Network Working Group
Email Address Internationalization E. Dainow
Intended status: Experimental
Internet-Draft K. Fujiwara
Expires: January 8, 2010
Intended status: Informational JPRS
July 8, 2009
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 This Memo
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Copyright (c) 2009 2012 IETF Trust and the persons identified as the
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This Code Components extracted from this document provides some guidelines for email clients that support
Email Address Internationalization (EAI) must
include Simplified BSD License text as outlined described in RFC 4952. A
number of interoperability cases between different versions Section 4.e of email
components are reviewed. Recommendations are made to improve
interoperability and usability and to minimize discrepancies between
the display of composed Trust Legal Provisions and received email are provided without warranty as
described in different language
environments. the Simplified BSD License.
Table of Contents
1. Conventions used in this document..............................3 document . . . . . . . . . . . . . . 3
2.1. Terminology...............................................3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Version Interoperability.......................................4
3.2.1. Display of Downgraded Messages As Received...........9
3.2.2. Downgraded Display...................................9 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Alternate Addresses...........................................10 Interoperability . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. Entry and Display of Alternate Addresses.................11
4.4. Mailbox Integration......................................12 Interoperability Scenarios . . . . . . . . . . . . . . . . 5
5. Character Encoding............................................13 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. Normalization.................................................13 Mailbox Integration . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations.......................................14 Character Encoding . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations...........................................14 Normalization . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments...............................................14 Security Considerations . . . . . . . . . . . . . . . . . . . 13
10.1. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
12. Normative References....................................14
10.2. Informative References..................................16
Author's Addresses...............................................16 References . . . . . . . . . . . . . . . . . . . . . 14
1. Conventions used in this document
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].
[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 [RFC5335] [RFC6532]
and to the protocols SMTP [RFC5336], [RFC6531], POP [draft-ietf-eai-pop] [I-D.ietf-eai-rfc5721bis]
and IMAP [draft-ietf-eai-imap-utf8]. [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
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 [draft-crocker-email-arch]. [RFC5598].
sender -> MUA -> MSA -> MTA
MTA -> MDA -> MS -> PIF -> MUA -> recipient
(where ... represents possible additional MTA relays)
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 [RFC4409], [RFC6409], [RFC5068].
"SMTPUTF8" is used to indicate email address internationalization as
specified by [RFC4952] [RFC6530] and related documents.
"ASCII" refers to the strict 7-bit ASCII character set [ASCII].
"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 [RFC5335]. [RFC6532].
"message/rfc822" is an email message that contains only 7 bit ASCII
and does not use any UTF8SMTP 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 [RFC3490], [RFC5890], MIME body
encoding [RFC2045] or MIME header encoding [RFC2047].
Not all the components in Internet email systems will get upgraded to
UTF8SMTP at the same time. There will be a transition period where
upgraded components should be backwards
Internationalized Email is not compatible with traditional legacy email components.
The following table characterizes the most typical (but not all the
those based on prior Internet email paths between users standards [RFC5321], [RFC5322].
Non-ASCII email addresses cannot be submitted in different organizations legacy SMTP commands
like MAIL FROM or
enterprises (E1, E2, etc.) and highlights RCPT TO. In addition the boundaries where
incompatibilities will occur most frequently.
Cases where email Internationalized Email
standard does not cross include a jurisdictional boundary between
sender and receiver are not shown. This includes email between users
within the same organization and email between users in different
organizations who use method to "downgrade" message/global to
An Internationalized message cannot be transmitted via SMTP if the same third party mail service.
| Sending |Receiving
Case| MUA |MSA |MTA...MTA |MTA...MTA |MDA |MUA |
1 | E1------------------|E2--------------> |
2 | E1--|E2-------------|E3-------------|E4 |
3a | E1--|E2-------------|E3--------------> |
3b | E1------------------|E2-------------|E3 |
It is assumed
receiving MTA does not announce SMTPUTF8 in all response to EHLO. There
are two failure cases that SMTP mail between MTAs uses DNS.
Lookup an email client may have to handle
described in Section 3.2 of [RFC6531].
a) If the MX record for the destination domain means that there client is only one boundary of incompatibility between MTAs.
Case 1 represents the larger organizations and expert users who
manage their own email infrastructure. In these environments there
will likely be submitting a coordinated effort message to upgrade all components of the
email system together. Each organization typically has several MTAs an MSA that act as virus scanners, spam filters, mail relays and gateways to
manage mail across different divisions and locations of does not
support SMTPUTF8, the
organization. The boundary of incompatibility is message will be rejected.
b) If the MSA does support SMTPUTF8 but a downstream MTA between does not,
organizations. If both enterprises support UTF8SMTP, they mail will be
able to send Internationalized email without the risk of
incompatibility or Downgrade.
For large organizations bounce. That is, a delivery status notification
(DSN) that allow end users to select and install
their own email client software, the MUA boundaries are also possible
incompatibilities. Users in this category would actually mail could not be
represented by cases 2 and 3.
Case 2 represents the home user and small delivered will be sent back to medium sized businesses
who use the
Incompatibility between Internationalized email infrastructure of and legacy systems is
expected to be important initially during a third party, such transition period but
less important over time as an ISP
(Internet Service Provider) or an outsourced provider. The mail
provider has an infrastructure similar more email systems upgrade to Case 1. The boundaries of support the
SMTPUTF8 extensions. To the extent that this incompatibility are is
deemed important at the MUA and between the MTAs of time an implementation is undertaken, the
Case 3 covers mixed client should provide methods to prevent or at least minimize
4.1. Interoperability Scenarios
The following scenarios cover the different cases where of sending mail
from an Internationalized server to a user with 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 outsourced address on a Legacy mail server, which must be
Case 1. The simple compatibility case
From: IA1 (or LA1)
The message will be successfully sent as long as the email
sends to message/rfc822 rather than message/global.
Case 2. The simple incompatibility case
The message will be rejected by the MSA or receives will bounce from a
downstream SMTP server.
If user in an organization that
manages its own email infrastructure. The boundaries of
incompatibility correspond to Cases 1 and 2. These cases may I1 also
apply to some applications within larger organizations. For example,
cell phone has an ASCII email address IA1 or LA1, there may utilize a mail gateway from be a third party
provider even though the rest of
simple workaround. If the email infrastructure is in-
For an MUA, client supports multiple email
accounts, the boundaries where version compatibility is most likely user just has to occur is between home/small office users switch the From address to an ASCII
address and their email
providers. it becomes Case 1.
Case 3. The general incompatibility cases
The worst general case scenario is Case 2, where three boundaries a mix of incompatibility are possible between sender Internationalized and recipient.
For legacy addresses.
While many combinations are possible, the two cases below essentially
cover all possibilities.
The message will be sent to I3 but it will bounce from LA2.
Switching the From address to an MUA that supports UTF8SMTP, there ASCII address as in Case 2 is not a matrix of possibilities
based on whether the email envelope and
solution, as the following case demonstrates.
From: IA1 (or LA1)
This message contain non-ASCII
characters and whether will bounce from LA2 since the MSA supports address in the UTF8SMTP extensions or
not. The following table shows 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 possible combinations.
Case|Envelope |Message |MSA is |MUA sends
| | |UTF8SMTP|
1 |UTF8SMTP |global |Yes |UTF8SMTP
2 |UTF8SMTP |rfc822 |Yes |UTF8SMTP
3 |ASCII |global |Yes |UTF8SMTP
4 |ASCII |rfc822 |Yes |Traditional email
5 |UTF8SMTP |global |No |Reject/Downgrade
6 |UTF8SMTP |rfc822 |No |Reject/Downgrade
7 |ASCII |global |No |Reject/Downgrade
8 |ASCII |rfc822 |No |Traditional email
The Envelope original list and then
using an ASCII address to the Message type are considered separately because
the Envelope may contain, for example, email bounced recipients.
If users know beforehand which addresses that are all
ASCII 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 Subject or other header fields incompatibility
problems outlined in the Message Section 4. There may
contain non-ASCII (Cases 3, 7).
Cases 2 and 6 be several ways to do
this. Following are unusual since a UTF8SMTP address in the envelope is
usually also in the message header. An example guidelines on some of when the ways that this can occur
is when an rfc822 message is forwarded with server-based forwarding
(as with a .forward file) to a UTF8SMTP address.
Messages containing non-ASCII characters SHOULD be sent using
UTF8SMTP extensions in preference very least, to older encoding methods, such as
IDNA [RFC3490] provide basic compatibility between
Internationalized and legacy systems, if all email addresses in the
SMTP envelope and MIME header encoding [RFC2047]. If the message
body contains non-ASCII characters, it SHOULD headers are ASCII, then a message/
rfc822 should be sent using 8BITMIME
[RFC1652] instead of MIME body encoding such as quoted-printable or
This could be considered a configuration error. If (Case 1 above).
For Case 2, the MSA does not email client should support UTF8SMTP, multiple email accounts
and allow the user should upgrade the MSA, or to switch the From address at any time during
composition of the message.
For Case 3, several mechanisms may be required to an
email provider that supports UTF8SMTP.
compatibility support. These are outlined in the MSA is a reasonable approach following sub-
5.1. Address Book
Each contact in the case of larger
organizations, where an IT group would address book should be expected able to synchronize MUA
and MSA versions. However, home/small office users may end up in this
situation when they have a computer that came with UTF8SMTP several email
client software and their Internet Service Provider (ISP) does not
In these cases, the MUA MUST NOT submit a message with UTF8SMTP
headers if the MSA does not support the UTF8SMTP extensions
If the message
addresses, each of which is not submitted, some guidance should be provided to
the user about how configured to correct the problem. It may also be desirable
to save this status and highlight it for the either an
Internationalized or a Legacy address.
The user before may not necessarily know if an ASCII address they compose enter in
their address book is on a message. This would provide advance warning that internationalized
email cannot be sent.
The MUA MAY support the "downgrade" option, which legacy server or not. If it is specified configured
option for all email components MUA, MSA, MTA Internationalized address and MDA. Downgrade
builds a message with all ASCII headers so that it is compatible with turns out to be wrong, then
email components that don't support the UTF8SMTP extensions.
Downgrade basically redirects mail from a UTF8SMTP address sent to an
Alternate ASCII Address [RFC5504].
It is not recommended that the MUA support Downgrade for cases 5-7. contact may bounce. The user should be encouraged to correct can then re-
configure the configuration and
upgrade address as Legacy so the MSA or switch email providers in order to get support for
The following shows an example client can provide
warnings of downgrading a "From" header with a
non-ASCII "Display-Name", non-ASCII email address and ASCII Alternate
From: Display-Name <eai-addr <alt-ascii-addr>>
Downgrade would change the From address 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 Alternate Address and
preserve the EAI type of each address in a new "Downgraded-From" header.
From: =?UTF-8?Q?Display-Name?= <alt-ascii-addr>
Note that the Display-Name in the From header is encoded using
traditional MIME email standards [RFC2047] with charset UTF-8. The
MUA at the recipient end headers does not need to support the UTF8SMTP
extensions conform to decode and display the original name.
Complete examples of Downgrade are shown in
message mode, then the Appendix of
The matrix of possibilities user is based on 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 email message type and
whether IMAP/POP and mode to see if the MUA support
When the UTF8SMTP extensions or
not(Y/N) [draft-ietf-eai-imap-utf8], [draft-ietf-eai-pop].
|Message |Message |/POP|Message | |Message
1 |global |global | Y |global | Y |global
2 |global |global | Y |downgraded| N |downgraded
3 |global |global | N | - |Y/N| -
4a |global |downgraded|Y/N |downgraded| Y |downgraded
4b |global |downgraded|Y/N |downgraded| Y |global
5 |global |downgraded|Y/N |downgraded| N |downgraded
6 |rfc822 |rfc822 |Y/N |rfc822 |Y/N|rfc822
Note that mode is switched, the cases email client switches addresses in which
message header fields to match the recipient receives mode, selecting from the message as
sent list of
addresses in each contact.
There are 1 (all UTF8SMTP), 6 (traditional email) and 4b (downgraded
In cases 2, 4a and where both modes provide warnings (see Example 5
below). In these cases, the recipient receives a downgraded message.
IMAP or POP must support Downgrade for this configuration. Direct
maildrop access for message/global is prohibited if user can remove the MUA does not
This is a configuration error. If IMAP or POP does not support
UTF8SMTP, then 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 for the MUA to receive global
An ASCII message may be received from either compose a UTF8SMTP message/rfc822 if any sender or a non-
address is possible that not ASCII.
Where both modes provide warnings, users will likely want to send the original message was a UTF8SMTP
got downgraded to ASCII in transit. A message can be identified as
downgraded because each mode in order to reach all recipients. The email
client should make it will have one or more headers that easy to do this. There are prefixed
(Case 4a) A UTF8SMTP compliant MUA MAY display many possible
designs to accomplish this. The following is one example.
An option is provided when composing email to add a downgraded second message
as received, or (Case 4b) it MAY apply a conversion to restore the
header section in the "Downgraded-" headers as specified in
3.2.1. Display of Downgraded Messages As Received
Cases 2, 4a, 5
When displaying a downgraded message as received, UTF8SMTP addresses other mode that had Alternate Addresses in the original email will not be shown
in the headers when reading, replying or forwarding email. Only the
Alternate Addresses will be shown.
If a UTF8SMTP address in the original email did not have an Alternate
Address, then allows the UTF8SMTP address will be displayed in an empty
group (using ":;") user to note that a UTF8SMTP address has been removed
[RFC5504]-Section 5.1.7. move
addresses between sections. This may appear is in any header such as To: or
Display-Name Internationalized Address eai-addr Removed:;
If a user replies to an email with such a group, many MUAs do not
handle this correctly. Observed behavior has ranged from refusing addition to
send the message due to an "invalid address", or attempting to send making individual
changes to the group address headers as in normal email composition. The
Subject and reporting a DSN failure, or deleting Body are common so the group
altogether. The user may resort to removing the group can compose a single message
but have it sent in order the two different modes to get
around these problems. Recipients of such email will not have different recipients.
Following is an
accurate record example of who the original recipients were. MUAs this for Case 3 above.
From: IA1 From: I1
To: LA2 <---> To:
Cc: <---> Cc: I3
5.3. Message Format
In Internationalized Mode, mail should be
upgraded to support groups, sent as defined in [RFC2822]-Section 3.4.
Note that even if an MUA does not support UTF8SMTP (Cases 2, 5), it message/global.
The aim of Internationalized Email is able 8 bit clean messages using
UTF-8 encoding to decode and display "Downgraded-" represent Unicode characters in header fields because
Downgrade uses 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 [RFC 2047][RFC 2231].
3.2.2. Downgraded Display
Support for conversion of "Downgraded-" headers is separate from
support for Downgrade. An MUA MAY support none or one [RFC2045] or both of
Conversion replaces the Alternate Addresses in email headers with the
original UTF8SMTP addresses that were recorded in the "Downgraded-"
If the MUA supports conversion of "Downgraded-" headers, the
following considerations MIME header encoding [RFC2047]. Any
encoding should be taken into account:
1. If based on UTF-8. In the MUA receives interest of
interoperability, charsets other than UTF-8 are prohibited in mail from an IMAP/POP server, the conversion
may have already been done but the message will still contain
addresses and "Downgraded-Rcpt-To" headers.
2. Conversion of Downgraded message headers is not a reliable, reversible
3. There is no authenticated binding between the original UTF8SMTP and
the downgraded Alternate Address. This introduces various security
concerns [draft-ietf-eai-downgraded-display]-Section 5.
4. Alternate Addresses
Alternate Addresses MAY be required for Downgrade, which may occur
anywhere on described in Section 7.1 of [RFC6530].
5.4. Error Handling
If a message is rejected by the path that MSA with a non-UTF8SMTP response code that
indicates incompatibility with legacy email component is
encountered [RFC5336]-Section 3.2. If Downgrade cannot be done described in
these cases, Section 3.2
of [RFC6531], the email may be returned ("bounced").
Downgrade is expected to compose window should be important initially during a transition
period but less important over time as more kept open so that the user
can make changes and retry. The email systems upgrade client should provide guidance
to the UTF8SMTP extensions. To user about switching the extent that Downgrade is deemed
important at Message Mode, reconfiguring the time type
of an implementation is undertaken, Alternate
Addresses [RFC5336] SHOULD be supported.
An Alternate Address address in the address book or adding an ASCII legacy address
for a contact in the sender MAY be provided, so that after
Downgrade there is address book.
Similarly, if a return path for message bounces, the email client could parse the
delivery status notifications
Email addresses are generally created and set up on message disposition notifications
[RFC6533] to determine if the server side,
not by failure was a compatibility problem and
if so, which addresses caused the MUA. An email provider may wish to problem.
The following examples illustrate most of the different possible
Suppose the user (Sender) has set up an Alternate
Address automatically for each UTF8SMTP the following email account that is created.
While in some environments it may be difficult or unfamiliar for a
user to enter ASCII characters, selecting
containing two email addresses, an Alternate Address for
the user's UTF8SMTP Internationalized address SHOULD NOT be done automatically.
Automatic generation often results in usability problems when names
that are difficult to read or pronounce are produced. Any generation
of and an Alternate Address should be presented to the user as a
suggestion that can be changed.
A UTF8SMTP implementation of
ASCII address on an MSA/MTA may provide Internationalized server.
Sender: I0, IA0
Examples are not provided for the ability to
bind an Alternate Address to a UTF8SMTP address. In this case, following cases:
a) Sender: I0, LA0
MUA may not need 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 Alternate Addresses for an option to
switch the sender.
However, users may wish Message Mode to use different Alternate Addresses than
those created for their UTF8SMTP Legacy.
c) Sender: LA0
If the Sender has only accounts on Legacy servers, then it cannot
send Internationalized messages. The email account, such as when they
already have client cannot provide an ASCII
option to switch the Message Mode to Internationalized.
The address on another book has the following contacts with email system. addresses.
Contact1: I1, IA1
This message can be sent in Internationalized mode.
In general, Legacy mode the MUA SHOULD allow users to save email client would flag Contact2, who does not
have an Alternate Address
for the sender's UTF8SMTP address, typically under "Account"
settings. The configured value of this field ASCII address.
This message can be sent in either Internationalized or Legacy mode.
This message cannot be sent in Internationalized mode. Contact4
would be flagged since it is used as an ALT-
ADDRESS parameter not on the SMTP command "MAIL FROM:" and an Internationalized server.
This message can be sent in Legacy mode.
This message headers.
There are two cases where Downgrade can occur:
1. Mail be sent from a UTF8SMTP user to a non-UTF8SMTP user.
2. Mail in either Internationalized mode or Legacy
This message cannot be sent from a UTF8SMTP user to a UTF8SMTP user where a non-
UTF8SMTP component in either mode.
Internationalized mode would flag Contact4 which is on a Legacy
server. The user can remove Contact4 or use the path.
Downgrade in Case 1 provides backwards compatibility with recipients send anyway option.
Legacy mode would flag Contact2 who are does not UTF8SMTP. In this case, the recipient has have an ASCII
The user would have to remove Contact2 in order to send this message.
In summary, the guidelines outlines in Section 4 and an Alternate Address Section 5 will
provide the following compatibility solutions:
1. When there is not required.
In Case 2, Downgrade REQUIRES an Alternate Address ASCII address for all contacts in the recipient.
However, this case could be considered message,
then a configuration error. As
reviewed in section 3, when DNS is used single legacy compatible message can be sent to determine all
2. When some contacts in the transport
path from sender to receiver, mail does message do not get routed through have an
email relay of a third party. If the sender ASCII address
and recipient both some have
UTF8SMTP addresses, then one of their MTA mail relays was not
upgraded to UTF8SMTP. Users should only be set up with UTF8SMTP ASCII addresses if all the mail relays within the organization support
If it is decided that it is important to support Downgrade for Case
2, on legacy servers, then the MUA SHOULD allow the user to enter and edit an optional
Alternate Address wherever a UTF8SMTP recipient address
message can be
entered and edited. This would typically be split into two. One message headers when
composing email and entries stored in is sent as an "Address Book".
Internationalized message to recipients on Internationalized servers.
The recipient Alternate Address, if provided in an email composition, other is used sent as an ALT-ADDRESS parameter a legacy compatible message to recipients on the SMTP command "RCPT TO:"
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 message headers where the recipient their address is used.
4.3. Entry book and Display of Alternate Addresses
The following applies configuring the type
correctly as Internationalized or Legacy.
When replying to both sender and recipient Alternate
Alternate Address fields MUST NOT contain non-ASCII addresses.
If an email, the main email address is message may have addresses that are
not UTF8SMTP, then entering an in the address book. The user may also enter addresses directly
during message composition that are not in the Alternate Address field SHOULD NOT be allowed [RFC5336]-
Section 3.4. address book.
The following is one example of how to display Alternate Addresses, email client may determine by using the UTF8SMTP "double angle bracket" format defined in
Display-Name <eai-addr <alt-ascii-addr>>
It would inspection that some addresses are
Internationalized. If an address contains any non-ASCII character,
then it must be helpful to display Internationalized. However, an indicator ASCII address may be
on UTF8SMTP 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
addresses client to flag unknown
address types in a message so that do not have an Alternate Address. This would alert the user is not lead to the possibility believe
that the message may bounce. In the example
above, an empty double bracket could be displayed in will not bounce just because there were no
b) Address Removal
When email addresses are removed from a highlighted
color, reminding the user 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 missing alternate address,
of removed recipients by using an empty group. This technique is
Display-Name <eai-addr < >>
When sending Section 3.1.8 of [I-D.ietf-eai-popimap-downgrade].
This is not an ideal solution, since replies to the message, message will not
reach everyone intended. But at least it provides the MUA would have necessary
contact information to remove empty double
Since Downgrade and Alternate Addresses recipients who may not be widely used after
a transition period, such an indicator should be configurable so that
a user is able to turn it off.
4.4. use other
methods to reply to all intended.
6. Mailbox Integration
If Alternate Addresses are supported, it may be desirable to combine
mail for the UTF8SMTP more than one email address and is used for the Alternate Address into one sender user, emails
may arrive at different email accounts. There are several ways to
provide mailbox integration so that the user is able to view all related mail can be managed in
For example, if location, such as a message single 'Inbox' folder.
If integration is sent from a UTF8SMTP address to a list
of recipients, some of done on the messages may be downgraded. Replies to
downgraded messages will be delivered to server, through the Alternate Address, so
all use of aliases,
then the replies to a message may be split across two different
Mailbox integration is email client does not generally handled by an MUA. Many existing
MTAs/MDAs can need to do this with a anything. All mail "alias" or "forward". One address
is selected as the primary mailbox and will be
received at the other address is
configured as an alias.
Forwarding allows an email address on client from one address.
The email provider to be
integrated into the client should provide mailbox on another email provider. Mailbox integration can make it easier for users to migrate from an old email
system that does cases where
server side integration is not support UTF8SMTP to available and for more flexibility on
the part of the user. Many email clients already provide a newer one that does. All
convenient way to do is forward their old manage multiple email address accounts.
An option to an Alternate
Address that was created on their new view all mail service.
5. 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
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 UTF8SMTP 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
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.
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.
For example, in the Japanese environment, special consideration is
needed for the "@" symbol used to separate the local name from the
domain name in email addresses. Normalization is necessary to replace
FULLWIDTH COMMERCIAL AT (U+FF20) with ASCII "@", COMMERCIAL AT
(U+0040) for proper parsing of email addresses.
Normalization of email headers is specified in [RFC 5335]-Section
4.1. 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, an email address
where * represents 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
IDEOGRAPHIC FULL STOP (U+3002), as used in some
Asian languages, would display as
For message bodies that contain UTF-8 characters (message/global), (U+3002) or FILLWIDTH FULL STOP (U+FF0E). In
email addresses, "@" is needed to separate the "Net-Unicode" standardized text transmission format specified in
[RFC5198] SHOULD be followed. It covers both normalization local name from the
domain name and "." to separate domain name labels. Normalization is
necessary to replace FULLWIDTH COMMERCIAL AT (U+FF20) with ASCII "@",
IDEOGRAPHIC FULL STOP (U+3002) with ASCII "." and
control characters that may affect display of text.
7. FILLWIDTH FULL STOP
(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
10. IANA Considerations
IANA changes are covered by the normative references for Email
Address Internationalization (EAI).
12. Normative References
[ANSI.X3-4.1968] American National Standards Institute (formerly United States
of America Standards Institute),
Institute, "USA Code for
ANSI X3.4-1968, X3.4, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[draft-ietf-eai-downgraded-display] Fujiwara, K., "Displaying
Downgraded Messages for Email Address
(work in progress), March 2009.
[I-D.ietf-eai-5738bis] Resnick, P. and P., Newman, C., and S.
Shen, "IMAP Support for UTF-8", draft-ietf-eai-imap-utf8-04
draft-ietf-eai-5738bis-09 (work in
[draft-ietf-eai-pop] Newman, C. and 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
in progress), June
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994. 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,
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a
transformation format of ISO
10646", STD 63, RFC 3629,
[RFC4952] Klensin, J.
[RFC5068] Hutzler, C., Crocker, D., Resnick,
P., Allman, E., and Y. Ko, "Overview T. Finch,
"Email Submission Operations:
Access and Framework for
Internationalized Email", Accountability
Requirements", BCP 134, RFC 4952, July 5068,
[RFC5198] Klensin, J. and M. Padlipsky, M.,
"Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5321] Klensin, J., "Internationalized Email Headers", "Simple Mail Transfer
Protocol", RFC 5335,
November 5321, October 2008.
[RFC5336] Yao, J. and W. Mao, "SMTP extension for internationalized
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5336, September 5322, October 2008.
[RFC5504] Yoneya, Y. and K. Fujiwara, "Downgrading mechanism for
Email Address Internationalization", RFC 5504, March 2009.
10.2. Informative References
[RFC5598] Crocker, D., "Internet Mail
draft-crocker-email-arch-14 (work in progress), June 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, J.,
"Message Submission for Mail",
STD 72, RFC 4409, 2006.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E. 6409, November 2011.
[RFC6530] Klensin, J. and
Finch, 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., "Email Submission Operations: Access Newman, C., and
Accountability Requirements", A.
Delivery Status and Disposition
Notifications", RFC 5068, November 2007. 6533,
4141 Yonge Street
Toronto, Ontario M2P 2A8
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