Email Address Internationalization                             E. Dainow
(EAI)                                                            Afilias
Internet-Draft                                               K. Fujiwara
Intended status: Informational                                      JPRS
Expires: March 11, 2013                                September 7, 2012

             Guidelines for Internationalized Email Clients


   This document provides some guidelines for email clients that support
   Email Address Internationalization (EAI) as outlined in [RFC6530].  A
   number of interoperability cases between different versions of email
   components are reviewed.  Recommendations are made to improve
   interoperability and usability and to minimize discrepancies between
   the display of composed and received email in different language

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 11, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   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.  Conventions used in this document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Interoperability . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Interoperability Scenarios . . . . . . . . . . . . . . . .  5
   5.  Compatibility Support  . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Address Book . . . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Message Mode . . . . . . . . . . . . . . . . . . . . . . .  7
     5.3.  Message Format . . . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Error Handling . . . . . . . . . . . . . . . . . . . . . .  8
     5.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.6.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Mailbox Integration  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Character Encoding . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Normalization  . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 14

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1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   [RFC6530] Overview and Framework for Internationalized Email
   describes changes to electronic mail (email) to fully support
   internationalized characters.  The fundamental change is to remove
   the ASCII only restriction on email addresses and allow them to
   contain UTF-8 characters.  Additional documents provide detailed
   specifications for the extensions required to email headers [RFC6532]
   and to the protocols SMTP [RFC6531], POP [I-D.ietf-eai-rfc5721bis]
   and IMAP [I-D.ietf-eai-5738bis].

   This document provides guidelines for email clients that support
   these specifications for Email Address Internationalization (EAI).
   It does not introduce any protocol extensions that are not defined in
   the above documents.  It highlights the extensions that are important
   to the design and implementation of email clients and makes a number
   of recommendations intended to improve interoperability and

3.  Terminology

   A number of different acronyms are typically used to describe the
   major functional components of email.

   Mail User Agent (MUA)
   Message Submission Agent (MSA)
   Message Transfer Agent (MTA)
   Message Delivery Agent (MDA)
   Message Store (MS)

   The architecture of modern email systems can range from simple, with
   all components running on one server, to very complex, with
   components being distributed across multiple, geographically
   dispersed machines.  Nevertheless, the above terminology is generally
   sufficient to represent different architectures from a functional
   point of view.  For a comprehensive description of email architecture
   see [RFC5598].

   sender -> MUA -> MSA -> MTA
                           MTA -> MDA -> MS -> PIF -> MUA -> recipient

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   In this context, an "Email Client" is an MUA that has an interface to
   an MSA to send email and an interface to the MS to retrieve email.
   The interface to retrieve mail (PIF) is a POP or IMAP server or
   direct access to the File system.  The MUA also provides a User
   Interface (UI) that allows an end user to read (display) and write
   (compose) their email.

   A common email architecture includes the MSA function within the MTA.
   An improved architecture that better addresses security concerns is a
   separate MSA component as shown here [RFC6409], [RFC5068].

   "SMTPUTF8" is used to indicate email address internationalization as
   specified by [RFC6530] and related documents.

   "ASCII" refers to the strict 7-bit ASCII character set

   "UTF-8", Unicode Transformation Format/8-bit is a character encoding
   scheme that can represent any character in the Unicode standard
   [RFC3629].  It contains ASCII as a subset.

   "message/global" is an email message that contains UTF-8 characters
   beyond 7-bit ASCII in message headers and/or body parts [RFC6532].

   "message/rfc822" is an email message that contains only 7 bit ASCII
   and does not use any SMTPUTF8 extensions.  Note that the original
   message (as composed by the user) may contain non-ASCII characters
   that have been encoded into ASCII using IDNA [RFC5890], MIME body
   encoding [RFC2045] or MIME header encoding [RFC2047].

4.  Interoperability

   Internationalized Email is not compatible with legacy email systems,
   those based on prior Internet email standards [RFC5321], [RFC5322].
   Non-ASCII email addresses cannot be submitted in legacy SMTP commands
   like MAIL FROM or RCPT TO.  In addition the Internationalized Email
   standard does not include a method to "downgrade" message/global to

   An Internationalized message cannot be transmitted via SMTP if the
   receiving MTA does not announce SMTPUTF8 in response to EHLO.  There
   are two failure cases that an email client may have to handle
   described in Section 3.2 of [RFC6531].

   a) If the client is submitting a message to an MSA that does not
   support SMTPUTF8, the message will be rejected.

   b) If the MSA does support SMTPUTF8 but a downstream MTA does not,

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   then the mail will bounce.  That is, a delivery status notification
   (DSN) that the mail could not be delivered will be sent back to the

   Incompatibility between Internationalized email and legacy systems is
   expected to be important initially during a transition period but
   less important over time as more email systems upgrade to support the
   SMTPUTF8 extensions.  To the extent that this incompatibility is
   deemed important at the time an implementation is undertaken, the
   email client should provide methods to prevent or at least minimize
   these failures.

4.1.  Interoperability Scenarios

   The following scenarios cover the different cases of sending mail
   from an Internationalized server to a legacy server.

   'I' indicates an Internationalized address (a non-ASCII address on an
   Internationalized mail server).

   'IA' indicates an ASCII address on an Internationalized server.

   'LA' indicates an address on a Legacy mail server, which must be

   Case 1.  The simple compatibility case

   From:    IA1 (or LA1)
   To:      LA2
   Subject: ...
   Body     ...

   The message will be successfully sent as long as the email client
   sends message/rfc822 rather than message/global.

   Case 2.  The simple incompatibility case

   From:    I1
   To:      LA2
   Subject: ...
   Body     ...

   The message will be rejected by the MSA or will bounce from a
   downstream SMTP server.

   If user I1 also has an ASCII email address IA1 or LA1, there may be a
   simple workaround.  If the email client supports multiple email
   accounts, the user just has to switch the From address to an ASCII

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   address and it becomes Case 1.

   Case 3.  The general incompatibility cases

   The general case is a mix of Internationalized and legacy addresses.
   While many combinations are possible, the two cases below essentially
   cover all possibilities.

   From: I1
   To:   LA2
   Cc:   I3

   The message will be sent to I3 but it will bounce from LA2.

   Switching the From address to an ASCII address as in Case 2 is not a
   solution, as the following case demonstrates.

   From: IA1 (or LA1)
   To:   LA2
   Cc:   I3

   This message will bounce from LA2 since the address in the Cc header
   cannot be transmitted to a legacy server.

   In these cases, users will likely send the message twice in order to
   reach all intended recipients.  First, to the original list and then
   using an ASCII address to the bounced recipients.

   If users know beforehand which addresses are on legacy servers, they
   can avoid bounced messages by removing those addresses, but they
   still have to send a second email to reach recipients that were

5.  Compatibility Support

   An email client can provide support to minimize the incompatibility
   problems outlined in Section 4.  There may be several ways to do
   this.  Following are guidelines on some of the ways that this can be

   At the very least, to provide basic compatibility between
   Internationalized and legacy systems, if all email addresses in the
   SMTP envelope and the message headers are ASCII, then a message/
   rfc822 should be sent (Case 1 above).

   For Case 2, the email client should support multiple email accounts
   and allow the user to switch the From address at any time during
   composition of the message.

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   For Case 3, several mechanisms may be required to provide
   compatibility support.  These are outlined in the following sub-

5.1.  Address Book

   Each contact in the address book should be able to have several email
   addresses, each of which is configured to be either an
   Internationalized or a Legacy address.

   The user may not necessarily know if an ASCII address they enter in
   their address book is on a legacy server or not.  If it is configured
   as an Internationalized address and that turns out to be wrong, then
   email sent to that contact may bounce.  The user can then re-
   configure the address as Legacy so the email client can provide
   warnings of a possible bounce on subsequent messages.

5.2.  Message Mode

   Message composition should have "Message Mode" option to specify
   "Internationalized Mode" or "Legacy Mode".

   If the type of each address in the headers does not conform to the
   message mode, then the user is given a warning about those addresses
   that don't match the mode.  In a graphical user interface this might
   be done by setting such addresses to a different color such as red.

   The user would typically first change the message mode to see if the
   warnings disappear.

   When the mode is switched, the email client switches addresses in
   message header fields to match the mode, selecting from the list of
   addresses in each contact.

   There are cases where both modes provide warnings (see Example 5
   below).  In these cases, the user can remove the addresses that don't
   conform to the mode.

   For Internationalized mode, the user has an additional option to send
   the message anyway, without removing flagged addresses.  They would
   have to handle bounced messages from Legacy servers later.  The
   option to send anyway cannot be provided in Legacy mode, as it is not
   possible to compose a message/rfc822 if any sender or recipient
   address is not ASCII.

   Where both modes provide warnings, users will likely want to send the
   message in each mode in order to reach all recipients.  The email
   client should make it easy to do this.  There are many possible

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   designs to accomplish this.  The following is one example.

   An option is provided when composing email to add a second message
   header section in the other mode that allows the user to move
   addresses between sections.  This is in addition to making individual
   changes to address headers as in normal email composition.  The
   Subject and Body are common so the user can compose a single message
   but have it sent in the two different modes to different recipients.

   Following is an example of this for Case 3 above.

   Legacy                   Internationalized
   From: IA1                From: I1
   To:   LA2       <--->    To:
   Cc:             <--->    Cc:   I3
   Subject: ...
   Body:    ...

5.3.  Message Format

   In Internationalized Mode, mail should be sent as message/global.
   The aim of Internationalized Email is 8 bit clean messages using
   UTF-8 encoding to represent Unicode characters in header fields and
   the message body.

   In Legacy Mode, mail must be sent as message/rfc822.  This may
   include non-ASCII characters that are encoded into ASCII using MIME
   body encoding [RFC2045] or MIME header encoding [RFC2047].  Any
   encoding should be based on UTF-8.  In the interest of
   interoperability, charsets other than UTF-8 are prohibited in mail
   addresses and message headers described in Section 7.1 of [RFC6530].

5.4.  Error Handling

   If a message is rejected by the MSA with a response code that
   indicates incompatibility with legacy email described in Section 3.2
   of [RFC6531], the compose window should be kept open so that the user
   can make changes and retry.  The email client should provide guidance
   to the user about switching the Message Mode, reconfiguring the type
   of an address in the address book or adding an ASCII legacy address
   for a contact in the address book.

   Similarly, if a message bounces, the email client could parse the
   delivery status notifications and message disposition notifications
   [RFC6533] to determine if the failure was a compatibility problem and

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   if so, which addresses caused the problem.

5.5.  Examples

   The following examples illustrate most of the different possible

   Suppose the user (Sender) has set up the following email account
   containing two email addresses, an Internationalized address and an
   ASCII address on an Internationalized server.

   Sender: I0, IA0

   Examples are not provided for the following cases:

   a) Sender: I0, LA0

   If the Sender has both Internationalized and Legacy addresses, then
   this is equivalent to the above.

   b) Sender: I0

   If the Sender has only Internationalized addresses, then it cannot
   send Legacy messages.  The email client cannot provide an option to
   switch the Message Mode to Legacy.

   c) Sender: LA0

   If the Sender has only accounts on Legacy servers, then it cannot
   send Internationalized messages.  The email client cannot provide an
   option to switch the Message Mode to Internationalized.

   The address book has the following contacts with email addresses.

   Contact1: I1, IA1
   Contact2: I2
   Contact3: IA3
   Contact4: LA4

   Example 1:

   From: Sender
   To:   Contact1
   CC:   Contact2

   This message can be sent in Internationalized mode.

   In Legacy mode the email client would flag Contact2, who does not

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   have an ASCII address.

   Example 2:

   From: Sender
   To:   Contact1
   CC:   Contact3

   This message can be sent in either Internationalized or Legacy mode.

   Example 3:

   From: Sender
   To:   Contact1
   CC:   Contact4

   This message cannot be sent in Internationalized mode.  Contact4
   would be flagged since it is not on an Internationalized server.

   This message can be sent in Legacy mode.

   Example 4:

   From: Sender
   To:   Contact2
   CC:   Contact3

   This message can be sent in either Internationalized mode or Legacy

   Example 5:

   From: Sender
   To:   Contact2
   CC:   Contact4

   This message cannot be sent in either mode.

   Internationalized mode would flag Contact4 which is on a Legacy
   server.  The user can remove Contact4 or use the send anyway option.

   Legacy mode would flag Contact2 who does not have an ASCII address.
   The user would have to remove Contact2 in order to send this message.

5.6.  Limitations

   In summary, the guidelines outlines in Section 4 and Section 5 will
   provide the following compatibility solutions:

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   1.  When there is an ASCII address for all contacts in the message,
   then a single legacy compatible message can be sent to all

   2.  When some contacts in the message do not have an ASCII address
   and some have only ASCII addresses on legacy servers, then the
   message can be split into two.  One message is sent as an
   Internationalized message to recipients on Internationalized servers.
   The other is sent as a legacy compatible message to recipients on
   legacy servers.

   These guidelines have a number of limitations.

   a) Unknown Address Types

   Message Mode is effective only if users are fairly disciplined about
   keeping addresses in their address book and configuring the type
   correctly as Internationalized or Legacy.

   When replying to an email, the message may have addresses that are
   not in the address book.  The user may also enter addresses directly
   during message composition that are not in the address book.

   The email client may determine by inspection that some addresses are
   Internationalized.  If an address contains any non-ASCII character,
   then it must be Internationalized.  However, an ASCII address may be
   on either an Internationalized server or a Legacy server and there is
   no way software can determine this automatically.

   In such cases, it may be useful for the email client to flag unknown
   address types in a message so that the user is not lead to believe
   that the message will not bounce just because there were no
   incompatibility warnings.

   b) Address Removal

   When email addresses are removed from a message to meet compatibility
   requirements, recipients do not see everyone who was intended to be
   part of the conversation.  The email client can provide the address
   of removed recipients by using an empty group.  This technique is
   described in Section 3.1.8 of [I-D.ietf-eai-popimap-downgrade].

   This is not an ideal solution, since replies to the message will not
   reach everyone intended.  But at least it provides the necessary
   contact information to recipients who may be able to use other
   methods to reply to all intended.

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6.  Mailbox Integration

   If more than one email address is used for the sender user, emails
   may arrive at different email accounts.  There are several ways to
   provide mailbox integration so the user is able to view all mail in
   one location, such as a single 'Inbox' folder.

   If integration is done on the server, through the use of aliases,
   then the email client does not need to do anything.  All mail will be
   received at the client from one address.

   The email client should provide mailbox integration for cases where
   server side integration is not available and for more flexibility on
   the part of the user.  Many email clients already provide a
   convenient way to manage multiple email accounts.

   An option to view all mail from a group of accounts in one integrated
   folder should also be provided.

7.  Character Encoding

   Email message bodies may be composed and displayed using many
   different character encoding schemes.  Numerous character encodings
   have been developed over time in order to best represent different
   language scripts.  In recent years there has been a trend to prefer
   Unicode as a "universal" character set and UTF-8 as the preferred
   encoding method.

   A good general principle to follow is to minimize character
   conversions.  This will reduce the chance that the received message
   is displayed differently from how it was composed.  Displaying
   received mail SHOULD use the character encoding of the received mail.

   Since older MUAs may not be able to parse UTF-8, the MUA SHOULD try
   to reply to mail using the character encoding of the received mail.
   This may not be possible if the sender adds new characters that
   cannot be encoded in the original encoding.  For example, if the
   received message is encoded in ISO-2022-JP and characters in ISO-
   8859-1 are added to the message, the text cannot be carried in ISO-
   2022-JP and conversion to UTF-8 may be the best solution.

   For new mail, A SMTPUTF8 compliant MUA SHOULD use UTF-8 as the
   default encoding if the message type is global or if the envelope
   contains non-ASCII addresses.  If email clients utilize this default,
   character conversions will be minimized and there will be less chance
   that someone will receive mail in an unrecognized encoding.

   If the message type is rfc822, other considerations may apply, such

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   as using the system locale/language.

   Notwithstanding the above, there may be cases where the default does
   not work well.  There SHOULD be options for the user to reset the
   default character encoding.  There SHOULD also be options to change
   the encoding when reading or writing individual email messages.

8.  Normalization

   Different sequences of UTF-8 characters may represent the same thing.
   Normalization is a process that converts all canonically equivalent
   sequences to a single unique form.

   Normalization of email headers is specified in Section 3.1 of
   [RFC6532].  The MUA SHOULD normalize all email addresses in the
   envelope and message headers.

   For message bodies that contain UTF-8 characters (message/global),
   the "Net-Unicode" standardized text transmission format specified in
   [RFC5198] SHOULD be followed.  It covers both normalization and
   control characters that may affect display of text.

   If the MUA saves email addresses (such as in an address book), they
   SHOULD be stored in normalized form.

   Other normalizations may be needed in specific language environments.
   For example, in the Japanese environment, special considerations are
   needed for the "@" and "." symbols.  Most Japanese input methods
   convert "@" to FULLWIDTH COMMERCIAL AT (U+FF20) and "." to either
   email addresses, "@" is needed to separate the local name from the
   domain name and "." to separate domain name labels.  Normalization is
   necessary to replace FULLWIDTH COMMERCIAL AT (U+FF20) with ASCII "@",
   (U+FF0E) with ASCII ".".

9.  Security Considerations

   This document does not introduce any security considerations beyond
   those already covered by the normative references for Email Address
   Internationalization (EAI).

10.  IANA Considerations

   IANA changes are covered by the normative references for Email
   Address Internationalization (EAI).

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11.  Acknowledgments

12.  Normative References

   [ANSI.X3-4.1968]                  American National Standards
                                     Institute, "USA Code for
                                     Information Interchange",
                                     ANSI X3.4, 1968.

   [I-D.ietf-eai-5738bis]            Resnick, P., Newman, C., and S.
                                     Shen, "IMAP Support for UTF-8",
                                     draft-ietf-eai-5738bis-09 (work in
                                     progress), August 2012.

   [I-D.ietf-eai-popimap-downgrade]  Fujiwara, K., "Post-delivery
                                     Message Downgrading for
                                     Internationalized Email Messages",
                                     (work in progress), August 2012.

   [I-D.ietf-eai-rfc5721bis]         Gellens, R., Newman, C., Yao, J.,
                                     and K. Fujiwara, "POP3 Support for
                                     draft-ietf-eai-rfc5721bis-07 (work
                                     in progress), July 2012.

   [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.

   [RFC3629]                         Yergeau, F., "UTF-8, a
                                     transformation format of ISO
                                     10646", STD 63, RFC 3629,
                                     November 2003.

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   [RFC5068]                         Hutzler, C., Crocker, D., Resnick,
                                     P., Allman, E., and T. Finch,
                                     "Email Submission Operations:
                                     Access and Accountability
                                     Requirements", BCP 134, RFC 5068,
                                     November 2007.

   [RFC5198]                         Klensin, J. and M. Padlipsky,
                                     "Unicode Format for Network
                                     Interchange", RFC 5198, March 2008.

   [RFC5321]                         Klensin, J., "Simple Mail Transfer
                                     Protocol", RFC 5321, October 2008.

   [RFC5322]                         Resnick, P., Ed., "Internet Message
                                     Format", RFC 5322, October 2008.

   [RFC5598]                         Crocker, D., "Internet Mail
                                     Architecture", RFC 5598, July 2009.

   [RFC5890]                         Klensin, J., "Internationalized
                                     Domain Names for Applications
                                     (IDNA): Definitions and Document
                                     Framework", RFC 5890, August 2010.

   [RFC6409]                         Gellens, R. and J. Klensin,
                                     "Message Submission for Mail",
                                     STD 72, RFC 6409, November 2011.

   [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.

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Authors' Addresses

   Ernie Dainow
   Afilias Canada
   4141 Yonge Street
   Toronto, Ontario  M2P 2A8


   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065

   Phone: +81 3 5215 8451

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