]> ICANN

6 Rond Point Schumann, Bd. 1 Brussels 1040 Belgium arnt@gulbrandsen.priv.no

CNNIC

No.4 South 4th Zhongguancun Street Beijing 100190 China yaojk@cnnic.cn

Applications mailmaint email This document specifies rules for email addresses that are flexible enough to express the addresses typically used with SMTPUTF8, while avoiding confusing or risky elements. This is one of a pair of documents: This is simple to implement, contains only globally viable rules and is intended to be usable for software such an MTA. Its companion defines has more complex rules, takes regional usage into account and aims to allow only addresses that are readable and cut-and-pastable in some community.

Introduction - and - 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 , and allows almost all UTF8 strings as localparts. While this certainly allows everything users want to use, it is also flexible enought 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. excludes things that have been described as security risks.
3. Looks safe at first glance to implementers (including ones with little unicode expertise) and are fairly easy to use in unit tests.
4. Contain no regional rules.

These goals are somewhat aspirational.

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 when, and only when, they appear in all capitals, as shown here.

Terminology Script, in this document, refers to the unicode script property (see ). 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.

Rules Based on the above goals, the following rules are formulated:

1. An address MUST NOT contain an a-label (e.g. xn--dmi-0na).
2. An address MUST contain only code points in the PRECIS IdentifierClass.
3. An address MUST consist entirely of a sequence of composite characters, ZWJ and ZWNJ. ("c" followed by "combining hook below" is an example of a composite character, "d" is another example; see for the definition.)
4. An address MOT NOT contain more than one script, disregarding ASCII. (Disregarding ASCII, the word Orléans contains only an é, which is one script, namely Latin.)

Examples example@example.com is permitted, because 1) it does not contain any a-label, 2) it consists entirely of permissible code points, 4) it consists of 19 composite characters, and 4) 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) does not apply, 3) it consists entirely of permissible code points, 4) it consists of 12 composite characters, 5) 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 permitted, because 4) U+200E and U+200F are not parts of composite characters. 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 illegal becasue it contains Han 阿 and the Latin non-ASCII letter ø. TODO: add more examples and rationales again.

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

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 can make it difficult to cut and paste.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] This document provides a list of terms used in the IETF when discussing internationalization. The purpose is to help frame discussions of internationalization in the various areas of the IETF and to help introduce the main concepts to IETF participants. This memo documents an Internet Best Current Practice. Full use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is a replacement for RFC 4952; it reflects additional issues identified since that document was published. [STANDARDS-TRACK] Internet mail was originally limited to 7-bit ASCII. MIME added support for the use of 8-bit character sets in body parts, and also defined an encoded-word construct so other character sets could be used in certain header field values. However, full internationalization of electronic mail requires additional enhancements to allow the use of Unicode, including characters outside the ASCII repertoire, in mail addresses as well as direct use of Unicode in header fields like "From:", "To:", and "Subject:", without requiring the use of complex encoded-word constructs. This document specifies an enhancement to the Internet Message Format and to MIME that allows use of Unicode in mail addresses and most header field content. This specification updates Section 6.4 of RFC 2045 to eliminate the restriction prohibiting the use of non-identity content-transfer- encodings on subtypes of "message/". [STANDARDS-TRACK] Delivery status notifications (DSNs) are critical to the correct operation of an email system. However, the existing Draft Standards (RFC 3461, RFC 3464, RFC 6522) are presently limited to ASCII text in the machine-readable portions of the protocol. This specification adds a new address type for international email addresses so an original recipient address with non-ASCII characters can be correctly preserved even after downgrading. This also provides updated content return media types for delivery status notifications and message disposition notifications to support use of the new address type. This document extends RFC 3461, RFC 3464, RFC 3798, and RFC 6522. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Until now, there has been no standard method for domain names to use characters outside the ASCII repertoire. This document defines internationalized domain names (IDNs) and a mechanism called Internationalizing Domain Names in Applications (IDNA) for handling them in a standard fashion. IDNs use characters drawn from a large repertoire (Unicode), but IDNA allows the non-ASCII characters to be represented using only the ASCII characters already allowed in so-called host names today. This backward-compatible representation is required in existing protocols like DNS, so that IDNs can be introduced with no changes to the existing infrastructure. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] This document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] The Internet Message Format (RFC 5322) allows "group" syntax in some email header fields, such as "To:" and "CC:", but not in "From:" or "Sender:". This document updates RFC 5322 to relax that restriction, allowing group syntax in those latter fields, as well as in "Resent-From:" and "Resent-Sender:", in certain situations. This document specifies a method for IMAP and POP servers to serve internationalized messages to conventional clients. The specification is simple, easy to implement, and provides only rudimentary results. n.d. n.d. n.d.

Acknowledgments The authors wish to thank John C. Klensin, [your name here, please] [oh 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.

Instructions to the RFC editor Please remove all mentions of the Protocol Police before publication (including this sentence). Please remove the Open Issues section.

Open issues

1. PRECIS IdentifierClass?
2. More examples.
3. 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.
4. More words on the relationship between this and the companion. There are several parallel differences, maybe this warrants a section of its own.
5. Should this even mention the requirements placed on domains by IDNA, ICANN, web browsers and others?