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1. Introduction

The document defines a Simple Mail Transfer Protocol [RFC5321] extension so servers can advertise the ability to accept and process internationalized email addresses (see Section 1.1) and internationalized email headers [RFC6532].

An extended overview of the extension model for internationalized email addresses and the email header appears in RFC 6530 [RFC6530], referred to as "the framework document" in this specification. A thorough understanding of the information in that document and in the base Internet email specifications [RFC5321] [RFC5322] is necessary to understand and implement this specification.

1.1. Terminology​

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 RFC 2119 [RFC2119].

The terms "UTF-8 string" or "UTF-8 character" are used to refer to Unicode characters, which may or may not be members of the ASCII subset, in UTF-8 [RFC3629], a standard Unicode Encoding Form. All other specialized terms used in this specification are defined in the framework document or in the base Internet email specifications. In particular, the terms "ASCII address", "internationalized email address", "non-ASCII address", "SMTPUTF8", "internationalized message", and "message" are used in this document according to the definitions in the framework document [RFC6530].

Strings referred to in this document, including ASCII strings, MUST be expressed in UTF-8.

This specification uses Augmented BNF (ABNF) rules [RFC5234]. Some basic rules in this document are identified in Section 3.3 as being defined (under the same names) in RFC 5234 [RFC5234], RFC 5321 [RFC5321], RFC 5890 [RFC5890], or RFC 6532 [RFC6532].

1.2. Changes Made to Other Specifications​

This specification extends some syntax rules defined in RFC 5321 and permits internationalized email addresses in the envelope and in trace fields, but it does not modify RFC 5321. It permits data formats defined in RFC 6532 [RFC6532], but it does not modify RFC 5322. It does require that the 8BITMIME extension [RFC6152] be announced by the SMTPUTF8-aware SMTP server and used with "BODY=8BITMIME" by the SMTPUTF8-aware SMTP client, but it does not modify the 8BITMIME specification in any way.

This specification replaces an earlier, experimental, approach to the same problem [RFC5336]. Section 6 of RFC 6530 [RFC6530] describes the changes in approach between RFC 5336 [RFC5336] and this specification. Anyone trying to convert an implementation from the experimental specification to the specification in this document will need to review those changes carefully.