Network Working Group Y. Abel, Ed.
Request for Comments: 5335 TWNIC
Updates: 2045, 2822 September 2008
Internationalized Email Headers
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Full internationalization of electronic mail requires not only the
capabilities to transmit non-ASCII content, to encode selected
information in specific header fields, and to use non-ASCII
characters in envelope addresses. It also requires being able to
express those addresses and the information based on them in mail
header fields. This document specifies an experimental variant of
Internet mail that permits the use of Unicode encoded in UTF-8,
rather than ASCII, as the base form for Internet email header field.
This form is permitted in transmission only if authorized by an SMTP
extension, as specified in an associated specification. This
specification Updates section 6.4 of RFC 2045 to conform with the
1.1. Role of This Specification
Full internationalization of electronic mail requires several
o The capability to transmit non-ASCII content, provided for as part
of the basic MIME specification [RFC2045], [RFC2046].
o The capability to use international characters in envelope
addresses, discussed in [RFC4952] and specified in [RFC5336].
o The capability to express those addresses, and information related
to them and based on them, in mail header fields, defined in this
This document specifies an experimental variant of Internet mail that
permits the use of Unicode encoded in UTF-8 [RFC3629], rather than
ASCII, as the base form for Internet email header fields. This form
is permitted in transmission, if authorized by the SMTP extension
specified in [RFC5336] or by other transport mechanisms capable of
1.2. Relation to Other Standards
This document updates Section 6.4 of RFC 2045. It removes the
blanket ban on applying a content-transfer-encoding to all subtypes
of message/, and instead specifies that a composite subtype MAY
specify whether or not a content-transfer-encoding can be used for
that subtype, with "cannot be used" as the default.
This document also updates [RFC2822] and MIME ([RFC2045]), and the
fact that an Experimental specification updates a Standards-Track
specification means that people who participate in the experiment
have to consider those standards updated.
Allowing use of a content-transfer-encoding on subtypes of messages
is not limited to transmissions that are authorized by the SMTP
extension specified in [RFC5336]. Message/global permits use of a
2. Background and History
Mailbox names often represent the names of human users. Many of
these users throughout the world have names that are not normally
expressed with just the ASCII repertoire of characters, and would
like to use more or less their real names in their mailbox names.
These users are also likely to use non-ASCII text in their common
names and subjects of email messages, both received and sent. This
protocol specifies UTF-8 as the encoding to represent email header
The traditional format of email messages [RFC2822] allows only ASCII
characters in the header fields of messages. This prevents users
from having email addresses that contain non-ASCII characters. It
further forces non-ASCII text in common names, comments, and in free
text (such as in the Subject: field) to be encoded (as required by
MIME format [RFC2047]). This specification describes a change to the
email message format that is related to the SMTP message transport
change described in the associated document [RFC4952] and [RFC5336],
and that allows non-ASCII characters in most email header fields.
These changes affect SMTP clients, SMTP servers, mail user agents
(MUAs), list expanders, gateways to other media, and all other
processes that parse or handle email messages.
As specified in [RFC5336], an SMTP protocol extension "UTF8SMTP" is
used to prevent the transmission of messages with UTF-8 header fields
to systems that cannot handle such messages.
Use of this SMTP extension helps prevent the introduction of such
messages into message stores that might misinterpret, improperly
display, or mangle such messages. It should be noted that using an
ESMTP extension does not prevent transferring email messages with
UTF-8 header fields to other systems that use the email format for
messages and that may not be upgraded, such as unextended POP and
IMAP servers. Changes to these protocols to handle UTF-8 header
fields are addressed in [EAI-POP] and [IMAP-UTF8] .
The objective for this protocol is to allow UTF-8 in email header
fields. Issues such as how to handle messages containing UTF-8
header fields that have to be delivered to systems that have not been
upgraded to support this capability are discussed in [DOWNGRADE].
A plain ASCII string is also a valid UTF-8 string; see [RFC3629]. In
this document, ordinary ASCII characters are UTF-8 characters if they
are in headers which contain <utf8-xtra-char>s.
Unless otherwise noted, all terms used here are defined in [RFC2821],
[RFC2822], [RFC4952], or [RFC5336].
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 [RFC2119].
4. Changes on Message Header Fields
SMTP clients can send header fields in UTF-8 format, if the UTF8SMTP
extension is advertised by the SMTP server or is permitted by other
This protocol does NOT change the [RFC2822] rules for defining header
field names. The bodies of header fields are allowed to contain
UTF-8 characters, but the header field names themselves must contain
only ASCII characters.
To permit UTF-8 characters in field values, the header definition in
[RFC2822] must be extended to support the new format. The following
ABNF is defined to substitute those definitions in [RFC2822].
The syntax rules not covered in this section remain as defined in
4.1. UTF-8 Syntax and Normalization
UTF-8 characters can be defined in terms of octets using the
following ABNF [RFC5234], taken from [RFC3629]:
UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail /
%xE1-EC 2(UTF8-tail) /
%xED %x80-9F UTF8-tail /
UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) /
%xF1-F3 3( UTF8-tail ) /
%xF4 %x80-8F 2( UTF8-tail )
UTF8-tail = %x80-BF
These are normatively defined in [RFC3629], but kept in this document
for reasons of convenience.
See [RFC5198] for a discussion of normalization; the use of
normalization form NFC is RECOMMENDED.
4.2. Changes on MIME Headers
This specification updates Section 6.4 of [RFC2045]. [RFC2045]
prohibits applying a content-transfer-encoding to all subtypes of
message/. This specification relaxes the rule -- it allows newly
defined MIME types to permit content-transfer-encoding, and it allows
content-transfer-encoding for message/global (see Section 4.6).
Background: Normally, transfer of message/global will be done in
8-bit-clean channels, and body parts will have "identity" encodings,
that is, no decoding is necessary. In the case where a message
containing a message/global is downgraded from 8-bit to 7-bit as
described in [RFC1652], an encoding may be applied to the message; if
the message travels multiple times between a 7-bit environment and an
environment implementing UTF8SMTP, multiple levels of encoding may
occur. This is expected to be rarely seen in practice, and the
potential complexity of other ways of dealing with the issue are
thought to be larger than the complexity of allowing nested encodings
4.3. Syntax Extensions to RFC 2822
The following rules are intended to extend the corresponding rules in
[RFC2822] in order to allow UTF-8 characters.
FWS = <see [RFC2822], folding white space>
CFWS = <see [RFC2822], folding white space>
ctext =/ UTF8-xtra-char
utext =/ UTF8-xtra-char
comment = "(" *([FWS] utf8-ccontent) [FWS] ")"
word = utf8-atom / utf8-quoted-string
This means that all the [RFC2822] constructs that build upon these
will permit UTF-8 characters, including comments and quoted strings.
We do not change the syntax of <atext> in order to allow UTF8
characters in <addr-spec>. This would also allow UTF-8 characters in
<message-id>, which is not allowed due to the limitation described in
Section 4.5. Instead, <utf8-atext> is added to meet this
Note, however, this does not remove any constraint on the character
set of protocol elements; for instance, all the allowed values for
timezone in the Date: headers are still expressed in ASCII. And
also, none of this revised syntax changes what is allowed in a
<msg-id>, which will still remain in pure ASCII.
4.4. Change on addr-spec Syntax
Internationalized email addresses are represented in UTF-8. Thus,
all header fields containing <mailbox>es are updated to permit UTF-8
as well as an additional, optional all-ASCII alternate address. Note
that Message Submission Servers ("MSAs") and Message Transfer Agents
(MTAs) may downgrade internationalized messages as needed. The
procedure for doing so is described in [DOWNGRADE].
mailbox = name-addr / addr-spec / utf8-addr-spec
angle-addr =/ [CFWS] "<" utf8-addr-spec [ alt-address ] ">"
[CFWS] / obs-angle-addr
utf8-addr-spec = utf8-local-part "@" utf8-domain
utf8-local-part= utf8-dot-atom / utf8-quoted-string / obs-local-part
utf8-domain = utf8-dot-atom / domain-literal / obs-domain
alt-address = FWS "<" addr-spec ">"
Below are a few examples of possible <mailbox> representations.
; traditional mailbox format
; UTF8SMTP but no ALT-ADDRESS parameter provided,
; message will bounce if UTF8SMTP extension is not supported
; without DISPLAY_NAME and quoted string
; UTF8SMTP but no ALT-ADDRESS parameter provided,
; message will bounce if UTF8SMTP extension is not supported
"DISPLAY_NAME" <non-ASCII@non-ASCII <ASCII@ASCII>>
; UTF8SMTP with ALT-ADDRESS parameter provided,
; ALT-ADDRESS can be used if downgrade is necessary
4.5. Trace Field Syntax
"For" fields containing internationalized addresses are allowed, by
use of the new uFor syntax. UTF-8 information may be needed in
Received fields. Such information is therefore allowed to preserve
the integrity of those fields. The uFor syntax retains the original
UTF-8 email address between email address internationalization (EAI)-
aware MTAs. Note that, should downgrading be required, the uFor
parameter is dropped per the procedure specified in [DOWNGRADE].
The "Return-Path" header provides the email return address in the
mail delivery. Thus, the header is augmented to carry UTF-8
addresses (see the revised syntax of <angle-addr> in Section 4.4 of
this document). This will not break the rule of trace field
integrity, because the header is added at the last MTA and described
The <item-value> on "Received:" syntax is augmented to allow UTF-8
email address in the "For" field. <angle-addr> is augmented to
include UTF-8 email address. In order to allow UTF-8 email addresses
in an <addr-spec>, <utf8-addr-spec> is added to <item-value>.
item-value =/ utf8-addr-spec
Internationalized messages must only be transmitted as authorized by
[RFC5336] or within a non-SMTP environment which supports these
messages. A message is a "message/global message", if
o it contains UTF-8 header values as specified in this document, or
o it contains UTF-8 values in the headers fields of body parts.
The type message/global is similar to message/rfc822, except that it
contains a message that can contain UTF-8 characters in the headers
of the message or body parts. If this type is sent to a 7-bit-only
system, it has to be encoded in MIME [RFC2045]. (Note that a system
compliant with MIME that doesn't recognize message/global would treat
it as "application/octet-stream" as described in Section 5.2.4 of
Alternatively, SMTP servers and other systems which transfer a
message/global body part MAY choose to down-convert it to a message/
rfc822 body part using the rules described in [DOWNGRADE].
Type name: message
Subtype name: global
Required parameters: none
Optional parameters: none
Encoding considerations: Any content-transfer-encoding is permitted.
The 8-bit or binary content-transfer-encodings are recommended
Security considerations: See Section 5.
Interoperability considerations: The media type provides
functionality similar to the message/rfc822 content type for email
messages with international email headers. When there is a need
to embed or return such content in another message, there is
generally an option to use this media type and leave the content
unchanged or down-convert the content to message/rfc822. Both of
these choices will interoperate with the installed base, but with
different properties. Systems unaware of international headers
will typically treat a message/global body part as an unknown
attachment, while they will understand the structure of a message/
rfc822. However, systems that understand message/global will
provide functionality superior to the result of a down-conversion
to message/rfc822. The most interoperable choice depends on the
Published specification: RFC 5335
Applications that use this media type: SMTP servers and email
clients that support multipart/report generation or parsing.
Email clients which forward messages with international headers as
Magic number(s): none
File extension(s): The extension ".u8msg" is suggested.
Macintosh file type code(s): A uniform type identifier (UTI) of
"public.utf8-email-message" is suggested. This conforms to
"public.message" and "public.composite-content", but does not
necessarily conform to "public.utf8-plain-text".
Person & email address to contact for further information: See the
Author's Address section of this document.
Intended usage: COMMON
Restrictions on usage: This is a structured media type which embeds
other MIME media types. The 8-bit or binary content-transfer-
encoding MUST be used unless this media type is sent over a 7-bit-
Author: See the Author's Address section of this document.
Change controller: IETF Standards Process
5. Security Considerations
If a user has a non-ASCII mailbox address and an ASCII mailbox
address, a digital certificate that identifies that user may have
both addresses in the identity. Having multiple email addresses as
identities in a single certificate is already supported in PKIX
(Public Key Infrastructure for X.509 Certificates) and OpenPGP.
Because UTF-8 often requires several octets to encode a single
character, internationalized local parts may cause mail addresses to
become longer. As specified in [RFC2822], each line of characters
MUST be no more 998 octets, excluding the CRLF.
Because internationalized local parts may cause email addresses to be
longer, processes that parse, store, or handle email addresses or
local parts must take extra care not to overflow buffers, truncate
addresses, or exceed storage allotments. Also, they must take care,
when comparing, to use the entire lengths of the addresses.
In this specification, a user could provide an ASCII alternative
address for a non-ASCII address. However, it is possible these two
addresses go to different mailboxes, or even different people. This
configuration may be based on a user's personal choice or on
administration policy. We recognize that if ASCII and non-ASCII
email is delivered to two different destinations, based on MTA
capability, this may violate the principle of least astonishment, but
this is not a "protocol problem".
The security impact of UTF-8 headers on email signature systems such
as Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is
discussed in RFC 4952, Section 9. A subsequent document [DOWNGRADE]
will cover the impact of downgrading on these systems.
6. IANA Considerations
IANA has registered the message/global MIME type using the
registration form contained in Section 4.4.
This document incorporates many ideas first described in Internet-
Draft form by Paul Hoffman, although many details have changed from
that earlier work.
The author especially thanks Jeff Yeh for his efforts and
contributions on editing previous versions.
Most of the content of this document is provided by John C Klensin.
Also, some significant comments and suggestions were received from
Charles H. Lindsey, Kari Hurtta, Pete Resnick, Alexey Melnikov, Chris
Newman, Yangwoo Ko, Yoshiro Yoneya, and other members of the JET team
(Joint Engineering Team) and were incorporated into the document.
The editor sincerely thanks them for their contributions.
8.1. Normative References
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-
MIMEtransport", RFC 1652, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for
Network Interchange", RFC 5198, March 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5336] Yao, J., Ed. and W. Mao, Ed., "SMTP Extension for
Internationalized Email Addresses", RFC 5336,
8.2. Informative References
[DOWNGRADE] Fujiwara, K. and Y. Yoneya, "Downgrading mechanism for
Email Address Internationalization", Work in Progress,
[EAI-POP] Newman, C. and R. Gellens, "POP3 Support for UTF-8",
Work in Progress, July 2008.
[IMAP-UTF8] Resnick, P. and C. Newman, "IMAP Support for UTF-8",
Work in Progress, April 2008.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
Abel Yang (editor)
4F-2, No. 9, Sec 2, Roosvelt Rd.
Phone: +886 2 23411313 ext 505
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