Downgrading Mechanism for Email Address Internationalization
RFC 5504

Network Working Group                                   K. Fujiwara, Ed.
Request for Comments: 5504                                Y. Yoneya, Ed.
Category: Experimental                                              JPRS
                                                              March 2009

      Downgrading Mechanism for Email Address Internationalization

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.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Provisions Relating to IETF Documents in effect on the date of
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Abstract

   Traditional mail systems handle only ASCII characters in SMTP
   envelope and mail header fields.  The Email Address
   Internationalization (UTF8SMTP) extension allows UTF-8 characters in
   SMTP envelope and mail header fields.  To avoid rejecting
   internationalized email messages when a server in the delivery path
   does not support the UTF8SMTP extension, some sort of converting
   mechanism is required.  This document describes a downgrading
   mechanism for Email Address Internationalization.  Note that this is
   a way to downgrade, not tunnel.  There is no associated up-conversion
   mechanism, although internationalized email clients might use
   original internationalized addresses or other data when displaying or
   replying to downgraded messages.

Fujiwara & Yoneya             Experimental                      [Page 1]
RFC 5504                   UTF8SMTP Downgrade                 March 2009

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. New Header Fields Definition ....................................5
      3.1. Envelope Information Preservation Header Fields ............5
      3.2. Address Header Fields' Preservation Header Fields ..........6
      3.3. Unknown Header Fields' Preservation Header Fields ..........6
   4. SMTP Downgrading ................................................7
      4.1. Path Element Downgrading ...................................7
      4.2. ORCPT downgrading ..........................................8
   5. Email Header Fields Downgrading .................................8
      5.1. Downgrading Method for Each ABNF Element ...................8
           5.1.1. RECEIVED Downgrading ................................9
           5.1.2. UNSTRUCTURED Downgrading ............................9
           5.1.3. WORD Downgrading ....................................9
           5.1.4. COMMENT Downgrading .................................9
           5.1.5. MIME-VALUE Downgrading ..............................9
           5.1.6. DISPLAY-NAME Downgrading ............................9
           5.1.7. MAILBOX Downgrading .................................9
           5.1.8. ENCAPSULATION Downgrading ..........................10
           5.1.9. TYPED-ADDRESS Downgrading ..........................10
      5.2. Downgrading Method for Each Header Field ..................10
           5.2.1. Address Header Fields That Contain <address>s ......10
           5.2.2. Address Header Fields with Typed Addresses .........11
           5.2.3. Downgrading Non-ASCII in Comments ..................11
           5.2.4. Received Header Field ..............................11
           5.2.5. MIME Content Header Fields .........................12
           5.2.6. Non-ASCII in <unstructured> ........................12
           5.2.7. Non-ASCII in <phrase> ..............................12
           5.2.8. Other Header Fields ................................12
   6. MIME Body-Part Header Field Downgrading ........................12
   7. Security Considerations ........................................13
   8. Implementation Notes ...........................................14
      8.1. RFC 2047 Encoding .........................................14
      8.2. Trivial Downgrading .......................................15
      8.3. 7bit Transport Consideration ..............................15
   9. IANA Considerations ............................................16
   10. Acknowledgements ..............................................18
   11. References ....................................................18
      11.1. Normative References .....................................18