SMTPUTF8 Email Addresses

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Table of Contents

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. For example, the domain extracted by code may not match that shown to users on-screen.¶

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.¶

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 to 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.¶

The term a-label is defined in [RFC5890] section 2.3.2.1.¶

4. Rules

The following three rules apply to the [I-D.ietf-emailcore-rfc5322bis] mailbox production, as indirectly extended by [RFC6532].¶

1. An atom in an address MUST NOT be an a-label (e.g. xn--dmi-0na).¶

2. An address MUST contain only code points in the "A", "H" and "K" classes, the code points allowed by the "F" class, and "." and "@". These classes are defined in [RFC8264] section 9: A (LetterDigits) in 9.1, H (JoinControl) in 9.8, K (ASCII7) in 9.11 and F (Exceptions) in 9.6. All but K are also defined in [RFC5892] section 2 (2.1, 2.8 and 2.6 respectively). (A contains letters and digits, H contains join controls, K contains ASCII and F contains a few exceptions.)¶

3. An address MUST NOT contain more than one script, when ASCII is disregarded. (For example: In the word Orléans, Orl and ans are ASCII and é is non-ASCII. Since é is a single letter, the word contains only one script.)¶

5. Examples

example@example.com is permitted, because 1) it does not contain any a-label, 2) it consists entirely of permissible code points and 3) it contains no non-ASCII code points at all.¶

The address dømi@dømi.fo is permitted, because 1) it does not contain any a-label, 2) it consists entirely of code points in the "A" and "K" classes and 3) it consists entirely of 'Latin' and 'Common' code points (and ./@).¶

The address U+200E '@' U+200F '.' U+200E is not permitted, because 2) U+200E and U+200F are in the "C" class (IgnorableProperties, [RFC5892] section 2.3), not A/H/K/F.¶

阿Q正传@阿Q正传.example is permitted because it contains ASCII and Han, dømi@dømi.fo is legal because it contains ASCII and Latin, but 阿Q正传@dømi.fo is not, because it contains both Han (阿) and Latin non-ASCII (ø).¶

6. IANA Considerations

This document does not require any actions from the IANA.¶

7. Security Considerations

When a program renders a non-ASCII email address string on-screen or stores it as a string (e.g. in a system log file), the address shown to the eventual human may not be as intended. One possible way to diminish the risk is to replace impermissible code points with U+FFFD, REPLACEMENT CHARACTER. There are also other options.¶

The rules in this document permit a number of code points that can make an address difficult to cut and paste.¶

8. References

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, (your name here, please)¶

(Wow, the ack section is already outdated)¶

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. 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 D. Open issues

1. Wording to identify destiny; I think this should probably become a proposed standard and modify a couple of RFCs, but I'm uncertain about some details and left that open now.¶

2. More words on the relationship between this and the companion. There are several parallel differences, maybe this warrants a section of its own.¶

3. Should this even mention the requirements placed on domains by IDNA, ICANN, web browsers and others?¶

4. SHOULD or MUST not contain a-labels?¶

Authors' Addresses