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-ToE-mail address for reporting abuse
Also-ControlUsenet control message and a normal article at the same time
Alternate-RecipientControls whether the message may be forwarded to alternate recipients
Apparently-ToRecipients derived from message envelope when there is no To header
ApprovedModerator of the Usenet newsgroup, or marks certain control messages
Approved-ByModerator of a mailing list, who approved the message
Article-NamesReference to specially important articles for a particular Usenet newsgroup
Article-UpdatesSimilar to Supersedes but does not cause the referenced article to be physically deleted
AutoforwardedHas been automatically forwarded
Auto-ForwardedMisspelling of Autoforwarded
bccRecipient(s) not to be disclosed to other recipients ("blind carbon copy")
ccSecondary, informational recipient(s)
CommentsText comments added to the message
Content-AliasUsed in addition to Content-Location if this content part can be retrieved through more than one URI
Content-AlternativeInformation on where an alternative variant of this document might be found
Content-BaseBase to be used for resolving relative URIs within this content part
Content-ClassType information of the content in some class hierarchy
Content-ConversionWhether the body may be converted from a charset to another
Content-DescriptionDescription of a particular body part of a message
Content-DispositionWhether MIME body part is to be shown inline or as an attachment; may also suggest a file name
Content-FeaturesMore detailed information about the Content-Type
Content-IDUnique ID of one body part of the content of a message
Content-IdentifierA text string which identifies the content of a message
Content-LanguageCode for natural language used in the message
Content-LengthSize in bytes of the message text
Content-LocationURI with which the content of this content part might be retrievable
Content-MD5Checksum of content
Content-ReturnWhether the content of a message is to be returned with non-delivery notifications
Content-SGML-EntityInformation about the SGML entity declaration corresponding to the body
Content-Transfer-EncodingCoding method used in a MIME message body
Content-TypeData type and format of content
ControlOn Usenet, indicates a control message
ConversionWhether the body may be converted from a charset to another
Conversion-With-LossWhether the body may be converted from a charset to another if information will be lost
DateThe time when the message was written (or submitted)
Delivered-ToUsed for loop detection
Delivery-DateThe time when a message was delivered to its recipient
Discarded-X400-IPMS-ExtensionsX.400 IPM extensions which could not be mapped to Internet mail format
Discarded-X400-MTS-ExtensionsX.400 MTS extensions which could not be mapped to Internet mail format
Disclose-RecipientsControls whether recipients are to be told the names of other recipients
Disposition-Notification-OptionsOptions for notifications to be sent when the message is received
Disposition-Notification-ToRequests for notification when the message is received, and specifies the address for them
DistributionLimitation on where the Usenet article can be distributed
DL-Expansion-History-IndicationTrace of distribution lists passed
EncodingMixed usage: content type encoding, length, or boundary information
Errors-ToAddress to which notifications are to be sent and a request to get delivery notifications
ExpiresSuggested expiration time
Expiry-DateTime at which a message loses its validity
FaxFax number of the originator
FccName of file in which a copy of the message is stored
Followup-ToUsenet group(s) to which followups to the article should be sent
For-ApprovalPrimary recipients, who are requested to approve the information in this message or its attachments
For-CommentPrimary recipients, who are requested to comment on the information in this message or its attachments
For-HandlingPrimary recipients, who are requested to handle the information in this message or its attachments
FromAuthor(s) or person(s) taking responsibility for the message
Generate-Delivery-ReportWhether a delivery report is wanted at successful delivery
ImportanceA hint from the originator to the recipients about how important a message is
In-Reply-ToReference to message which this message is a reply to
Incomplete-CopyBody parts are missing
Injector-InfoInformation about article's injection to Usenet
KeywordsKeywords to be used as an aid in determining if the message is interesting to the reader
LanguageCode for natural language used in the message
LinesNumber of lines in the body of the message
List-ArchiveURL to use to browse the archives of the mailing list
List-DigestURL to use to subscribe to the digest version of the mailing list
List-IDURN of the the mailing list expander
List-OwnerURL to use to send E-mail to the owner of the mailing list
List-PostURL to use to post to the mailing list
List-SoftwareInformation about the software uses in the mailing list expander
List-SubscribeURL to use to subscribe to the mailing list
List-UnsubscribeURL to use to unsubscribe the mailing list
List-URLURL for various information about the mailing list
Mail-Copies-ToSpecial address for copies of posted followups
Mail-Reply-Requested-ByAddress to which notifications are to be sent and a request to get delivery notifications
Mail-System-VersionClient software used by the originator
MailerClient software used by the originator
Mailing-ListInformation about mailing list
Message-IDUnique ID for the message
Message-TypeIndicates that this is a delivery report gatewayed from X.400
MIME-VersionSpecifies the version of MIME that the message format complies with
NewsgroupsUsenet group(s) to which the article was posted
NNTP-Posting-DateTime of posting to Usenet
NNTP-Posting-HostHost that posted the article to Usenet
NNTP-Posting-TimeTime of posting to Usenet
NNTP-Proxy-RelayHost that relayed the message to Usenet
ObsoletesReference to previous message being corrected and replaced
OrganizationThe organization to which the sender belongs, or to which the machine belongs
Original-Encoded-Information-TypesWhich body part types occur in this message
Original-RecipientOriginal recipient information for inclusion in disposition notifications
Originating-ClientClient software used by the originator
OriginatorUsed the same way as Sender
Originator-InfoInformation about the authentication of the originator
PathList of MTAs which the (Usenet) message has passed
PhonePhone number of originator
PICS-LabelRatings label to control selection (filtering) of messages according to the PICS protocol
Posted-ToNewsgroup to which the message has been posted /td>
PrecedenceMixed usage: priority value, control of automatic replies or return-of-content faciliries, prevention of mailing list loops
Prevent-NonDelivery-ReportWhether non-delivery report is wanted at delivery error
PriorityPriority for message delivery
Read-Receipt-ToAddress to which notifications are to be sent and a request to get delivery notifications
ReceivedTrace of MTAs which the message has passed
ReferencesIn E-mail: reference to other related messages; in Usenet: reference to replied-to-articles
ReplacesSpecifies previous message being corrected and replaced
Reply-ByLatest time at which a reply is requested
Reply-ToSuggested E-mail address for replies
Resent-bccThe bcc of a forwarded message
Resent-ccThe cc of a forwarded message
Resent-DateThe Date of a forwarded message
Resent-FromThe From of a forwarded message
Resent-Message-IDThe Message-ID of a forwarded message
Resent-Reply-ToThe Reply-To of a forwarded message
Resent-SenderThe Sender of a forwarded message
Resent-SubjectThe Subject of a forwarded message
Resent-ToThe To of a forwarded message
Return-PathData passed from MAIL FROM: envelope
Return-Receipt-RequestedAddress to which notifications are to be sent and a request to get delivery notifications
Return-Receipt-ToAddress to which notifications are to be sent and a request to get delivery notifications
See-AlsoReferences to related articles (other than precursors) in Usenet
SenderThe person or agent submitting the message to the network, if other than shown by the From header
SensitivityHow sensitive it is to disclose this message to other people than the specified recipients
Speech-ActSpeech act categorization of the message
StatusStatus of delivery for the message when stored
SubjectText that provides a summary, or indicates the nature, of the message
SummaryShort text describing a longer article
SupersedesSpecifies previous message being corrected and replaced
TelefaxFax number of the originator
ToPrimary recipient(s)
Translated-ByMailbox of the person who made the translation
Translation-OfReference to a message which the current message is a translation of
User-AgentClient software used by the originator
X-Abuse-InfoInformation for reporting abuse
X-Accept-LanguageLanguage preference(s)
X-AdminE-mail address of news server administration
X-Article-Creation-DateTime of original creation of the article
X-AttributionThe name under which the sender would like to appear in attributions when the message is quoted
X-Authenticated-IPApparently the IP address of the host of the sender
X-Authenticated-SenderApparently some identification of the sender
X-Authentication-WarningSome warning
X-CacheInformation about caching server software
X-CommentsComments, usually some disclaimer
X-Complaints-ToAn E-mail address for sending complaints on the adequacy of the Usenet message
X-Confirm-reading-toAddress to which notifications are to be sent and a request to get delivery notifications
X-Envelope-FromSender, as extracted from the envelope
X-Envelope-ToRecipient, as extracted from the envelope
X-FacePicture of sender's face (a 48×48 bitmap)
X-FolderFolder for saving the message
X-Http-ProxyInformation about using Deja for posting, including the sender's IP address
X-Http-User-AgentWeb browser used for posting
X-IMAPUID as defined in IMAP
X-Last-UpdatedTime of most recent update
X-List-HostAs List-Software
X-ListserverAs List-Software
X-LoopUsed for loop detection
X-MailerClient software used by the originator
X-Mailer-InfoURL for info about client software used by the originator
X-Mailing-ListInformation about mailing list
X-MIME-AutoconvertedInformation about conversion of this message on the path from sender to recipient
X-MimeOLEInformation about client software used by the originator
X-MIMETrackInfo about the message passing through a router
X-MSMail-PriorityYet another priority indication
X-MyDeja-InfoInternal user id
X-NewsreaderCient software used by the originator
X-NNTP-Posting-HostPosting host
X-No-ArchiveDo not archive the message in publicly available archives
X-NoticeA comment on reporting abuse
X-Orig-Message-IDOriginal Message-ID
X-Original-Envelope-FromE-mail address of original sender
X-Original-NNTP-Posting-HostName or IP address of the original posting host
X-Original-TraceInformation (host, time, etc.) about the original posting
X-OriginalArrivalTimeTime when the message was delivered into the message transport system
X-Originating-IPApparently the IP address of the host of the sender
X-PMFLAGSInternal flags indicating message status
X-Posted-ByAttempted identification of the sender
X-Posting-AgentClient software used by the originator
X-PriorityPriority for message delivery
X-RCPT-TOIndication of recipient on the SMTP envelope
X-ReportInformation about reporting abuse
X-Report-Abuse-ToInformation about reporting abuse
X-SenderUsed similarly to Sender but often indicating that you may not be able to send E-mail to this address
X-Server-DateSome time denotation
X-TraceInformation about posting
X-UIDLUnique ID for the message, local to a particular local mailbox store
X-URISometimes used with the same meaning as Content-Location, sometimes to indicate the Web home page of the sender or of his organisation
X-URLSimilar usage as X-URI
X-X-SenderSimilar to X-Sender
X400-Content-ReturnPossible future name for Content-Return
XrefIdentification of a message within a server