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rfc:rfc4021

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

G. Klyne, et al. Standards Track [Page 1] RFC 4021 Mail and MIME Header Fields March 2005

           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

G. Klyne, et al. Standards Track [Page 2] RFC 4021 Mail and MIME Header Fields March 2005

           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.

G. Klyne, et al. Standards Track [Page 3] RFC 4021 Mail and MIME Header Fields March 2005

 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

G. Klyne, et al. Standards Track [Page 4] RFC 4021 Mail and MIME Header Fields March 2005

 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?

G. Klyne, et al. Standards Track [Page 5] RFC 4021 Mail and MIME Header Fields March 2005

 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.

G. Klyne, et al. Standards Track [Page 6] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 7] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 8] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 9] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 10] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 11] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 12] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 13] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 14] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 15] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 16] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 17] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 18] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 19] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 20] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 21] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 22] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 23] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 24] RFC 4021 Mail and MIME Header Fields March 2005

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.)

G. Klyne, et al. Standards Track [Page 25] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 26] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 27] RFC 4021 Mail and MIME Header Fields March 2005

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'.

G. Klyne, et al. Standards Track [Page 28] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 29] RFC 4021 Mail and MIME Header Fields March 2005

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:'.

G. Klyne, et al. Standards Track [Page 30] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 31] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 32] RFC 4021 Mail and MIME Header Fields March 2005

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].

G. Klyne, et al. Standards Track [Page 33] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 34] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 35] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 36] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 37] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 38] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 39] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 40] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 41] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 42] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 43] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 44] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 45] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 46] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 47] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 48] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 49] RFC 4021 Mail and MIME Header Fields March 2005

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].

G. Klyne, et al. Standards Track [Page 50] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 51] RFC 4021 Mail and MIME Header Fields March 2005

 [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>.

G. Klyne, et al. Standards Track [Page 52] RFC 4021 Mail and MIME Header Fields March 2005

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

G. Klyne, et al. Standards Track [Page 53] RFC 4021 Mail and MIME Header Fields March 2005

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.

G. Klyne, et al. Standards Track [Page 54]

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