Interoperable Email Addresses for SMTPUTF8
draft-ietf-mailmaint-interoperable-addresses-02

mailmaint                                                 A. Gulbrandsen
Internet-Draft                                                     ICANN
Intended status: Standards Track                                  J. Yao
Expires: 17 April 2026                                             CNNIC
                                                         14 October 2025

               Interoperable Email Addresses for SMTPUTF8
            draft-ietf-mailmaint-interoperable-addresses-02

Abstract

   This document specifies rules for email addresses that are flexible
   enough to express the addresses typically used with SMTPUTF8, while
   avoiding elements that harm human-to-human interoperation.

   This is one of a pair of documents: this contains recommendations for
   what addresses humans should use, such that address provisioning
   systems can restrain themselves to addresses that email valdidators
   accept.  (This set can also be described in other ways, including
   "simple to cut-and-paste" and "understandable by some community".)
   Its companion defines simpler rules, accepts more addresses, and is
   intended for software like MTAs.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Mail Maintenance
   Working Group mailing list (mailmaint@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/mailmaint/.

   Source for this draft and an issue tracker can be found at
   https://github.com/arnt/mailmaint-smtputf8.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 17 April 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  An oversimplified description of current SMTPUTF8 email
           addresses . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  An oversimplified description of IDNs and the domain name
           system  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  IDNA2008  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Registry rules  . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Web browser rules . . . . . . . . . . . . . . . . . . . .   6
   6.  Rules for interoperable email addresses . . . . . . . . . . .   6
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Additional local rules  . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Testing  . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  11
   Appendix C.  Rationales for each condition  . . . . . . . . . . .  11
   Appendix D.  Instructions to the RFC editor . . . . . . . . . . .  12
   Appendix E.  Open issues  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   [RFC6530]-[RFC6533] and [RFC6854]-[RFC6858] extend various aspects of
   the email system to support non-ASCII both in localparts and domain
   parts.  In addition, some email software supports unicode in domain
   parts by using encoded domain parts in the SMTP transaction ("RCPT
   TO:<info@xn--dmi-0na.fo<") and presenting the unicode version
   (dømi.fo in this case) in the user interface.

   The email address syntax extension is in [RFC6532], and allows almost
   all UTF8 strings as localparts.  While this certainly allows
   everything users want to use, it is also flexible enough to allow
   many things that users and implementers find surprising and sometimes
   worrying.

   The flexibility has caused considerable reluctance to support the
   full syntax in contexts such as web form address validation.

   This document attempts to describe rules that:

   1.  includes the addresses that users generally want to use for
       themselves and organizations want to provision for their
       employees.

   2.  includes the addresses that have been used significantly, even if
       not exactly what users wanted.

   3.  excludes confusable things.

   4.  excludes things that have been described as security risks.

   5.  Looks safe at first glance to implementers (including ones with
       limited knowledge of unicode).

   These goals are somewhat aspirational.  For example, lower-case L and
   upper-case i are confusable and cannot realistically be disallowed,
   the protocol poLIce would arrest us all.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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3.  Terminology

   Script, in this document, refers to the unicode script property (see
   [UAX24]).  Each code point is assigned to one script ("a" is Latin),
   except that some are assigned to "Common" or a few other special
   values.  Fraktur and /etc/rc.local aren't scripts in this document,
   but Latin is.

   Latin refers those code points that have the script property "Latin"
   in Unicode.  Orléans in France and Münster in Germany both have Latin
   names in this document.  It also refers to combinations of those code
   points and combining characters, and to strings that contain no code
   points from other scripts.

   Han, Cyrillic etc. refer to those code points that have the
   respective script property in Unicode, as well as to strings that
   contain no code points from other scripts.

   ASCII refers to the first 128 code points within unicode, which
   includes the letters A-Z but not É or Ü. It also refers to strings
   that contain only ASCII code points.

   Non-ASCII refers to unicode code points except the first 128, and
   also to strings that contain at least one such code point.

   By way of example, the address info@dømi.fo is latin and non-ASCII,
   its localpart is latin and ASCII, and its domain part is latin and
   non-ASCII. 中国 is a Han string in this document, but 阿Q正传 is neither a
   Latin string nor a Han string, because it contains a Latin Q and
   three Han code points.

   ZWJ, "zero-width joiner", is the unicode codepoint U+200D.  ZWNJ,
   "zero-width non-joiner", is the unicode codepoint U+200C.

   The terms a-label and u-label are defined in [RFC5890] section
   2.3.2.1.

   PRECIS is described in [RFC8264], and IdentifierClass in section 4.2
   of that document.

4.  An oversimplified description of current SMTPUTF8 email addresses

   This section is informative.

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   In the countries the authors have visited, the norm is that some
   users and organizations want to use their daily script(s), but not
   others.  The name on your office door will be readable to your
   colleagues, no matter how your parents wrote it in the place where
   you were born.

   While the syntax defined in [RFC6532] technically supports addresses
   with an Indic localpart and a Japanese or Arabic domain part,
   organizations such as a university or a company don't want to use
   that kind of naming: The script that's used for the organization's
   domain is the one it wants to use when provisioning email addresses.

   Note that even when an organization emphatically doesn't want to
   provision localparts from scripts other than its own, that
   organization generally wants the ability to correspond worldwide.

   There are a few countries in which ASCII localparts are used with
   non-ASCII domains (reputedly because of email software that supports
   [RFC3490]/[RFC5891] but not [RFC6532]).  So far, the authors have
   only seen this in countries that use left-to-right scripts such as
   Cyrillic and Han.

   In some countries, ASCII non-letters is used together with non-Latin
   scripts.  Arabic is an example: the Arabic digits 0-9 are used often
   (more so in some countries than others).  Chinese domain names use
   the ASCII dot instead of the Chinese full-width dot.  ASCII
   punctuation (notably the hyphen) is used with several scripts.

   In the authors' experience, left-to-right and right-to-left writing
   is mixed in only a few cases (that are relevant to email addresses):

   *  ASCII digits (123) and right-to-left letters

   *  Arabic Indic digits (١٢٣) and Arabic letters

   *  Testcase domains/addresses

   *  single punctuation characters (often neutral in direction)

5.  An oversimplified description of IDNs and the domain name system

   This section is informative.

   The use of non-ASCII in domain names is restricted by several
   factors: IDNA2008 rules, registry rules and web browsers.

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   For a domain such as example.com, IDNA2008 restricts both labels
   (slightly), ICANN policy restricts "com" and the .com registry's
   policy restricts "example", and the browsers' rules apply to both.
   For a domain such as beispiel.example.com, IDNA2008 and the browsers'
   rules apply to "beispiel" as well.  Neither ICANN's nor the .com
   registry's rules apply to "beispiel".

5.1.  IDNA2008

   Using non-ASCII more or less requires IDNA2008, ie. if a domain name
   is to be practically usable, it needs to be usable with IDNA2008,
   which restricts the set of code points slightly.

5.2.  Registry rules

   The LGR, Label Generation Rules, are a set of rules largely developed
   by per-language and per-script committees and collected by ICANN.
   Most notably, a script or language has to be used by a current
   community in order to get an LGR.  An LGR contains that which is
   currently used by the community.

   The merged repertoire of all code points in all LGRs is called the
   Common LGR, and contains more than 30,000 code points at the time of
   writing.

   Registering a domain name in a public TLD requires adhering to the
   registry's policy, which is generally either an LGR, a small
   registry-chosen set of code points, or restricted by rules outside
   the DNS (such as for .bank, which is effectively restricted by what
   names banking regulators allow banks to have).

5.3.  Web browser rules

   The main browsers have rules for which domains are displayed in a
   human-readable manner in the address bar.  (One author has a runic
   domain, all of the main browsers display xn--something in the address
   bar instead of runes.)  Each of the three big browser vendors
   maintains its own rule set.  At the time of writing, all three may be
   described as broadly similar to the Common LGR and different in
   detail.

6.  Rules for interoperable email addresses

   Based on the above descriptions, the following rules are formulated
   for interoperable email addresses.

   1.  An atom in an interoperable address MUST NOT be an a-label.

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   2.  If the address matches the grammar in [RFC5322], then in order to
       be interoperable it MUST also match the grammar specified by the
       W3C WHATWG [TYPE_EMAIL] spec.

   3.  An address MUST conform to the PRECIS IdentifierClass.

   4.  If an address contains any right-to-left code points, then it MAY
       contain ASCII digits and MUST NOT contain any other left-to-right
       code points.

   5.  If an address contains any non-ASCII code point, then one of the
       following conditions MUST apply:

       1.  All code points share one unicode script property, or have
           the script property Common, or are ASCII digits (or ./@).

       2.  The localpart consists entirely of ASCII, and the domain part
           consists of code points that share one unicode script
           property, or have the script property Common, or are ASCII
           digits (or ./@).

       3.  All code points are have the script properties Han or Common,
           or are ASCII.

7.  Examples

   The address example@example.com is interoperable, because 1) it does
   not contain any a-label, 2) it matches the WHATWG [TYPE_EMAIL] spec,
   3) it conforms to the IdentifierClass, and the last two conditions do
   not apply.

   The address dømi@dømi.fo is interoperable, because 1) it does not
   contain any a-label, 2) does not apply, 3) it consists entirely of
   permissible code points, 4) does not apply and 6) it consists
   entirely of 'Latin' and 'Common' code points (and ./@).

   The address U+200E '@' U+200F '.'  U+200E is not interoperable,
   because 4) U+200E and U+200F are not permitted in the PRECIS
   IdentifierClass.

   The address U+627 '@' '2' '.' '3' is interoperable.  (U+627 is an
   arabic letter, written right-to-left.)  It is interoperable because
   1) it does not contain any a-label, 2) does not apply, 3) it consists
   entirely of permissible code points, 4) the only right-to-left code
   points used are ASCII digits and 6) all code points are 'Arabic' or
   ASCII digits (or @/).

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   (Note that it's interoperable even though its top-level domain cannot
   exist: The rules in this document don't require network access, and
   therefore do not require checking whether a domain exists.)

   The address info@xn--dmi-0na.fo is not interoperable, because 1) it
   contains the a-label xn--dmi-0na.

   The address 名字@例子.中国 is interoperable, because 1) it does not contain
   any a-label, 2) does not apply, 3) it consists entirely of
   permissible code points, 4) does not apply and 5) it consists
   entirely of 'Han' code points (and ./@).

   The address info@例子.中国 is interoperable, because 1) it does not
   contain any a-label, 2) does not apply, 3) it consists entirely of
   permissible code points, 4) does not apply and 5) the localpart is
   ASCII and the domain part consists entirely of 'Han' code points (and
   .).

   The address dømi@例子.中国 is not interoperable, because 5) it contains
   both 'Latin' and 'Han' code points and the localpart is non-ASCII.

   The address 阿Q@阿Q正传@.中国 is interoperable, because 1) it does not
   contain any a-label, 2) does not apply, 3) it consists entirely of
   permissible code points, 4) does not apply and 5) the address
   consists entirely of ASCII and Han code points (and .).

8.  Additional local rules

   Software that provisions addresses using a particular script may
   apply additional restrictions.

   1.  This document permits the use of all combining characters with
       letters in this script (formally, unicode \p{Common} is permitted
       in any script).  In the case of scripts that do not use
       diacritical marks, it may be advisable to never create localparts
       with combining codepoints.

   2.  If users are permitted to choose their own email addresses, it
       may be advisable to reserve names that sound official in the
       relevant languages, analogously with the list of English names in
       [RFC2142].

9.  IANA Considerations

   This document does not require any actions from the IANA.

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10.  Security Considerations

   When a program renders a unicode string on-screen or audibly and
   includes a substring supplied by a potentially malevolent source, the
   included substring can affect the rendering of a surprisingly large
   part of the overall string.

   This document describes rules that make it difficult for an attacker
   to use email addresses for such an attack.  Implementers should be
   aware of other possible vectors for the same kind of attack, such as
   subject fields and email address display-names.

   If an address is signed using DKIM and (against the rules of this
   document) mixes left-to-right and right-to-left writing, parts of
   both the localpart and the domain part can be rendered on the same
   side of the '@'.  This can create the appearance that a different
   domain signed the message.

   The rules in this document permit a number of code points that

11.  References

11.1.  Normative References

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/rfc/rfc5322>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5890>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/rfc/rfc6365>.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/rfc/rfc6530>.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <https://www.rfc-editor.org/rfc/rfc6532>.

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   [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov,
              "Internationalized Delivery Status and Disposition
              Notifications", RFC 6533, DOI 10.17487/RFC6533, February
              2012, <https://www.rfc-editor.org/rfc/rfc6533>.

   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8264>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

11.2.  Informative References

   [RFC2142]  Crocker, D., "Mailbox Names for Common Services, Roles and
              Functions", RFC 2142, DOI 10.17487/RFC2142, May 1997,
              <https://www.rfc-editor.org/rfc/rfc2142>.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, DOI 10.17487/RFC3490, March 2003,
              <https://www.rfc-editor.org/rfc/rfc3490>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5891>.

   [RFC6854]  Leiba, B., "Update to Internet Message Format to Allow
              Group Syntax in the "From:" and "Sender:" Header Fields",
              RFC 6854, DOI 10.17487/RFC6854, March 2013,
              <https://www.rfc-editor.org/rfc/rfc6854>.

   [RFC6858]  Gulbrandsen, A., "Simplified POP and IMAP Downgrading for
              Internationalized Email", RFC 6858, DOI 10.17487/RFC6858,
              March 2013, <https://www.rfc-editor.org/rfc/rfc6858>.

   [UAX24]    Whistler, K., "Unicode Script Property", 31 July 2025,
              <https://unicode.org/reports/tr24>.

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   [UMLAUT]   "Metal Umlaut", n.d.,
              <https://en.wikipedia.org/wiki/Metal_umlaut>.

   [TYPE_EMAIL]
              "WHATWG input type=email", n.d.,
              <https://html.spec.whatwg.org/multipage/input.html#email-
              state-(type=email)>.

Appendix A.  Testing

   This is a set of test addresses in JSON format.

   Below is a verbatim copy of https://github.com/arnt/mailmaint-
   smtputf8/tests.json as it was on (date here).  It contains a number
   of strange and unusual code points, so cutting and pasting this may
   not work.  Rather, it is recommended to either use the rfcstrip tool
   or download the tests using a command such as curl
   https://github.com/arnt/mailmaint-smtputf8/tests.json > tests.json.

   Note to IETF reviewers: The tests will be included here shortly
   before publication (and after IETF Last Call).

Appendix B.  Acknowledgments

   The authors wish to thank John C.  Klensin, Jeremy Harris, Daniel
   Eggert, (your name here, please)

   Dømi.fo and 例子.中国 are reserved by nic.fo and CNNIC for use in
   examples and documentation.

   阿Q正传@ is a famous Chinese novella, 阿Q is the main character.

Appendix C.  Rationales for each condition

   This section is informative.  Each of the five conditions has a
   separate rationale.

   1.  A-labels are confusing for many readers, and can potentially be
       used to confuse and attack readers.  Being visibly safe is one of
       the five goals.

   2.  Many existing address validators use the WHATWG rules; if this
       specification is exactly compatible with WHATWG [TYPE_EMAIL] for
       all the addresses that WHATWG covers, then it's possible to
       extend a WHATWG-compliant validator without risk of accidentally
       rejecting formerly accepted addresses.

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   3.  Unicode contains many code points that could perhaps be used for
       attacks.  The PRECIS IdentifierClass constrains and the
       repertoire to the plainest code points, which makes the
       specification visibly trustworthy.

   4.  Mixed-direction text can be confusing, and confusion has been
       used to attack users before.  This rule tries to gain safety at
       first glance by constraining mixed-direction text in addresses to
       that which is known to be necessary.

   5.  This rule permits addresse that are e.g. all-Chinese or all-Thai,
       and rejects addresses that mix e.g. Thai and Chinese.  This
       restricts the scope for visually confusable code points.  Since
       some communities are known to mix ASCII localparts with IDNs,
       combining left-to-right text with ASCII is allowed in the way
       that is currently used.

   Note that metal umlauts ("Mötley Crüe") are allowed (see [UMLAUT]).
   This is an unintentional feature.

Appendix D.  Instructions to the RFC editor

   Please remove all mentions of the Protocol Police before publication
   (including this sentence).

   Please remove the Open Issues section.

Appendix E.  Open issues

   1.  Metal umlauts might be a problem.  Accents are used sometimes
       with non-latin, but very seldom and might be seen as surprising
       to native users of e.g. cyrillic, even if Åквариум exists.
       https://krebsonsecurity.com/2022/11/disneyland-malware-team-its-
       a-puny-world-after-all/ is worth considering.  Not entirely clear
       how to subdivide the Common script so it can be used with some
       scripts, not with others.

Authors' Addresses

   Arnt Gulbrandsen
   ICANN
   6 Rond Point Schumann, Bd. 1
   1040 Brussels
   Belgium
   Email: arnt@gulbrandsen.priv.no

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   Jiankang Yao
   CNNIC
   No.4 South 4th Zhongguancun Street
   Beijing
   100190
   China
   Email: yaojk@cnnic.cn

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