path: root/doc/rfc/rfc4021.txt

Network Working Group                                           G. Klyne
Request for Comments: 4021                          University of Oxford
Category: Standards Track                                       J. Palme
                                                 Stockholm University/KT
                                                              March 2005


              Registration of Mail and MIME Header Fields

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines the initial IANA registration for permanent
   mail and MIME message header fields, per RFC 3864.

Table of Contents
   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Structure of This Document . . . . . . . . . . . . . . .  3
       1.2.  Document Terminology and Conventions . . . . . . . . . .  4
   2.  Registration Templates . . . . . . . . . . . . . . . . . . . .  4
       2.1.  Permanent Mail Header Field Registrations  . . . . . . .  4
             2.1.1.  Header Field: Date . . . . . . . . . . . . . . .  6
             2.1.2.  Header Field: From . . . . . . . . . . . . . . .  7
             2.1.3.  Header Field: Sender . . . . . . . . . . . . . .  7
             2.1.4.  Header Field: Reply-To . . . . . . . . . . . . .  8
             2.1.5.  Header Field: To . . . . . . . . . . . . . . . .  8
             2.1.6.  Header Field: Cc . . . . . . . . . . . . . . . .  9
             2.1.7.  Header Field: Bcc  . . . . . . . . . . . . . . .  9
             2.1.8.  Header Field: Message-ID . . . . . . . . . . . . 10
             2.1.9.  Header Field: In-Reply-To  . . . . . . . . . . . 10
             2.1.10. Header Field: References . . . . . . . . . . . . 11
             2.1.11. Header Field: Subject  . . . . . . . . . . . . . 11
             2.1.12. Header Field: Comments . . . . . . . . . . . . . 12
             2.1.13. Header Field: Keywords . . . . . . . . . . . . . 12
             2.1.14. Header Field: Resent-Date  . . . . . . . . . . . 13
             2.1.15. Header Field: Resent-From  . . . . . . . . . . . 13
             2.1.16. Header Field: Resent-Sender  . . . . . . . . . . 14



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             2.1.17. Header Field: Resent-To  . . . . . . . . . . . . 14
             2.1.18. Header Field: Resent-Cc  . . . . . . . . . . . . 15
             2.1.19. Header Field: Resent-Bcc . . . . . . . . . . . . 15
             2.1.20. Header Field: Resent-Reply-To  . . . . . . . . . 16
             2.1.21. Header Field: Resent-Message-ID  . . . . . . . . 16
             2.1.22. Header Field: Return-Path  . . . . . . . . . . . 17
             2.1.23. Header Field: Received . . . . . . . . . . . . . 17
             2.1.24. Header Field: Encrypted  . . . . . . . . . . . . 18
             2.1.25. Header Field: Disposition-Notification-To  . . . 18
             2.1.26. Header Field: Disposition-Notification-Options . 19
             2.1.27. Header Field: Accept-Language  . . . . . . . . . 19
             2.1.28. Header Field: Original-Message-ID  . . . . . . . 20
             2.1.29. Header Field: PICS-Label . . . . . . . . . . . . 20
             2.1.30. Header Field: Encoding . . . . . . . . . . . . . 21
             2.1.31. Header Field: List-Archive . . . . . . . . . . . 21
             2.1.32. Header Field: List-Help  . . . . . . . . . . . . 22
             2.1.33. Header Field: List-ID  . . . . . . . . . . . . . 22
             2.1.34. Header Field: List-Owner . . . . . . . . . . . . 23
             2.1.35. Header Field: List-Post  . . . . . . . . . . . . 23
             2.1.36. Header Field: List-Subscribe . . . . . . . . . . 24
             2.1.37. Header Field: List-Unsubscribe . . . . . . . . . 24
             2.1.38. Header Field: Message-Context  . . . . . . . . . 25
             2.1.39. Header Field: DL-Expansion-History . . . . . . . 25
             2.1.40. Header Field: Alternate-Recipient  . . . . . . . 26
             2.1.41. Header Field: Original-Encoded-Information-Types 26
             2.1.42. Header Field: Content-Return . . . . . . . . . . 27
             2.1.43. Header Field: Generate-Delivery-Report . . . . . 27
             2.1.44. Header Field: Prevent-NonDelivery-Report . . . . 28
             2.1.45. Header Field: Obsoletes  . . . . . . . . . . . . 28
             2.1.46. Header Field: Supersedes . . . . . . . . . . . . 29
             2.1.47. Header Field: Content-Identifier . . . . . . . . 29
             2.1.48. Header Field: Delivery-Date  . . . . . . . . . . 30
             2.1.49. Header Field: Expiry-Date  . . . . . . . . . . . 30
             2.1.50. Header Field: Expires  . . . . . . . . . . . . . 31
             2.1.51. Header Field: Reply-By . . . . . . . . . . . . . 31
             2.1.52. Header Field: Importance . . . . . . . . . . . . 32
             2.1.53. Header Field: Incomplete-Copy  . . . . . . . . . 32
             2.1.54. Header Field: Priority . . . . . . . . . . . . . 33
             2.1.55. Header Field: Sensitivity  . . . . . . . . . . . 33
             2.1.56. Header Field: Language . . . . . . . . . . . . . 34
             2.1.57. Header Field: Conversion . . . . . . . . . . . . 34
             2.1.58. Header Field: Conversion-With-Loss . . . . . . . 35
             2.1.59. Header Field: Message-Type . . . . . . . . . . . 35
             2.1.60. Header Field: Autosubmitted  . . . . . . . . . . 36
             2.1.61. Header Field: Autoforwarded  . . . . . . . . . . 36
             2.1.62. Header Field: Discarded-X400-IPMS-Extensions . . 37
             2.1.63. Header Field: Discarded-X400-MTS-Extensions  . . 37
             2.1.64. Header Field: Disclose-Recipients  . . . . . . . 38



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             2.1.65. Header Field: Deferred-Delivery  . . . . . . . . 38
             2.1.66. Header Field: Latest-Delivery-Time . . . . . . . 39
             2.1.67. Header Field: Originator-Return-Address  . . . . 39
             2.1.68. Header Field: X400-Content-Identifier  . . . . . 40
             2.1.69. Header Field: X400-Content-Return  . . . . . . . 40
             2.1.70. Header Field: X400-Content-Type  . . . . . . . . 41
             2.1.71. Header Field: X400-MTS-Identifier  . . . . . . . 41
             2.1.72. Header Field: X400-Originator  . . . . . . . . . 42
             2.1.73. Header Field: X400-Received  . . . . . . . . . . 42
             2.1.74. Header Field: X400-Recipients  . . . . . . . . . 43
             2.1.75. Header Field: X400-Trace . . . . . . . . . . . . 43
       2.2.  Permanent MIME Header Field Registrations  . . . . . . . 44
             2.2.1.  Header Field: MIME-Version . . . . . . . . . . . 44
             2.2.2.  Header Field: Content-ID . . . . . . . . . . . . 45
             2.2.3.  Header Field: Content-Description  . . . . . . . 45
             2.2.4.  Header Field: Content-Transfer-Encoding  . . . . 46
             2.2.5.  Header Field: Content-Type . . . . . . . . . . . 46
             2.2.6.  Header Field: Content-Base . . . . . . . . . . . 47
             2.2.7.  Header Field: Content-Location . . . . . . . . . 47
             2.2.8.  Header Field: Content-features . . . . . . . . . 48
             2.2.9.  Header Field: Content-Disposition  . . . . . . . 48
             2.2.10. Header Field: Content-Language . . . . . . . . . 49
             2.2.11. Header Field: Content-Alternative  . . . . . . . 49
             2.2.12. Header Field: Content-MD5  . . . . . . . . . . . 50
             2.2.13. Header Field: Content-Duration . . . . . . . . . 50
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 50
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 51
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 51
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 51
       6.2.  Informative References . . . . . . . . . . . . . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 54

1.  Introduction

   This document defines IANA registration for a number of mail message
   and MIME header fields, per registration procedures for message
   header fields [1].

   The main body of this document is automatically generated from RDF/N3
   data.  Some experimental HTML registry pages have been prepared from
   the same data and can be found at [27].

1.1.  Structure of This Document

   Section 2.1 contains the templates for initial registration of mail
   message header fields.



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   Section 2.2 contains templates for initial registration of MIME
   header fields.

1.2.  Document Terminology and Conventions

   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 BCP 14, RFC 2119 [9].

2.  Registration Templates

   Header field registry entries are summarized in tabular form for
   convenience of reference and presented in full in the following
   sections.

2.1.  Permanent Mail Header Field Registrations


   Header name         Protocol
   -----------         --------
   Date                Mail      Message date and time
   From                Mail      Mailbox of message author
   Sender              Mail      Mailbox of message sender
   Reply-To            Mail      Mailbox for replies to message
   To                  Mail      Primary recipient mailbox
   Cc                  Mail      Carbon-copy recipient mailbox
   Bcc                 Mail      Blind-carbon-copy recipient
                                 mailbox
   Message-ID          Mail      Message identifier
   In-Reply-To         Mail      Identify replied-to message(s)
   References          Mail      Related message identifier(s)
   Subject             Mail      Topic of message
   Comments            Mail      Additional comments about the
                                 message
   Keywords            Mail      Message key words and/or phrases
   Resent-Date         Mail      Date and time message is resent
   Resent-From         Mail      Mailbox of person for whom message
                                 is resent
   Resent-Sender       Mail      Mailbox of person who actually
                                 resends the message
   Resent-To           Mail      Mailbox to which message is resent
   Resent-Cc           Mail      Mailbox(es) to which message is
                                 cc'ed on resend
   Resent-Bcc          Mail      Mailbox(es) to which message is
                                 bcc'ed on resend
   Resent-Reply-To     Mail      Resent reply-to
   Resent-Message-ID   Mail      Message identifier for resent
                                 message



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   Return-Path         Mail      Message return path
   Received            Mail      Mail transfer trace information
   Encrypted           Mail      Message encryption information
   Disposition-Notification-To
                       Mail      Mailbox for sending disposition
                                 notification
   Disposition-Notification-Options
                       Mail      Disposition notification options
   Accept-Language     Mail      Language(s) for auto-responses
   Original-Message-ID Mail      Original message identifier
   PICS-Label          Mail      PICS rating label
   Encoding            Mail      Message encoding and other
                                 information
   List-Archive        Mail      URL of mailing list archive
   List-Help           Mail      URL for mailing list information
   List-ID             Mail      Mailing list identifier
   List-Owner          Mail      URL for mailing list owner's
                                 mailbox
   List-Post           Mail      URL for mailing list posting
   List-Subscribe      Mail      URL for mailing list subscription
   List-Unsubscribe    Mail      URL for mailing list
                                 unsubscription
   Message-Context     Mail      Type or context of message
   DL-Expansion-History
                       Mail      Trace of distribution lists passed
   Alternate-Recipient Mail      Controls forwarding to alternate
                                 recipients
   Original-Encoded-Information-Types
                       Mail      Body part types in message
   Content-Return      Mail      Return content on non-delivery?
   Generate-Delivery-Report
                       Mail      Request delivery report generation
   Prevent-NonDelivery-Report
                       Mail      Non-delivery report required?
   Obsoletes           Mail      Reference message to be replaced
   Supersedes          Mail      Reference message to be replaced
   Content-Identifier  Mail      Message content identifier
   Delivery-Date       Mail      Message delivery time
   Expiry-Date         Mail      Message expiry time
   Expires             Mail      Message expiry time
   Reply-By            Mail      Time by which a reply is requested
   Importance          Mail      Message importance
   Incomplete-Copy     Mail      Body parts are missing
   Priority            Mail      Message priority
   Sensitivity         Mail      Message content sensitivity
   Language            Mail      X.400 message content language
   Conversion          Mail      Conversion allowed?




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   Conversion-With-Loss
                       Mail      Lossy conversion allowed?
   Message-Type        Mail      Message type: delivery report?
   Autosubmitted       Mail      Automatically submitted indicator
   Autoforwarded       Mail      Automatically forwarded indicator
   Discarded-X400-IPMS-Extensions
                       Mail      X.400 IPM extensions discarded
   Discarded-X400-MTS-Extensions
                       Mail      X.400 MTS extensions discarded
   Disclose-Recipients Mail      Disclose names of other
                                 recipients?
   Deferred-Delivery   Mail      Deferred delivery information
   Latest-Delivery-Time
                       Mail      Latest delivery time requested
   Originator-Return-Address
                       Mail      Originator return address
   X400-Content-Identifier
                       Mail      Message content identifier
   X400-Content-Return Mail      Return content on non-delivery?
   X400-Content-Type   Mail      X400 content type
   X400-MTS-Identifier Mail      X400 MTS-Identifier
   X400-Originator     Mail      X400 Originator
   X400-Received       Mail      X400 Received
   X400-Recipients     Mail      X400 Recipients
   X400-Trace          Mail      X400 Trace

2.1.1.  Header Field: Date

   Description:
      Message date and time

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.1)

   Related information:
      Specifies the date and time at which the creator of the message
      indicated that the message was complete and ready to enter the
      mail delivery system.  Defined as standard by RFC 822.





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2.1.2.  Header Field: From

   Description:
      Mailbox of message author

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.2)

   Related information:
      Specifies the author(s) of the message; that is, the mailbox(es)
      of the person(s) or system(s) responsible for the writing of the
      message.  Defined as standard by RFC 822.

2.1.3.  Header Field: Sender

   Description:
      Mailbox of message sender

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.2)

   Related information:
      Specifies the mailbox of the agent responsible for the actual
      transmission of the message.  Defined as standard by RFC 822.











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2.1.4.  Header Field: Reply-To

   Description:
      Mailbox for replies to message

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org) Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.2)

   Related information:
      When the "Reply-To:" field is present, it indicates the
      mailbox(es) to which the author of the message suggests that
      replies be sent.  Defined as standard by RFC 822.

2.1.5.  Header Field: To

   Description:
      Primary recipient mailbox

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.3)

   Related information:
      Contains the address(es) of the primary recipient(s) of the
      message.  Defined as standard by RFC 822.












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2.1.6.  Header Field: Cc

   Description:
      Carbon-copy recipient mailbox

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.3)

   Related information:
      Contains the addresses of others who are to receive the message,
      though the content of the message may not be directed at them.
      Defined as standard by RFC 822.

2.1.7.  Header Field: Bcc

   Description:
      Blind-carbon-copy recipient mailbox

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.3)

   Related information:
      Contains addresses of recipients of the message whose addresses
      are not to be revealed to other recipients of the message.
      Defined as standard by RFC 822.










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2.1.8.  Header Field: Message-ID

   Description:
      Message identifier

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.4)

   Related information:
      Contains a single unique message identifier that refers to a
      particular version of a particular message.  If the message is
      resent without changes, the original Message-ID is retained.
      Defined as standard by RFC 822.

2.1.9.  Header Field: In-Reply-To

   Description:
      Identify replied-to message(s)

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.4)

   Related information:
      The message identifier(s) of the original message(s) to which the
      current message is a reply.  Defined as standard by RFC 822.










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2.1.10.  Header Field: References

   Description:
      Related message identifier(s)

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.4)

   Related information:
      The message identifier(s) of other message(s) to which the current
      message may be related.  In RFC 2822, the definition was changed
      to say that this header field contains a list of all Message-IDs
      of messages in the preceding reply chain.  Defined as standard by
      RFC 822.

2.1.11.  Header Field: Subject

   Description:
      Topic of message

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.5)

   Related information:
      Contains a short string identifying the topic of the message.
      Defined as standard by RFC 822.









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2.1.12.  Header Field: Comments

   Description:
      Additional comments about the message

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.5)

   Related information:
      Contains any additional comments on the text of the body of the
      message.  Warning: Some mailers will not show this field to
      recipients.  Defined as standard by RFC 822.

2.1.13.  Header Field: Keywords

   Description:
      Message key words and/or phrases

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.5)

   Related information:
      Contains a comma-separated list of important words and phrases
      that might be useful for the recipient.  Defined as standard by
      RFC 822.










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2.1.14.  Header Field: Resent-Date

   Description:
      Date and time message is resent

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the date and time that a message is reintroduced into the
      message transfer system.  Defined as standard by RFC 822.

2.1.15.  Header Field: Resent-From

   Description:
      Mailbox of person for whom message is resent

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the mailbox of the agent who has reintroduced the message
      into the message transfer system, or on whose behalf the message
      has been resent.  Defined as standard by RFC 822.











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2.1.16.  Header Field: Resent-Sender

   Description:
      Mailbox of person who actually resends the message

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the mailbox of the agent who has reintroduced the message
      into the message transfer system, if this is different from the
      Resent-From value.  Defined as standard by RFC 822.

2.1.17.  Header Field: Resent-To

   Description:
      Mailbox to which message is resent

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the mailbox(es) to which the message has been resent.
      Defined as standard by RFC 822.











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2.1.18.  Header Field: Resent-Cc

   Description:
      Mailbox(es) to which message is cc'ed on resend

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the mailbox(es) to which message is cc'ed on resend.
      Defined as standard by RFC 822.

2.1.19.  Header Field: Resent-Bcc

   Description:
         Mailbox(es) to which message is bcc'ed on resend

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains the mailbox(es) to which message is bcc'ed on resend.
      Defined as standard by RFC 822.












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2.1.20.  Header Field: Resent-Reply-To

   Description:
      Resent reply-to

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18]

   Related information:
      Resent Reply-to.  Defined by RFC 822, obsoleted by RFC 2822.

2.1.21.  Header Field: Resent-Message-ID

   Description:
      Message identifier for resent message

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.6)

   Related information:
      Contains a message identifier for a resent message.  Defined as
      standard by RFC 822.













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2.1.22.  Header Field: Return-Path

   Description:
      Message return path

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.7)

   Related information:
      Return path for message response diagnostics.  See also RFC 2821
      [17].  Defined as standard by RFC 822.

2.1.23.  Header Field: Received

   Description:
      Mail transfer trace information

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2822 [18] (section 3.6.7)

   Related information:
      Contains information about receipt of the current message by a
      mail transfer agent on the transfer path.  See also RFC 2821 [17].
      Defined as standard by RFC 822.











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2.1.24.  Header Field: Encrypted

   Description:
      Message encryption information

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 822 [2]

   Related information:
      Defined by RFC 822, but was found to be inadequately specified,
      was not widely implemented, and was removed in RFC 2822.  Current
      practice is to use separate encryption, such as S/MIME or OpenPGP,
      possibly in conjunction with RFC 1847 MIME security multiparts.

2.1.25.  Header Field: Disposition-Notification-To

   Description:
      Mailbox for sending disposition notification

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2298 [12]

   Related information:
      Indicates that the sender wants a disposition notification when
      this message is received (read, processed, etc.) by its
      recipients.









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2.1.26.  Header Field: Disposition-Notification-Options

   Description:
      Disposition notification options

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2298 [12]

   Related information:
      For optional modifiers on disposition notification requests.

2.1.27.  Header Field: Accept-Language

   Description:
      Language(s) for auto-responses

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3282 [21]

   Related information:
      Indicates a language that the message sender requests to be used
      for responses.  Accept-language was not designed for email but has
      been considered useful as input to the generation of automatic
      replies.  Some problems have been noted concerning its use with
      email, including but not limited to determination of the email
      address to which it refers; cost and lack of effective
      internationalization of email responses; interpretation of
      language subtags; and determining what character set encoding
      should be used.






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2.1.28.  Header Field: Original-Message-ID

   Description:
      Original message identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3297 [22]

   Related information:
      Original message identifier used with resend of message with
      alternative content format; identifies the original message data
      to which it corresponds.

2.1.29.  Header Field: PICS-Label

   Description:
      PICS rating label

   Applicable protocol: Mail [18]

   Status: standard

   Author/change controller:
      W3C  (mailto:web-human@w3.org)
      World Wide Web Consortium

   Specification document(s):
      PICS-labels [24]

   Related information:
      Ratings label to control selection (filtering) of messages
      according to the PICS protocol.  Specified for general use with
      RFC 822 message format, with HTTP-specific extensions.










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2.1.30.  Header Field: Encoding

   Description:
      Message encoding and other information

   Applicable protocol: Mail [18]

   Status: experimental

   Author/change controller:
      Albert K. Costanzo  (mailto:AL@AKC.COM)
      AKC Consulting Inc.

   Specification document(s):
      RFC 1505 [4]

   Related information:
      Used in several different ways by different mail systems.  Some
      use it for a kind of content-type information, some for encoding
      and length information, some for a kind of boundary information,
      and some in other ways.

2.1.31.  Header Field: List-Archive

   Description:
      URL of mailing list archive

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to use to browse the archives of the mailing list
      from which this message was relayed.










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2.1.32.  Header Field: List-Help

   Description:
      URL for mailing list information

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to use to get information about the mailing list
      from which this message was relayed.

2.1.33.  Header Field: List-ID

   Description:
      Mailing list identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2919 [20]

   Related information:
      Stores an identification of the mailing list through which this
      message was distributed.












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2.1.34.  Header Field: List-Owner

   Description:
      URL for mailing list owner's mailbox

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to send e-mail to the owner of the mailing list
      from which this message was relayed.

2.1.35.  Header Field: List-Post

   Description:
      URL for mailing list posting

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to use to send contributions to the mailing list
      from which this message was relayed.












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2.1.36.  Header Field: List-Subscribe

   Description:
      URL for mailing list subscription

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to use to get a subscription to the mailing list
      from which this message was relayed.

2.1.37.  Header Field: List-Unsubscribe

   Description:
      URL for mailing list unsubscription

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2369 [13]

   Related information:
      Contains the URL to use to unsubscribe the mailing list from which
      this message was relayed.












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2.1.38.  Header Field: Message-Context

   Description:
      Type or context of message

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3458 [23]

   Related information:
      Provides information about the context and presentation
      characteristics of a message.  Can have the values 'voice-
      message', 'fax-message', 'pager-message', 'multimedia-message',
      'text-message', or 'none'.

2.1.39.  Header Field: DL-Expansion-History

   Description:
      Trace of distribution lists passed

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Trace of distribution lists passed. (MIXER X.400 mapping; not for
      general use.)










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2.1.40.  Header Field: Alternate-Recipient

   Description:
      Controls forwarding to alternate recipients

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Controls whether this message may be forwarded to an alternate
      recipient, such as a postmaster, if delivery to the intended
      recipient is not possible.  Default: Allowed.  RFC 2156 (MIXER),
      not for general use.

2.1.41.  Header Field: Original-Encoded-Information-Types

   Description:
      Body part types in message

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Which body part types occur in this message.  RFC 2156 (MIXER);
      not for general use.










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2.1.42.  Header Field: Content-Return

   Description:
      Return content on non-delivery?

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Indicates whether the content of a message is to be returned with
      non-delivery notifications.  Introduced by RFC 1327 and
      subsequently changed by RFC 2156 to avoid confusion with MIME
      defined fields.

2.1.43.  Header Field: Generate-Delivery-Report

   Description:
      Request delivery report generation

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether a delivery report is wanted at successful
      delivery.  Default is not to generate such a report.  RFC 2156
      (MIXER); not for general use.









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2.1.44.  Header Field: Prevent-NonDelivery-Report

   Description:
      Non-delivery report required?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether a non-delivery report is wanted on delivery
      error.  Default is to generate such a report.  RFC 2156 (MIXER);
      not for general use.

2.1.45.  Header Field: Obsoletes

   Description:
      Reference message to be replaced

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Reference to a previous message being corrected and replaced.
      Compare to 'Supersedes:',f used in Usenet News.  Introduced by RFC
      1327 and subsequently renamed by RFC 2156 to 'Supersedes'.










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2.1.46.  Header Field: Supersedes

   Description:
      Reference message to be replaced

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Reference to a previous message being corrected and replaced.
      Renamed version of obsolete 'Obsoletes' header field.  RFC 2156
      (MIXER); not for general use.

2.1.47.  Header Field: Content-Identifier

   Description:
      Message content identifier

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      A text string that identifies the content of a message.
      Introduced by RFC 1327 and subsequently changed by RFC 2156 to
      avoid confusion with MIME defined fields.  Gateways that reverse
      map may support the old field.









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2.1.48.  Header Field: Delivery-Date

   Description:
      Message delivery time

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The time when a message was delivered to its recipient.  RFC 2156
      (MIXER); not for general use.

2.1.49.  Header Field: Expiry-Date

   Description:
      Message expiry time

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Time at which a message loses its validity.  Introduced by RFC
      1327 and subsequently changed by RFC 2156 to 'Expires:'.












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2.1.50.  Header Field: Expires

   Description:
      Message expiry time

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Time at which a message loses its validity.  Renamed version of
      obsolete Expiry-Date header field.  RFC 2156 (MIXER), not for
      general use.

2.1.51.  Header Field: Reply-By

   Description:
      Time by which a reply is requested

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Latest time by which a reply is requested (not demanded).  RFC
      2156 (MIXER); not for general use.











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2.1.52.  Header Field: Importance

   Description:
      Message importance

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      A hint from the originator to the recipients about how important a
      message is.  Values: High, normal, or low.  Not used to control
      transmission speed.  Proposed for use with RFC 2156 (MIXER) [10]
      and RFC 3801 (VPIM) [14].

2.1.53.  Header Field: Incomplete-Copy

   Description:
      Body parts are missing

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Body parts are missing.  RFC 2156 (MIXER); not for general use.











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2.1.54.  Header Field: Priority

   Description:
      Message priority

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be 'normal', 'urgent', or 'non-urgent' and can influence
      transmission speed and delivery.  RFC 2156 (MIXER); not for
      general use.

2.1.55.  Header Field: Sensitivity

   Description:
      Message content sensitivity

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      How sensitive it is to disclose this message to people other than
      the specified recipients.  Values: Personal, private, and company
      confidential.  The absence of this header field in messages
      gatewayed from X.400 indicates that the message is not sensitive.
      Proposed for use with RFC 2156 (MIXER) [10] and RFC 3801 (VPIM)
      [14].







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2.1.56.  Header Field: Language

   Description:
      X.400 message content language

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can include a code for the natural language used in a message;
      e.g., 'en' for English.  See also 'Content-Language'.  RFC 2156
      (MIXER); not for general use.

2.1.57.  Header Field: Conversion

   Description:
      Conversion allowed?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The body of this message may not be converted from one character
      set to another.  Values: prohibited and allowed.  RFC 2156
      (MIXER); not for general use.










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2.1.58.  Header Field: Conversion-With-Loss

   Description:
      Lossy conversion allowed?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The body of this message may not be converted from one character
      set to another if information will be lost.  Values: prohibited
      and allowed.  RFC 2156 (MIXER); not for general use.

2.1.59.  Header Field: Message-Type

   Description:
      Message type: delivery report?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Only used with the value 'Delivery Report' to indicate that this
      is a delivery report gatewayed from X.400.  RFC 2156 (MIXER); not
      for general use.










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2.1.60.  Header Field: Autosubmitted

   Description:
      Automatically submitted indicator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Has been automatically submitted.  RFC 2156 (MIXER); not for
      general use.

2.1.61.  Header Field: Autoforwarded

   Description:
      Automatically forwarded indicator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Has been automatically forwarded.  RFC 2156 (MIXER), not for
      general use.












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2.1.62.  Header Field: Discarded-X400-IPMS-Extensions

   Description:
      X.400 IPM extensions discarded

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be used in Internet mail to indicate X.400 IPM extensions that
      could not be mapped to Internet mail format.  RFC 2156 (MIXER);
      not for general use.

2.1.63.  Header Field: Discarded-X400-MTS-Extensions

   Description:
      X.400 MTS extensions discarded

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be used in Internet mail to indicate X.400 MTS extensions that
      could not be mapped to Internet mail format.  RFC 2156 (MIXER);
      not for general use.










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2.1.64.  Header Field: Disclose-Recipients

   Description:
      Disclose names of other recipients?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Tells whether recipients are to be told the names of other
      recipients of the same message.  This is primarily an X.400
      facility.  In X.400, this is an envelope attribute and refers to
      disclosure of the envelope recipient list.  Disclosure of other
      recipients is done in Internet mail via the To:, cc:, and bcc:
      header fields.  Not for general use.

2.1.65.  Header Field: Deferred-Delivery

   Description:
      Deferred delivery information

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Provides information about deferred delivery service to the
      recipient.  RFC 2156 (MIXER); not for general use.








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2.1.66.  Header Field: Latest-Delivery-Time

   Description:
      Latest delivery time requested

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Provides the recipient with information about requested delivery
      but will not be acted on by the SMTP infrastructure.  RFC 2156
      (MIXER); not for general use.

2.1.67.  Header Field: Originator-Return-Address

   Description:
      Originator return address

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Originator return address.  RFC 2156 (MIXER); not for general use.












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2.1.68.  Header Field: X400-Content-Identifier

   Description:
      Message content identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      A text string that identifies the content of a message.  Renamed
      version of obsolete Content-Identifier field.  RFC 2156 (MIXER);
      not for general use.

2.1.69.  Header Field: X400-Content-Return

   Description:
      Return content on non-delivery?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether the content of a message is to be returned with
      non-delivery notifications.  Renamed version of obsolete Content-
      Return field.  RFC 2156 (MIXER); not for general use.










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2.1.70.  Header Field: X400-Content-Type

   Description:
      X400 content type

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 content type.  RFC 2156 (MIXER); not for general use.

2.1.71.  Header Field: X400-MTS-Identifier

   Description:
      X400 MTS-Identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 MTS-Identifier.  RFC 2156 (MIXER); not for general use.














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2.1.72.  Header Field: X400-Originator

   Description:
      X400 Originator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Originator.  RFC 2156 (MIXER); not for general use.

2.1.73.  Header Field: X400-Received

   Description:
      X400 Received

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Received.  RFC 2156 (MIXER); not for general use.














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2.1.74.  Header Field: X400-Recipients

   Description:
      X400 Recipients

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Recipients.  RFC 2156 (MIXER); not for general use.

2.1.75.  Header Field: X400-Trace

   Description:
      X400 Trace

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Trace.  RFC 2156 (MIXER), not for general use.














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2.2.  Permanent MIME Header Field Registrations

      Header name         Protocol
      -----------         --------
      MIME-Version        MIME      MIME version number
      Content-ID          MIME      Identify content body part
      Content-Description MIME      Description of message body part
      Content-Transfer-Encoding
                          MIME      Content transfer encoding applied
      Content-Type        MIME      MIME content type
      Content-Base        MIME      Base to be used for resolving
                                    relative URIs within this content
                                    part
      Content-Location    MIME      URI for retrieving a body part
      Content-features    MIME      Indicates content features of a
                                    MIME body part
      Content-Disposition MIME      Intended content disposition and
                                    file name
      Content-Language    MIME      Language of message content
      Content-Alternative MIME      Alternative content available
      Content-MD5         MIME      MD5 checksum of content
      Content-Duration    MIME      Time duration of content

2.2.1.  Header Field: MIME-Version

   Description:
      MIME version number

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 4)

   Related information:
      An indicator that this message is formatted according to the MIME
      standard, and an indication of which version of MIME is used.









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2.2.2.  Header Field: Content-ID

   Description:
      Identify content body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 7)

   Related information:
      Specifies a Unique ID for one MIME body part of the content of a
      message.

2.2.3.  Header Field: Content-Description

   Description:
      Description of message body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 8)

   Related information:
      Description of a particular body part of a message; for example, a
      caption for an image body part.












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2.2.4.  Header Field: Content-Transfer-Encoding

   Description:
      Content transfer encoding applied

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 6)

   Related information:
      Coding method used in a MIME message body part.

2.2.5.  Header Field: Content-Type

   Description:
      MIME content type

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 5)

   Related information:
      Format of content (character set, etc.)  Note that the values for
      this header field are defined in different ways in RFC 1049 and in
      MIME (RFC 2045).  The 'MIME-version' header field will show
      whether Content-Type is to be interpreted according to RFC 1049 or
      according to MIME.  The MIME definition should be used in
      generating mail.  RFC 1049 has 'historic' status.  RFC 1766 [5]
      defines a parameter 'difference' to this header field.  Various
      other Content-Type define various additional parameters.  For
      example, the parameter 'charset' is mandatory for all textual
      Content-Types.  See also RFC 1049, RFC 1123: 5.2.13, and RFC 1766:
      4.1.




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2.2.6.  Header Field: Content-Base

   Description:
      Base to be used for resolving relative URIs within this content
      part.

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2110 [8]

   Related information:
      Base to be used for resolving relative URIs within this content
      part.  See also Content-Location.  This header was included in the
      first version of MHTML and HTTP 1.1 but removed in the second
      version (RFC 2557).

2.2.7.  Header Field: Content-Location

   Description:
      URI for retrieving a body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2557 [16]

   Related information:
      URI using which the content of this body-part part was retrieved,
      might be retrievable, or which otherwise gives a globally unique
      identification of the content.








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2.2.8.  Header Field: Content-features

   Description:
      Indicates content features of a MIME body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2912 [19] (section 3)

   Related information:
      The 'Content-features:' header can be used to annotate a MIME body
      part with a media feature expression, to indicate features of the
      body part content.  See also RFC 2533, RFC 2506, and RFC 2045.

2.2.9.  Header Field: Content-Disposition

   Description:
      Intended content disposition and file name

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2183 [11]

   Related information:
      Indicates whether a MIME body part is to be shown inline or is an
      attachment; can also indicate a suggested filename for use when
      saving an attachment to a file.










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2.2.10.  Header Field: Content-Language

   Description:
      Language of message content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3282 [21]

   Related information:
      Can include a code for the natural language used in a message;
      e.g., 'en' for English.  Can also contain a list of languages for
      a message containing more than one language.

2.2.11.  Header Field: Content-Alternative

   Description:
      Alternative content available

   Applicable protocol: MIME [7]

   Status: work-in-progress

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3297 [22]

   Related information:
      Information about the media features of alternative content
      formats available for the current message.











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2.2.12.  Header Field: Content-MD5

   Description:
      MD5 checksum of content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1864 [6]

   Related information:
      Checksum of content to ensure that it has not been modified.

2.2.13.  Header Field: Content-Duration

   Description:
      Time duration of content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2424 [15]

   Related information:
      Time duration of body part content, in seconds (e.g., for audio
      message).

3.  IANA Considerations

   Section 2 of this specification provides initial registrations of
   mail and MIME header fields in the "Permanent Message Header Field
   Registry", defined by registration procedures for message header
   fields [1].






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4.  Security Considerations

   No security considerations are introduced by this registration
   document beyond those already inherent in use of the mail message
   header fields referenced.

5.  Acknowledgements

   Most of the information in this document has been derived from Jacob
   Palme's work in RFC 2076 [25] and subsequent updates [26].  The
   authors also gratefully acknowledge contributions and constructive
   input from Mark Nottingham, Bruce Lilly, Keith Moore, and Charles
   Lindsey (the mention of whom is not intended to imply their
   unqualified support for material herein).

6.  References

6.1.  Normative References

   [1]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
        Procedures for Message Header Fields", BCP 90, RFC 3864,
        September 2004.

   [2]  Crocker, D., "Standard for the format of ARPA Internet text
        messages", STD 11, RFC 822, August 1982.

   [3]  Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO 10021
        and RFC 822", RFC 1327, May 1992.

   [4]  Costanzo, A., Robinson, D., and R. Ullmann, "Encoding Header
        Field for Internet Messages", RFC 1505, August 1993.

   [5]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        1766, March 1995.

   [6]  Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864,
        October 1995.

   [7]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [8]  Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
        Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
        1997.

   [9]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.



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   [10] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
        between X.400 and RFC 822/MIME", RFC 2156, January 1998.

   [11] Troost, R., Dorner, S., and K. Moore, "Communicating
        Presentation Information in Internet Messages: The Content-
        Disposition Header Field", RFC 2183, August 1997.

   [12] Hansen, T. and G. Vaudreuil, "Message Disposition Notification",
        RFC 3798, May 2004.

   [13] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax for
        Core Mail List Commands and their Transport through Message
        Header Fields", RFC 2369, July 1998.

   [14] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet Mail -
        version 2 (VPIMv2)", RFC 3801, June 2004.

   [15] Vaudreuil, G. and G. Parsons, "Content Duration MIME Header
        Definition", RFC 3803, June 2004.

   [16] Palme, J., Hopmann, A., and N. Shelness, "MIME Encapsulation of
        Aggregate Documents, such as HTML (MHTML)", RFC 2557, March
        1999.

   [17] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
        2001.

   [18] Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [19] Klyne, G., "Indicating Media Features for MIME Content", RFC
        2912, September 2000.

   [20] Chandhok, R. and G. Wenger, "List-Id: A Structured Field and
        Namespace for the Identification of Mailing Lists", RFC 2919,
        March 2001.

   [21] Alvestrand, H., "Content Language Headers", RFC 3282, May 2002.

   [22] Klyne, G., Iwazaki, R., and D. Crocker, "Content Negotiation for
        Messaging Services based on Email", RFC 3297, July 2002.

   [23] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
        Context for Internet Mail", RFC 3458, January 2003.

   [24] Miller, J., Krauskopf, T., Resnick, P. and W. Treese, "PICS
        Label Distribution Label Syntax and Communication Protocols",
        W3C Recommendation REC-PICS-labels, October 1996,
        <http://www.w3.org/TR/REC-PICS-labels>.



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6.2.  Informative References

   [25] Palme, J., "Common Internet Message Headers", RFC 2076, February
        1997.

   [26] Palme, J., "Common Internet Message Header Fields", Work in
        Progress.

URIs

   [27] <http://www.ninebynine.org/IETF/Messaging/HdrRegistry/
        Intro.html>

Authors' Addresses

   Graham Klyne
   Image Bioinformatics Research Group
   Department of Zoology, University of Oxford
   South Parks Road, Oxford OX1 3PS, UK

   Phone: +44-(0)1865-281991
   Fax:   +44-(0)1865-310447
   EMail: GK-IETF@ninebynine.org


   Jacob Palme
   Stockholm University/KTH
   Forum 100
   Kista  S-164 40
   Sweden

   Phone: +46-8-16 16 67
   Fax:   +46-8-783 08 29
   EMail: jpalme@dsv.su.se

















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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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