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 (C) The Internet Society (2005).
This document defines the initial IANA registration for permanent mail and MIME message header fields, per RFC 3864.
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 .
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
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?
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
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.
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.
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.
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.
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.
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.
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.
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.
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'.
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.
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.
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'.
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.
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)  and RFC 3801 (VPIM) .
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)  and RFC 3801 (VPIM) .
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.
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.
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
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  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.
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).
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.
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 .
Most of the information in this document has been derived from Jacob Palme's work in RFC 2076  and subsequent updates . 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).
 Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004.
 Crocker, D., "Standard for the format of ARPA Internet text messages", STD 11, RFC 822, August 1982.
 Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC 822", RFC 1327, May 1992.
 Costanzo, A., Robinson, D., and R. Ullmann, "Encoding Header Field for Internet Messages", RFC 1505, August 1993.
 Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995.
 Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864, October 1995.
 Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.
 Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)", RFC 2110, March 1997.
 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
 Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME", RFC 2156, January 1998.
 Troost, R., Dorner, S., and K. Moore, "Communicating Presentation Information in Internet Messages: The Content- Disposition Header Field", RFC 2183, August 1997.
 Hansen, T. and G. Vaudreuil, "Message Disposition Notification", RFC 3798, May 2004.
 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.
 Vaudreuil, G. and G. Parsons, "Voice Profile for Internet Mail - version 2 (VPIMv2)", RFC 3801, June 2004.
 Vaudreuil, G. and G. Parsons, "Content Duration MIME Header Definition", RFC 3803, June 2004.
 Palme, J., Hopmann, A., and N. Shelness, "MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)", RFC 2557, March 1999.
 Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001.
 Resnick, P., "Internet Message Format", RFC 2822, April 2001.
 Klyne, G., "Indicating Media Features for MIME Content", RFC 2912, September 2000.
 Chandhok, R. and G. Wenger, "List-Id: A Structured Field and Namespace for the Identification of Mailing Lists", RFC 2919, March 2001.
 Alvestrand, H., "Content Language Headers", RFC 3282, May 2002.
 Klyne, G., Iwazaki, R., and D. Crocker, "Content Negotiation for Messaging Services based on Email", RFC 3297, July 2002.
 Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message Context for Internet Mail", RFC 3458, January 2003.
 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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