Internet Message Header Fields

The following table lists Internet message header fields which have been defined in RFCs or actually used or both, in E-mail messages, Usenet messages, or other contexts. For each header, its name, a short description, and some reference is given; the reference tries to point to a document where the meaning of the header is specified or described. In several cases, the description is just an educated (?) guess. The use of the word nonstandard here typically means just lack of more specific information; it does not imply that other headers would necessarily be standard. unofficial sources.

Header field name	Description
Abuse-Reports-To	E-mail address for reporting abuse
Also-Control	Usenet control message and a normal article at the same time
Alternate-Recipient	Controls whether the message may be forwarded to alternate recipients
Apparently-To	Recipients derived from message envelope when there is no To header
Approved	Moderator of the Usenet newsgroup, or marks certain control messages
Approved-By	Moderator of a mailing list, who approved the message
Article-Names	Reference to specially important articles for a particular Usenet newsgroup
Article-Updates	Similar to Supersedes but does not cause the referenced article to be physically deleted
Autoforwarded	Has been automatically forwarded
Auto-Forwarded	Misspelling of Autoforwarded
bcc	Recipient(s) not to be disclosed to other recipients ("blind carbon copy")
cc	Secondary, informational recipient(s)
Comments	Text comments added to the message
Content-Alias	Used in addition to Content-Location if this content part can be retrieved through more than one URI
Content-Alternative	Information on where an alternative variant of this document might be found
Content-Base	Base to be used for resolving relative URIs within this content part
Content-Class	Type information of the content in some class hierarchy
Content-Conversion	Whether the body may be converted from a charset to another
Content-Description	Description of a particular body part of a message
Content-Disposition	Whether MIME body part is to be shown inline or as an attachment; may also suggest a file name
Content-Features	More detailed information about the Content-Type
Content-ID	Unique ID of one body part of the content of a message
Content-Identifier	A text string which identifies the content of a message
Content-Language	Code for natural language used in the message
Content-Length	Size in bytes of the message text
Content-Location	URI with which the content of this content part might be retrievable
Content-MD5	Checksum of content
Content-Return	Whether the content of a message is to be returned with non-delivery notifications
Content-SGML-Entity	Information about the SGML entity declaration corresponding to the body
Content-Transfer-Encoding	Coding method used in a MIME message body
Content-Type	Data type and format of content
Control	On Usenet, indicates a control message
Conversion	Whether the body may be converted from a charset to another
Conversion-With-Loss	Whether the body may be converted from a charset to another if information will be lost
Date	The time when the message was written (or submitted)
Delivered-To	Used for loop detection
Delivery-Date	The time when a message was delivered to its recipient
Discarded-X400-IPMS-Extensions	X.400 IPM extensions which could not be mapped to Internet mail format
Discarded-X400-MTS-Extensions	X.400 MTS extensions which could not be mapped to Internet mail format
Disclose-Recipients	Controls whether recipients are to be told the names of other recipients
Disposition-Notification-Options	Options for notifications to be sent when the message is received
Disposition-Notification-To	Requests for notification when the message is received, and specifies the address for them
Distribution	Limitation on where the Usenet article can be distributed
DL-Expansion-History-Indication	Trace of distribution lists passed
Encoding	Mixed usage: content type encoding, length, or boundary information
Errors-To	Address to which notifications are to be sent and a request to get delivery notifications
Expires	Suggested expiration time
Expiry-Date	Time at which a message loses its validity
Fax	Fax number of the originator
Fcc	Name of file in which a copy of the message is stored
Followup-To	Usenet group(s) to which followups to the article should be sent
For-Approval	Primary recipients, who are requested to approve the information in this message or its attachments
For-Comment	Primary recipients, who are requested to comment on the information in this message or its attachments
For-Handling	Primary recipients, who are requested to handle the information in this message or its attachments
From	Author(s) or person(s) taking responsibility for the message
Generate-Delivery-Report	Whether a delivery report is wanted at successful delivery
Importance	A hint from the originator to the recipients about how important a message is
In-Reply-To	Reference to message which this message is a reply to
Incomplete-Copy	Body parts are missing
Injector-Info	Information about article's injection to Usenet
Keywords	Keywords to be used as an aid in determining if the message is interesting to the reader
Language	Code for natural language used in the message
Lines	Number of lines in the body of the message
List-Archive	URL to use to browse the archives of the mailing list
List-Digest	URL to use to subscribe to the digest version of the mailing list
List-ID	URN of the the mailing list expander
List-Owner	URL to use to send E-mail to the owner of the mailing list
List-Post	URL to use to post to the mailing list
List-Software	Information about the software uses in the mailing list expander
List-Subscribe	URL to use to subscribe to the mailing list
List-Unsubscribe	URL to use to unsubscribe the mailing list
List-URL	URL for various information about the mailing list
Mail-Copies-To	Special address for copies of posted followups
Mail-Reply-Requested-By	Address to which notifications are to be sent and a request to get delivery notifications
Mail-System-Version	Client software used by the originator
Mailer	Client software used by the originator
Mailing-List	Information about mailing list
Message-ID	Unique ID for the message
Message-Type	Indicates that this is a delivery report gatewayed from X.400
MIME-Version	Specifies the version of MIME that the message format complies with
Newsgroups	Usenet group(s) to which the article was posted
NNTP-Posting-Date	Time of posting to Usenet
NNTP-Posting-Host	Host that posted the article to Usenet
NNTP-Posting-Time	Time of posting to Usenet
NNTP-Proxy-Relay	Host that relayed the message to Usenet
Obsoletes	Reference to previous message being corrected and replaced
Organization	The organization to which the sender belongs, or to which the machine belongs
Original-Encoded-Information-Types	Which body part types occur in this message
Original-Recipient	Original recipient information for inclusion in disposition notifications
Originating-Client	Client software used by the originator
Originator	Used the same way as Sender
Originator-Info	Information about the authentication of the originator
Path	List of MTAs which the (Usenet) message has passed
Phone	Phone number of originator
PICS-Label	Ratings label to control selection (filtering) of messages according to the PICS protocol
Posted-To	Newsgroup to which the message has been posted /td>
Precedence	Mixed usage: priority value, control of automatic replies or return-of-content faciliries, prevention of mailing list loops
Prevent-NonDelivery-Report	Whether non-delivery report is wanted at delivery error
Priority	Priority for message delivery
Read-Receipt-To	Address to which notifications are to be sent and a request to get delivery notifications
Received	Trace of MTAs which the message has passed
References	In E-mail: reference to other related messages; in Usenet: reference to replied-to-articles
Replaces	Specifies previous message being corrected and replaced
Reply-By	Latest time at which a reply is requested
Reply-To	Suggested E-mail address for replies
Resent-bcc	The bcc of a forwarded message
Resent-cc	The cc of a forwarded message
Resent-Date	The Date of a forwarded message
Resent-From	The From of a forwarded message
Resent-Message-ID	The Message-ID of a forwarded message
Resent-Reply-To	The Reply-To of a forwarded message
Resent-Sender	The Sender of a forwarded message
Resent-Subject	The Subject of a forwarded message
Resent-To	The To of a forwarded message
Return-Path	Data passed from MAIL FROM: envelope
Return-Receipt-Requested	Address to which notifications are to be sent and a request to get delivery notifications
Return-Receipt-To	Address to which notifications are to be sent and a request to get delivery notifications
See-Also	References to related articles (other than precursors) in Usenet
Sender	The person or agent submitting the message to the network, if other than shown by the From header
Sensitivity	How sensitive it is to disclose this message to other people than the specified recipients
Speech-Act	Speech act categorization of the message
Status	Status of delivery for the message when stored
Subject	Text that provides a summary, or indicates the nature, of the message
Summary	Short text describing a longer article
Supersedes	Specifies previous message being corrected and replaced
Telefax	Fax number of the originator
To	Primary recipient(s)
Translated-By	Mailbox of the person who made the translation
Translation-Of	Reference to a message which the current message is a translation of
User-Agent	Client software used by the originator
X-Abuse-Info	Information for reporting abuse
X-Accept-Language	Language preference(s)
X-Admin	E-mail address of news server administration
X-Article-Creation-Date	Time of original creation of the article
X-Attribution	The name under which the sender would like to appear in attributions when the message is quoted
X-Authenticated-IP	Apparently the IP address of the host of the sender
X-Authenticated-Sender	Apparently some identification of the sender
X-Authentication-Warning	Some warning
X-Cache	Information about caching server software
X-Comments	Comments, usually some disclaimer
X-Complaints-To	An E-mail address for sending complaints on the adequacy of the Usenet message
X-Confirm-reading-to	Address to which notifications are to be sent and a request to get delivery notifications
X-Envelope-From	Sender, as extracted from the envelope
X-Envelope-To	Recipient, as extracted from the envelope
X-Face	Picture of sender's face (a 48×48 bitmap)
X-Folder	Folder for saving the message
X-Http-Proxy	Information about using Deja for posting, including the sender's IP address
X-Http-User-Agent	Web browser used for posting
X-IMAP	UID as defined in IMAP
X-Last-Updated	Time of most recent update
X-List-Host	As List-Software
X-Listserver	As List-Software
X-Loop	Used for loop detection
X-Mailer	Client software used by the originator
X-Mailer-Info	URL for info about client software used by the originator
X-Mailing-List	Information about mailing list
X-MIME-Autoconverted	Information about conversion of this message on the path from sender to recipient
X-MimeOLE	Information about client software used by the originator
X-MIMETrack	Info about the message passing through a router
X-MSMail-Priority	Yet another priority indication
X-MyDeja-Info	Internal user id
X-Newsreader	Cient software used by the originator
X-NNTP-Posting-Host	Posting host
X-No-Archive	Do not archive the message in publicly available archives
X-Notice	A comment on reporting abuse
X-Orig-Message-ID	Original Message-ID
X-Original-Envelope-From	E-mail address of original sender
X-Original-NNTP-Posting-Host	Name or IP address of the original posting host
X-Original-Trace	Information (host, time, etc.) about the original posting
X-OriginalArrivalTime	Time when the message was delivered into the message transport system
X-Originating-IP	Apparently the IP address of the host of the sender
X-PMFLAGS	Internal flags indicating message status
X-Posted-By	Attempted identification of the sender
X-Posting-Agent	Client software used by the originator
X-Priority	Priority for message delivery
X-RCPT-TO	Indication of recipient on the SMTP envelope
X-Report	Information about reporting abuse
X-Report-Abuse-To	Information about reporting abuse
X-Sender	Used similarly to Sender but often indicating that you may not be able to send E-mail to this address
X-Server-Date	Some time denotation
X-Trace	Information about posting
X-UIDL	Unique ID for the message, local to a particular local mailbox store
X-URI	Sometimes used with the same meaning as Content-Location, sometimes to indicate the Web home page of the sender or of his organisation
X-URL	Similar usage as X-URI
X-X-Sender	Similar to X-Sender
X400-Content-Return	Possible future name for Content-Return
Xref	Identification of a message within a server