2 FETCH (UID 70 [...]
205 S: a OK [DOWNGRADED 70,105,108,109] Done
207 The message-set argument to DOWNGRADED contains UIDs.
209 Note that DOWNGRADED does not necessarily mention all the
210 internationalized messages in the mailbox. In the example above, we
211 know that UID 65 does not contain internationalized addresses in the
212 "From:", "To:", and "Cc:" fields. It may, for example, contain an
213 internationalized "Subject:".
2154. POP-Specific Details
217 The number of lines specified in the TOP command [RFC1939] refers to
218 the surrogate message. The message size reported by, for example,
219 LIST may refer to either the internationalized or the surrogate
226Gulbrandsen Standards Track [Page 4]
228RFC 6858 POP and IMAP for Internationalized Email March 2013
2315. Security Considerations
233 If the internationalized message uses any sort of signature that
234 covers header fields, the signature of the surrogate message almost
235 certainly is invalid and may be invalid in other cases. This is a
236 necessary limitation of displaying internationalized messages in
237 legacy clients, since those clients do not support internationalized
238 header fields. These cases are discussed in more detail in the POP
239 or IMAP Downgrade document [RFC6857]. Even though invalid, these
240 signatures should not be removed from the surrogate message, to
241 preserve as much of the information as possible from the original
244 If any excised information is significant, then that information does
245 not arrive at the recipient. Notably, the "Message-Id:",
246 "In-Reference-To:", and "References:" fields may be excised, which
247 might cause a lack of context when the recipient reads the message.
249 Some POP or IMAP clients, such as Fetchmail, download messages and
250 delete the versions on the server. This may lead to permanent loss
251 of information when the only remaining version of a message is the
254 Other clients cache messages for a very long time, even across client
255 upgrades, such as the stock Android client. When such a client is
256 internationalized, care must be taken so that it does not use an old
257 surrogate message from its cache rather than retrieve the real
258 message from the server.
2606. IANA Considerations
262 IANA has added DOWNGRADED to the "IMAP Response Codes" registry.
2667.1. Normative References
268 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
269 STD 53, RFC 1939, May 1996.
271 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
272 Extensions (MIME) Part One: Format of Internet Message
273 Bodies", RFC 2045, November 1996.
275 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
276 Part Three: Message Header Extensions for Non-ASCII Text",
277 RFC 2047, November 1996.
282Gulbrandsen Standards Track [Page 5]
284RFC 6858 POP and IMAP for Internationalized Email March 2013
287 [RFC2606] Eastlake, D., 3rd and A. Panitz, "Reserved Top Level DNS
288 Names", BCP 32, RFC 2606, June 1999.
290 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
291 4rev1", RFC 3501, March 2003.
2937.2. Informative References
295 [RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925,
298 [RFC6855] Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
299 Support for UTF-8", RFC 6855, March 2013.
301 [RFC6856] Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "Post
302 Office Protocol Version 3 (POP3) Support for UTF-8", RFC
305 [RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for
306 Internationalized Email Messages", RFC 6857, March 2013.
310 Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John
311 Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman, and
312 Joseph Yee. This specification was inspired by the principle stated
313 in Rule 12 of "The Twelve Networking Truths" [RFC1925].
322 Fax: +49 89 4502 9758
323 EMail: arnt@gulbrandsen.priv.no
338Gulbrandsen Standards Track [Page 6]