Network Working Group J. Palme Request for Comments: 2076 Stockholm University/KTH Category: Informational February 1997
This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
This memo contains a table of commonly occurring headers in headings of e-mail messages. The document compiles information from other RFCs such as RFC 822, RFC 1036, RFC 1123, RFC 1327, RFC 1496, RFC 1521, RFC 1766, RFC 1806, RFC 1864 and RFC 1911. A few commonly occurring headers which are not defined in RFCs are also included. For each header, the memo gives a short description and a reference to the RFC in which the header is defined.
6. Author's Address
Headers sorted by Internet RFC document in which they appear. 21 Appendix B:
Many different Internet standards and RFCs define headers which may occur on Internet Mail Messages and Usenet News Articles. The intention of this document is to list all such headers in one document as an aid to people developing message systems or interested in Internet Mail standards.
The document contains all headers which the author has found in the following Internet standards: , RFC 822 , RFC 1036 , RFC 1123 , RFC 1327 , RFC 1496 , RFC 1521 , RFC 1766 , RFC 1806 , RFC 1864 and RFC 1911. Note in particular that heading attributes defined in PEM (RFC 1421-1424) and MOSS (RFC 1848 ) are not included. PEM and MOSS headers only appear inside the body of a message, and thus are not headers in the RFC 822 sense. Mail attributes in envelopes, i.e. attributes controlling the message transport mechanism between mail and news servers, are not included. This means that attributes from SMTP , UUCP  and NNTP  are mainly not covered either. Headings used only in HTTP  are not included yet, but may be included in future version of this memo. A few additional headers which often can be found in e-mail headings but are not part of any Internet standard are also included.
For each header, the document gives a short description and a reference to the Internet standard or RFC, in which they are defined.
The header names given here are spelled the same way as when they are actually used. This is usually American but sometimes English spelling. One header in particular, "Organisation/Organization", occurs in e-mail headers sometimes with the English and other times with the American spelling.
The following words are used in this memo with the meaning specified below:
heading Formatted text at the top of a message, ended by a blank line
header = heading One field in the heading, beginning with a field
field name, colon, and followed by the field value(s)
It is my intention to continue updating this document after its
publication as an RFC. The latest version, which may be more up-to-
date (but also less fully checked out) will be kept available for
downloading from URL
Please e-mail me (Jacob Palme <email@example.com>) if you have noted headers which should be included in this memo but are not.
RFC 1327 defines a number of new headers in Internet mail, which are defined to map headers which X.400 has but which were previously not standardized in Internet mail. The fact that a header occurs in RFC 1327 indicates that it is recommended for use in gatewaying messages between X.400 and Internet mail, but does not mean that the header is recommended for messages wholly within Internet mail. Some of these headers may eventually see widespread implementation and use in Internet mail, but at the time of this writing (1996) they are not widely implemented or used.
Headers defined only in RFC 1036 for use in Usenet News sometimes appear in mail messages, either because the messages have been gatewayed from Usenet News to e-mail, or because the messages were written in combined clients supporting both e-mail and Usenet News in the same client. These headers are not standardized for use in Internet e-mail and should be handled with caution by e-mail agents.
"not for general Used to mark headers which are defined in RFC usage" 1327 for use in messages from or to Internet mail/X.400 gateways. These headers have not been standardized for general usage in the exchange of messages between Internet mail- based systems.
"not standardized Used to mark headers defined only in RFC 1036 for use in e-mail" for use in Usenet News. These headers have no standard meaning when appearing in e-mail, some of them may even be used in different ways by different software. When appearing in e-mail, they should be handled with caution. Note that RFC 1036, although generally used as a de-facto standard for Usenet News, is not an official IETF standard or even on the IETF standards track. "non-standard" This header is not specified in any of referenced RFCs which define Internet protocols, including Internet Standards, draft standards or proposed standards. The header appears here because it often appears in e- mail or Usenet News. Usage of these headers is not in general recommended. Some header proposed in ongoing IETF standards development work, but not yet accepted, are also marked in this way. "discouraged" This header, which is non-standard, is known to create problems and should not be generated. Handling of such headers in incoming mail should be done with great caution. "controversial" The meaning and usage of this header is controversial, i.e. different implementors have chosen to implement the header in different ways. Because of this, such headers should be handled with caution and understanding of the different possible interpretations. "experimental" This header is used for newly defined headers, which are to be tried out before entering the IETF standards track. These should only be used if both communicating parties agree on using them. In practice, some experimental protocols become de-facto-standards before they are made into IETF standards.
Used to convey the information Return-Path: RFC 821, from the MAIL FROM envelope RFC 1123: 5.2.13. attribute in final delivery, when the message leaves the SMTP environment in which "MAIL FROM" is used. Trace of MTAs which a message has Received: RFC 822: 4.3.2, passed. RFC 1123: 5.2.8. List of MTAs passed. Path: RFC 1036: 2.1.6, only in Usenet News, not in e- mail. Trace of distribution lists DL-Expansion- RFC 1327, not for passed. History- general usage. Indication:
An indicator that this message is MIME-Version: RFC 1521: 3. formatted according to the MIME standard, and an indication of which version of MIME is utilized. Special Usenet News actions only. Control: RFC 1036: 2.1.6, only in Usenet News, not in e- mail. Special Usenet News actions and a Also-Control: son-of-RFC1036 normal article at the same time. , non- standard, only in Usenet News, not in e-mail Which body part types occur in Original- RFC 1327, not for this message. Encoded- general usage. Information- Types:
Controls whether this message may Alternate- RFC 1327, not for be forwarded to alternate Recipient: general usage. recipients such as a postmaster if delivery is not possible to the intended recipient. Default: Allowed. Whether recipients are to be told Disclose- RFC 1327, not for the names of other recipients of Recipients: general usage. 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 in Internet mail done via the To:, cc: and bcc: headers. Whether a MIME body part is to be Content- RFC 1806, shown inline or is an attachment; Disposition: experimental can also indicate a suggested filename for use when saving an attachment to a file.
Authors or persons taking From: RFC 822: 4.4.1, responsibility for the message. RFC 1123: 5.2.15- 16, 5.3.7, Note difference from the "From " RFC 1036 2.1.1 header (not followed by ":") below. (1) This header should never From not standardized appear in e-mail being sent, and for use in e-mail should thus not appear in this memo. It is however included, since people often ask about it.
This header is used in the so-
called Unix mailbox format, also
known as Berkely mailbox format
or the MBOX format. This is a
format for storing a set of
messages in a file. A line
beginning with "From " is used to
separate successive messages in
This header will thus appear when
you use a text editor to look at
a file in the Unix mailbox
format. Some mailers also use
this format when printing
messages on paper.
The information in this header
should NOT be used to find an
address to which replies to a
message are to be sent.
(2) Used in Usenet News mail From RFC 976: 2.4 for transport, to indicate the path or use in Usenet News through which an article has gone >From when transferred to a new host.
Sometimes called "From_" header.
Name of the moderator of the Approved: RFC 1036: 2.2.11, newsgroup to which this article not standardized is sent; necessary on an article for use in e-mail. sent to a moderated newsgroup to allow its distribution to the newsgroup members. Also used on certain control messages, which are only performed if they are marked as Approved. The person or agent submitting Sender: RFC 822: 4.4.2, the message to the network, if RFC 1123: 5.2.15- other than shown by the From: 16, 5.3.7. header. Primary recipients. To: RFC 822: 4.5.1, RFC 1123: 5.2.15- 16, 5.3.7.
Secondary, informational cc: RFC 822: 4.5.2, recipients. (cc = Carbon Copy) RFC 1123. 5.2.15- 16, 5.3.7. Recipients not to be disclosed to bcc: RFC 822: 4.5.3, other recipients. (bcc = Blind RFC 1123: 5.2.15- Carbon Copy). 16, 5.3.7. Primary recipients, who are For-Handling: Non-standard requested to handle the information in this message or its attachments. Primary recipients, who are For-Comment: Non-standard requested to comment on the information in this message or its attachments. In Usenet News: group(s) to which Newsgroups: RFC 1036: 2.1.3, this article was posted. not standardized Some systems provide this header and controversial also in e-mail although it is not for use in e-mail. standardized there.
Unfortunately, the header can
appear in e-mail with two
different and contradictory
(a) Indicating the newsgroup
recipient of an article/message
sent to both e-mail and Usenet
(b) In a personally addressed
reply to an article in a news-
group, indicating the newsgroup
in which this discussion
Inserted by Sendmail when there Apparently- Non-standard, is no "To:" recipient in the To: discouraged, original message, listing mentioned in recipients derived from the RFC 1211. envelope into the message heading. This behavior is not quite proper, MTAs should not modify headings (except inserting Received lines), and it can in some cases cause Bcc recipients to be wrongly divulged to non-Bcc recipients. Geographical or organizational Distribution: RFC 1036: 2.2.7, limitation on where this article not standardized can be distributed. for use in e-mail. Fax number of the originator. Fax:, Non-standard. Telefax: Phone number of the originator. Phone: Non-standard. Information about the client Mail-System- Non-standard. software of the originator. Version:, Mailer:, Originating- Client:, X- Mailer, X- Newsreader
This header is meant to indicate Reply-To: RFC 822: 4.4.3, where the sender wants replies to RFC 1036: 2.2.1 go. Unfortunately, this is controversial. ambiguous, since there are different kinds of replies, which the sender may wish to go to different addresses. In particular, there are personal replies intended for only one person, and group replies, intended for the whole group of people who read the replied-to message (often a mailing list, anewsgroup name cannot appear here because of different syntax, see "Followup-To" below.).
Some mail systems use this header
to indicate a better form of the
e-mail address of the sender.
Some mailing list expanders puts
the name of the list in this
header. These practices are
controversial. The personal
opinion of the author of this RFC
is that this header should be
avoided except in special cases,
but this is a personal opinion
not shared by all specialists in
Used in Usenet News to indicate Followup-To: RFC 1036: 2.2.3, that future discussions (=follow- not standardized up) on an article should go to a for use in e-mail. different set of newsgroups than the replied-to article. The most common usage is when an article is posted to several newsgroups, and further discussions is to take place in only one of them.
In e-mail, this header may occur
in a message which is sent to
both e-mail and Usenet News, to
show where follow-up in Usenet
news is wanted. The header does
not say anything about where
follow-up in e-mail is to be
Note that the value of this
header must always be one or more
newsgroup names, never e-mail
Address to which notifications Errors-To:, Non-standard, are to be sent and a request to Return- discouraged. get delivery notifications. Receipt-To: Internet standards recommend, however, the use of RCPT TO and Return-Path, not Errors-To, for where delivery notifications are to be sent.
Whether non-delivery report is Prevent- RFC 1327, not for wanted at delivery error. Default NonDelivery- general usage. is to want such a report. Report: Whether a delivery report is Generate- RFC 1327, not for wanted at successful delivery. Delivery- general usage. Default is not to generate such a Report: report. Indicates whether the content of Content- RFC 1327, not for a message is to be returned with Return: general usage. non-delivery notifications. Possible future change of name X400-Content- non-standard for "Content-Return:" Return:
Unique ID of this message. Message-ID: RFC 822: 4.6.1 RFC 1036: 2.1.5. Unique ID of one body part of the Content-ID: RFC 1521: 6.1. content of a message. Base to be used for resolving Content-Base: Non-standard relative URIs within this content part. URI with which the content of Content- Non-standard this content part might be Location: retrievable. Reference to message which this In-Reply-To: RFC 822: 4.6.2. message is a reply to. In e-mail: reference to other References: RFC 822: 4.6.3 related messages, in Usenet News: RFC 1036: 2.1.5. reference to replied-to-articles. References to other related See-Also: Son-of-RFC1036 articles in Usenet News. , non-standard Reference to previous message Obsoletes: RFC 1327, not for being corrected and replaced. general usage. Compare to "Supersedes:" below. This field may in the future be replaced with "Supersedes:".
Commonly used in Usenet News in Supersedes: son-of-RFC1036 similar ways to the "Obsoletes" , non-standard header described above. In Usenet News, however, Supersedes causes a full deletion of the replaced article in the server, while "Supersedes" and "Obsoletes" in e- mail is implemented in the client and often does not remove the old version of the text. Only in Usenet News, similar to Article- son-of-RFC1036 "Supersedes:" but does not cause Updates: , non-standard the referenced article to be physically deleted. Reference to specially important Article- son-of-RFC1036 articles for a particular Usenet Names: , non-standard Newsgroup.
Search keys for data base Keywords: RFC 822: 4.7.1 retrieval. RFC 1036: 2.2.9. Title, heading, subject. Often Subject: RFC 822: 4.7.1 used as thread indicator for RFC 1036: 2.1.4. messages replying to or commenting on other messages. Comments on a message. Comments: RFC 822: 4.7.2. Description of a particular body Content- RFC 1521: 6.2. part of a message. Description: Organization to which the sender Organization: RFC 1036: 2.2.8, of this article belongs. not standardized for use in e-mail. See Organization above. Organisation: Non-standard. Short text describing a longer Summary: RFC 1036: 2.2.10, article. Warning: Some mail not standardized systems will not display this for use in e-mail, text to the recipient. Because of discouraged. this, do not use this header for text which you want to ensure that the recipient gets.
A text string which identifies Content- RFC 1327, not for the content of a message. Identifier: general usage.
The time when a message was Delivery- RFC 1327, not for delivered to its recipient. Date: general usage. In Internet, the date when a Date: RFC 822: 5.1, message was written, in X.400, RFC 1123: 5.2.14 the time a message was submitted. RFC 1036: 2.1.2. Some Internet mail systems also use the date when the message was submitted. A suggested expiration date. Can Expires: RFC 1036: 2.2.4, be used both to limit the time of not standardized an article which is not for use in e-mail. meaningful after a certain date, and to extend the storage of important articles. Time at which a message loses its Expiry-Date: RFC 1327, not for validity. This field may in the general usage. future be replaced by "Expires:". Latest time at which a reply is Reply-By: RFC 1327, not for requested (not demanded). general usage.
Can be "normal", "urgent" or "non- Priority: RFC 1327, not for urgent" and can influence general usage. transmission speed and delivery. Sometimes used as a priority Precedence: Non-standard, value which can influence controversial, transmission speed and delivery. discouraged. Common values are "bulk" and "first-class". Other uses is to control automatic replies and to control return-of-content facilities, and to stop mailing list loops.
A hint from the originator to the Importance: RFC 1327 and recipients about how important a RFC 1911, message is. Values: High, normal experimental or low. Not used to control transmission speed. How sensitive it is to disclose Sensitivity: RFC 1327 and this message to other people than RFC 1911, the specified recipients. Values: experimental Personal, private, company confidential. The absence of this header in messages gatewayed from X.400 indicates that the message is not sensitive. Body parts are missing. Incomplete- RFC 1327, not for Copy: general usage.
Can include a code for the Language: RFC 1327, not for natural language used in a general usage. message, e.g. "en" for English. Can include a code for the Content- RFC 1766, proposed natural language used in a Language: standard. message, e.g. "en" for English.
Inserted by certain mailers to Content- Non-standard, indicate the size in bytes of the Length: discouraged. message text. This is part of a format some mailers use when showing a message to its users, and this header should not be used when sending a message through the net. The use of this header in transmission of a message can cause several robustness and interoperability problems. Size of the message. Lines: RFC 1036: 2.2.12, not standardized for use in e-mail.
The body of this message may not Conversion: RFC 1327, not for be converted from one character general usage. set to another. Values: Prohibited and allowed. Non-standard variant of Content- Non-standard. Conversion: with the same values. Conversion: The body of this message may not Conversion- RFC 1327, not for be converted from one character With-Loss: general usage. set to another if information will be lost. Values: Prohibited and allowed.
Format of content (character set Content-Type: RFC 1049, etc.) Note that the values for RFC 1123: 5.2.13, this header are defined in RFC 1521: 4. different ways in RFC 1049 and in RFC 1766: 4.1 MIME (RFC 1521), look for the "MIME-version" header to understand if Content-Type is to be interpreted according to RFC 1049 or according to MIME. The MIME definition should be used in generating mail.
RFC 1766 defines a parameter
"difference" to this header.
Information from the SGML entity Content-SGML- non-standard declaration corresponding to the Entity: entity contained in the body of the body part. Coding method used in a MIME Content- RFC 1521: 5. message body. Transfer- Encoding: Only used with the value Message-Type: RFC 1327, not for "Delivery Report" to indicates general usage. that this is a delivery report gatewayed from X.400.
Used in several different ways by Encoding: RFC 1154, different mail systems. Some use RFC 1505, it for a kind of content-type experimental. information, some for encoding and length information, some for a kind of boundary information, some in other ways.
When manually forwarding a Resent-Reply- RFC 822: C.3.3. message, headers referring to the To:, forwarding, not to the original Resent-From:, message. Note: MIME specifies Resent- another way of resending Sender:, messages, using the "Message" Resent-From:, Content-Type. Resent-Date:, Resent-To:, Resent-cc:, Resent-bcc:, Resent- Message-ID:
Checksum of content to ensure Content-MD5: RFC 1864, proposed that it has not been modified. standard. Used in Usenet News to store Xref: RFC 1036: 2.2.13, information to avoid showing a only in Usenet reader the same article twice if News, not in e- it was sent to more than one mail. newsgroup. Only for local usage within one Usenet News server, should not be sent between servers.
Name of file in which a copy of Fcc: Non-standard. this message is stored. Has been automatically forwarded. Auto- RFC 1327, not for Forwarded: general usage.
Can be used in Internet mail to Discarded- RFC 1327, not for indicate X.400 IPM extensions X400-IPMS- general usage. which could not be mapped to Extensions: Internet mail format. Can be used in Internet mail to Discarded- RFC 1327, not for indicate X.400 MTS extensions X400-MTS- general usage. which could not be mapped to Extensions: Internet mail format. This field is used by some mail Status: Non-standard, delivery systems to indicate the should never status of delivery for this appear in mail in message when stored. Common transit. values of this field are: U message is not downloaded and not deleted. R message is read or downloaded. O message is old but not deleted. D to be deleted. N new (a new message also sometimes is distinguished by not having any "Status:" header.
Combinations of these characters
can occur, such as "Status: OR"
to indicate that a message is
downloaded but not deleted.
Harald Tveit Alvestrand, Ned Freed, Olle Jdrnefors, Keith Moore, Nick Smith and several other people have helped me with compiling this list. I especially thank Ned Freed and Olle Jdrnefors for their thorough review and many helpful suggestions for improvements. I alone take responsibility for any errors which may still be in the list.
An earlier version of this list has been published as part of .
STD 10, RFC 821, August 1982. Recommended
Internet text messages." STD 11, RFC 822, Recommended August 1982.
interchange of USENET messages", RFC 1036, cial IETF December 1987. standard, but in reality a de- facto standard for Usenet News
internet messages", RFC 1049, March 1988. Recommended, but can in the future be expected to be replaced by MIME
Internet Hosts -- Application and Support", Required STD-3, RFC 1123, October 1989.
X.400(1988) / ISO 10021 and RFC 822", RFC standard, 1327 May 1992. elective
Downgrading Messages from X.400/88 to standard, X.400/84 When MIME Content-Types are Present elective in the Messages", RFC 1496, August 1993.
Internet Mail Extensions) Part One: Standard, Mechanisms for Specifying and Describing the elective Format of Internet Message Bodies", RFC 1521, Sept 1993.
of Languages", RFC 1766, February 1995. standard, elective
Protocol: "A Proposed Standard for the Stream- standard Based Transmission of News", RFC 977, January 1986.
S. Murphy, "MIME Object Security Services", standard RFC 1848, March 1995.
Header, RFC 1864, October 1995. standard
standard, RFC 976, Januari 1986. cial IETF standard, but in reality a de- facto standard for Usenet News
Hypertext Transfer Protocol -- HTTP/1.0, IETF standard, RFC 1945, May 1996. but the defacto standard until the next version is published
Transmission, June 1994, RFC, but FTP://zoo.toronto.edu/pub/news.ps still widely FTP://zoo.toronto.edu/pub/news.txt.Z used and partly This document is often referenced under the almost a de- name "son-of-RFC1036". facto standard for Usenet News
Jacob Palme Phone: +46-8-16 16 67 Stockholm University/KTH Fax: +46-8-783 08 29 Electrum 230 E-mail: firstname.lastname@example.org S-164 40 Kista, Sweden
"From " (followed by space, not colon (:")
Not Internet standard
"From " (not followed by ":")
3.8 Expires Extension see Discarded-X400-IPMS-Extensions, Discarded- X400-MTS-Extensions 3.4 Fax 3.16 Fcc 3.4 Followup-To Forwarded, see Auto-Forwarded 3.4 For-Comment 3.4 For-Handling 3.4 From 3.4 Generate-Delivery-Report History, see DL-Expansion-History-Indication ID, see Content-ID and Message-ID Identifier, see Content-ID and Message-ID 3.9 Importance 3.6 In-Reply-To 3.9 Incomplete-Copy 3.7 Keywords 3.10 Language see also Content-Language Length see Content-Length 3.11 Lines 3.4 Mail-System-Version see also X-mailer 3.4 Mailer MD5 see Content-MD5 3.6 Message-ID 3.13 Message-Type 3.3 MIME-Version 3.4 Newsgroups Newsreader, see X-Newsreader 3.6 Obsoletes 3.7 Organisation 3.7 Organization 3.3 Original-Encoded-Information-Types 3.4 Originating-Client 3.2 Path 3.4 Phone 3.9 Precedence 3.4 Prevent-NonDelivery-Report 3.9 Priority 3.2 Received Recipient, see To, cc, bcc, Alternate-Recipient, Disclose- Recipient 3.6 References 3.8 Reply-By 3.4 Reply-To, see also In-Reply-To, References 3.14 Resent- Return see also Content-Return 3.2 Return-Path
3.5 Return-Receipt-To 3.6 See-Also 3.4 Sender 3.9 Sensitivity 3.16 Status 3.7 Subject 3.7 Summary 3.6 Supersedes 3.4 Telefax 3.4 To Transfer-Encoding see Content-Transfer-Encoding Type see Content-Type, Message-Type, Original-Encoded- Information-Types Version, see MIME-Version, X-Mailer 3.4 X400-Content-Return 3.4 X-Mailer see also Mail-System-Version 3.4 X-Newsreader 3.15 Xref