SMTP Extension for Internationalized Email Addresses
RFC 5336

Network Working Group                                        J. Yao, Ed.
Request for Comments: 5336                                   W. Mao, Ed.
Updates: 2821, 2822, 4952                                          CNNIC
Category: Experimental                                    September 2008

          SMTP Extension for Internationalized Email Addresses

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This document specifies an SMTP extension for transport and delivery
   of email messages with internationalized email addresses or header
   information.  Communication with systems that do not implement this
   specification is specified in another document.  This document
   updates some syntaxes and rules defined in RFC 2821 and RFC 2822, and
   has some material updating RFC 4952.

Yao & Mao                     Experimental                      [Page 1]
RFC 5336                   EAI SMTP Extension             September 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of This Specification . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Mail Transport-Level Protocol  . . . . . . . . . . . . . . . .  4
     3.1.  Framework for the Internationalization Extension . . . . .  4
     3.2.  The UTF8SMTP Extension . . . . . . . . . . . . . . . . . .  5
     3.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  7
     3.4.  The ALT-ADDRESS Parameter  . . . . . . . . . . . . . . . .  9
     3.5.  ALT-ADDRESS Parameter Usage and Response Codes . . . . . . 10
     3.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . . 11
     3.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 11
       3.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 12
       3.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 12
       3.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 12
       3.7.4.  UTF-8 Strings in Replies . . . . . . . . . . . . . . . 13
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Material Updating RFC 4952  . . . . . . . . . . . . . 20
     A.1.  Conventional Message and Internationalized Message . . . . 20
     A.2.  LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     A.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 20
     A.4.  Implementation Advice  . . . . . . . . . . . . . . . . . . 20
     A.5.  Applicability of SMTP Extension to Additional Uses . . . . 21

Yao & Mao                     Experimental                      [Page 2]
RFC 5336                   EAI SMTP Extension             September 2008

1.  Introduction

   An internationalized email address includes two parts, the local part
   and the domain part.  The ways email addresses are used by protocols
   are different from the ways domain names are used.  The most critical
   difference is that emails are delivered through a chain of clients
   and servers, while domain names are resolved by name servers looking
   up those names in their own tables.  In addition to this, the Simple
   Mail Transfer Protocol [RFC2821] provides a negotiation mechanism
   about service extension with which clients can discover server
   capabilities and make decisions for further processing.  An extended
   overview of the extension model for internationalized addresses and
   headers appears in [RFC4952], referred to as "the framework document"
   or just as "Framework" elsewhere in this specification.  This
   document specifies an SMTP extension to permit internationalized
   email addresses in envelopes, and UNICODE characters (encoded in
   UTF-8) [RFC3629] in headers.

1.1.  Role of This Specification

   The framework document specifies the requirements for, and describes
   components of, full internationalization of electronic mail.  A
   thorough understanding of the information in that document and in the
   base Internet email specifications [RFC2821] [RFC2822] is necessary
   to understand and implement this specification.