Network Working Group                          Jacob Palme
Internet Draft                                   Stockholm
draft-palme-mailext-headers-06.txt          University/KTH
Category: Informational                             Sweden
Revision of: RFC 2076                  Date: November 2001
                                        Expires: April 2002



                   Common Internet Message Header Fields

                         Status of this Memo

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as
Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time.  It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Copyright (C) The Internet Society 2001. All Rights Reserved.


                             Abstract

This memo contains tables of commonly occurring header
fields in headings of e-mail messages. The document
compiles information from other RFCs such as RFC 822, RFC
1036, RFC 1123, RFC 2156, RFC 1496, RFC 1766, RFC 2183, RFC
1864, RFC 2421 and RFC 2045. A few commonly occurring
header fields which are not defined in RFCs are also
included. For each header field, the memo gives a short
description and a reference to the RFC in which the header
field is defined.

The latest, revised version of this document can be found
at URL
http://www.dsv.su.se/jpalme/ietf/mail-headers/.
The version at that URL may be more recent than the version
published as an RFC.

Another list of headers can be found at URL
http://www.hut.fi/~jkorpela/headers.html [28]


                   Changes since previous version

This document is a revision of RFC 2076. The following new
header fields, not included in RFC 2076, have been added:
Abuse-Reports-To:, Also-Control, Approved-By, Content-
Alias, Content-Alternative, Content-Class, Content-
Conversion, Content-Features, Content-ID, Delivered-To,
Disposition-Notification-Options, Disposition-Notification-
To, Expiry-Date, For-Approval, List-Archive, List-Digest,
List-Help, List-ID, List-Owner, List-Post, List-Software,
List-Subscribe, List-Unsubscribe, List-URL, Mail-Copies-
To:, Original-Recipient, Originator, Originator-Info, Path,
PICS-Label, NNTP-Posting-Host, Posted-To:, Read-Receipt-To,
Received, Registered-Mail-Reply-Requested-By, Replaces,
Return-Receipt-Requested, Speech-Act, Translated-By.
Translation-Of, User-Agent, X-Confirm-Reading-To, X-
Complaints-To:, X-Envelope-From, X-Envelope-To, X-Face, X-
List-Host, X-Listserver, X-Loop, X-MIME-Autoconverted, X-No-
Archive, X-OriginalArrivalTime, X-Priority, X-Report-Abuse-
To, X-Sender, X-X-Sender, X-UIDL, X-URL, X-URI.


                          Table of contents

     Abstract                                              1
     Changes since previous version                        2
1. Introduction                                           3
2. Use of gatewaying header fields                        4
3. Table of header fields                                 5
     **** 3.1 Phrases used in the tables                   5
     **** 3.2 Trace information                            6
     **** 3.3 Format and control information               7
     **** 3.4 Sender and recipient indication              8
     **** 3.5 Response control                            14
     **** 3.6 Message identification and referral
              header fields                               16
     **** 3.7 Other textual header fields                 19
     **** 3.8 Header fields containing dates and
              times                                       20
     **** 3.9 Quality information                         20
     **** 3.10 Language information                       22
     **** 3.11 Size information                           22
     **** 3.12 Conversion control                         22
     **** 3.13 Encoding information                       23
     **** 3.14 Resent-header fields                       24
     **** 3.15 Security and reliability                   25
     **** 3.16 Mailing list control                       25
     **** 3.17 Miscellaneous                              27
4. Acknowledgments                                       28
Copyright and disclaimer                                 29
5. References                                            30
6. Author's address                                      32
Appendix A: Header fields sorted by Internet RFC
             document in which they appear.               33
     RFC 822                                              33
     RFC 976                                              33
     RFC 1049                                             33
     RFC 1036                                             33
     RFC 1123                                             34
     RFC 2156                                             34
     RFC 1505                                             34
     RFC 1766                                             35
     RFC 2183                                             35
     RFC 1864                                             35
     RFC 2045                                             35
     RFC 2110                                             35
     RFC 2298                                             35
     RFC 2369                                             35
     RFC 2421                                             36
     son-of-RFC1036 [21]                                  36
     RFC 2912                                             36
     RFC 2919:                                            36
     World Wide Web Consortium (W3C) Recommendations      36
     Not Internet standard (as of May 2001)               36
Appendix B: Alphabetical index                           38


                          1. Introduction

Many different Internet standards and RFCs define header
fields which may occur on Internet Mail Messages and Usenet
News Articles. The intention of this document is to list
all such header fields in one document as an aid to people
developing message systems or interested in Internet Mail
standards.

The document contains all header fields which the author
has found in the following Internet standards: RFC 822 [2],
RFC 1036 [3], RFC 1123 [5], RFC 2156 [7], RFC 1496 [8], RFC
2045 [11], RFC 1766 [12], RFC 2183 [14], RFC 1864[17] and
RFC 2421[20]. Note in particular that heading attributes
defined in PEM (RFC 1421-1424) and MOSS (RFC 1848 [16]) are
not included. PEM and MOSS header fields only appear inside
the body of a message, and thus are not header fields 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 [1], UUCP [18] and NNTP [15] are
mainly not covered either. Headings used only in HTTP [19]
are not included yet, but may be included in future version
of this memo. Some additional header fields which often can
be found in e-mail headings but are not part of any
Internet standard are also included.

The author does not promise that this document contains a
complete list of all heading fields which are specified in
any standard or used by any mailer.

For each header field, the document gives a short
description and a reference to the Internet standard or
RFC, in which they are defined.

The header field names given here are spelled the same way
as when they are actually used. This is usually American
but sometimes English spelling.  One header field in
particular, "Organisation/Organization", occurs in e-mail
header fields 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 field   One field in the heading, beginning with a
                field name, colon, and followed by the
                field value(s). The words "heading field"
                and "header" are also sometimes used with
                this meaning.

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
http://www.dsv.su.se/jpalme/ietf/mail-headers

Please e-mail me (Jacob Palme <jpalme@dsv.su.se>) if you
have noted header fields which should be included in this
memo but are not.


                  2. Use of gatewaying header fields

RFC 2156 defines a number of new header fields in Internet
mail, which are defined to map header fields which X.400
has but which were previously not standardized in Internet
mail. The fact that a header field occurs in RFC 2156
indicates that it is recommended for use in gatewaying
messages between X.400 and Internet mail, but does not mean
that the header field is recommended for messages wholly
within Internet mail. Some of these header fields may
eventually see widespread implementation and use in
Internet mail, but at the time of this writing (2001) they
are not widely implemented or used.

Header fields 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 header fields are not standardized for use in
Internet e-mail and should be handled with caution by e-
mail agents.


                       3. Table of header fields

**** 3.1 Phrases used in the tables

"not for general    Used to mark header fields which are
usage"              defined in RFC 2156 for use in
                     messages from or to Internet
                     mail/X.400 gateways. These header
                     fields have not been standardized for
                     general usage in the exchange of
                     messages between Internet mail-based
                     systems.

"not standardized   Used to mark header fields defined
for use in e-       only in RFC 1036 for use in Usenet
mail"               News. These header fields 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 field is not specified in
                     any of referenced RFCs which define
                     Internet protocols, including Internet
                     Standards, draft standards or proposed
                     standards. The header field appears
                     here because it often appears in e-
                     mail or Usenet News. Usage of these
                     header fields is not in general
                     recommended. Some header field
                     proposed in ongoing IETF standards
                     development work, but not yet
                     accepted, are also marked in this way.

"discouraged"       This header field, which is non-
                     standard, is known to create problems
                     and should not be generated. Handling
                     of such header fields in incoming mail
                     should be done with great caution.

"controversial"     The meaning and usage of this header
                     field is controversial, i.e. different
                     implementors have chosen to implement
                     the header field in different ways.
                     Because of this, such header fields
                     should be handled with caution and
                     understanding of the different
                     possible interpretations.

"experimental"      This header field is used for newly
                     defined header fields, 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.


**** 3.2 Trace information

Trace of distribution lists   DL-           RFC 2156, not
passed.                       Expansion-    for general
                               History:      usage.

List of MTAs passed.          Path:         RFC 1036:
                                             2.1.6, only in
                                             Usenet News,
                                             not in e-mail.

Trace of MTAs which a         Received:     RFC 822: 4.3.2,
message has passed.                         RFC 1123:
                                             5.2.8.

Used to convey the            Return-       RFC 821,
information from the MAIL     Path:         RFC 1123:
FROM envelope attribute in                  5.2.13.
final delivery, when the
message leaves the SMTP
environment in which "MAIL
FROM" is used.

The netnews host, to which    NNTP-         Non-standard,
this article was originally   Posting-      common in
posted. Useful for finding    Host:         netnews
the sender of spams. Since
this header is added by the
news server, it is a little
more difficult to forge than
other header fields.


**** 3.3 Format and control information

Special Usenet News commands  Also-         son-of-RFC1036
and a normal article at the   Control:      [21], non-
same time.                                  standard, only
                                             in Usenet News,
                                             not in e-mail

Controls whether this         Alternate-    RFC 2156, not
message may be forwarded to   Recipient:    for general
alternate recipients such as                usage.
a postmaster if delivery is
not possible to the intended
recipient. Default: Allowed.

Whether a MIME body part is   Content-      RFC 2183,
to be shown inline or is an   Disposition   experimental
attachment; can also          :
indicate a suggested
filename for use when saving
an attachment to a file.

Can have the values "voice-   Message-      Non-standard
message", "fax-message",      Context:
"pager-message", "multimedia-
message", "text-message",
"none"

Only in Usenet News,          Control:      RFC 1036:
contains commands to be                     2.1.6, only in
performed by News agents.                   Usenet News,
                                             not in e-mail.

Whether recipients are to be  Disclose-     RFC 2156, not
told the names of other       Recipients:   for general
recipients of the same                      usage.
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: header
fields.

An indicator that this        MIME-         RFC 2045: 4.
message is formatted          Version:
according to the MIME
standard, and an indication
of which version of MIME is
utilized.

Which body part types occur   Original-     RFC 2156, not
in this message.              Encoded-      for general
                               Information-  usage.
                               Types:


**** 3.4 Sender and recipient indication

Inserted by Sendmail when     Apparently-   Non-standard,
there is no "To:" recipient   To:           discouraged,
in the original message,                    mentioned in
listing recipients derived                  RFC 1211.
from the 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.

Name of the moderator of the  Approved:     RFC 1036:
newsgroup to which this                     2.2.11, not
article is sent; necessary                  standardized
on an article sent to a                     for use in e-
moderated newsgroup to allow                mail.
its distribution to the
newsgroup members. Also used
on certain control messages,
which are only performed if
they are marked as Approved.

Name of the moderator of a    Approved-     Non-standard,
mailing list, and who has     By:           used by some
approved this message for                   mailing list
distribution to the members                 expansion
of the list.                                systems.

Recipients not to be          bcc:          RFC 822: 4.5.3,
disclosed to other                          RFC 1123:
recipients. (bcc = Blind                    5.2.15-16,
Carbon Copy).                               5.3.7.

Secondary, informational      cc:           RFC 822: 4.5.2,
recipients. (cc = Carbon                    RFC 1123.
Copy)                                       5.2.15-16,
                                             5.3.7.

Geographical or               Distributio   RFC 1036:
organizational limitation on  n:            2.2.7, not
where this article can be                   standardized
distributed. Value can be a                 for use in e-
compete or incomplete domain                mail.
names, also various special
values are accepted like
"world", "usenet", "USA",
etc.

Fax number of the             Fax:,         Non-standard.
originator.                   Telefax:

Primary recipients, who are   For-          Non-standard
requested to approve the      Approval:
information in this message
or its attachments.

Primary recipients, who are   For-          Non-standard
requested to comment on the   Comment:
information in this message
or its attachments.

Primary recipients, who are   For-          Non-standard
requested to handle the       Handling:
information in this message
or its attachments.

(2) Used in Usenet News mail  From          RFC 976: 2.4
transport, to indicate the    or            for use in
path through which an         >From         Usenet News
article has gone when         (not
transferred to a new host.    followed by
                               a colon)
Sometimes called "From_"
header field.

(1) This header field should  From (not     not
never appear in e-mail being  followed by   standardized
sent, and should thus not     a colon)      for use in e-
appear in this memo. It is                  mail
however included, since
people often ask about it.

This header field 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 such files.

This header field 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 field should NOT be
used to find an address to
which replies to a message
are to be sent.

Authors or persons taking     From:         RFC 822: 4.4.1,
responsibility for the                      RFC 1123:
message.                                    5.2.15-16,
                                             5.3.7,
Note difference from the                    RFC 1036 2.1.1
"From " header field (not
followed by ":") below.


Information about the client  Mail-System-  Non-standard.
software of the originator.   Version:,
                               Mailer:,
                               Originating-
                               Client:, X-
                               Mailer, X-
                               Newsreader,
                               X-MimeOLE:,
                               User-Agent:

In Usenet News: group(s) to   Newsgroups:   RFC 1036:
which this article was                      2.1.3, not
posted.                                     standardized
Some systems provide this                   and
header field also in e-mail                 controversial
although it is not                          for use in e-
standardized there.                         mail.

Unfortunately, the header
field can appear in e-mail
with three different and
contradictory meanings:

(a) Indicating the newsgroup
recipient of an
article/message sent to both
e-mail and Usenet News
recipients.

(b) In a message adressed to
some mail to news gateways,
indicates the newsgroup(s)
that the message is to be
posted to.

(c) In a personally
addressed reply to an
article in a news-group,
indicating the newsgroup in
which this discussion
originated.

See also: "Posted-To:".

Sometimes used in Usenet      Originator:   Non-standard in
News in similar ways to                     Usenet News,
"Sender:"                                   Experimental in
                                             RFC 1528.
Also used in printing
protocols.

Contains information about    Originator-   Non-standard
the authentication of the     Info:         [25]
originator in a format which
is not easily used to send
email to, to avoid the
problems with "Sender" and
"X-Sender".

Phone number of the           Phone:        Non-standard.
originator.

The person or agent           Sender:       RFC 822: 4.4.2,
submitting the message to                   RFC 1123:
the network, if other than                  5.2.15-16,
shown by the From: header                   5.3.7, RFC
field. Should be                            1036.
authenticated,
according to RFC 822, but
what
kind of authentication is
not
clear. Some implementations
expect that the e-mail
address used in this field
can be used to reach the
sender, others do not. See
also "X-Sender".

Primary recipients.           To:           RFC 822: 4.5.1,
                                             RFC 1123:
                                             5.2.15-16,
                                             5.3.7.

If the sender in the          X-Envelope-   Non-standard.
envelope (SMTP "RCTP TO") is  From:
not the same as the senders
in the "From" or "Sender"
RFC822 header fields, some
mail servers add this to the
RFC822 header fields as an
aid to clients which would
otherwise not be able to
display this information.

If the recipient in the       X-Envelope-   Non-standard.
envelope (SMTP "MAIL FROM")   To:,
is not included in the CC     Envelope-
list, some mail servers add   To:
this to the RFC822 header
field as an aid to clients
which would otherwise not be
able to display the envelope
recipients.

48x48 bitmap with picture of  X-Face:       Non-Standard
the sender of this message.

Indication in the mail        X-RCPT-TO:    Non-standard
header of recipient on the
SMTP envelope.

Some mail software expect     X-Sender:     Non-standard
"Sender:" to be an e-mail
address which you can send
mail to. However, some mail
software has as the best
authenticated sender a POP
or IMAP account, which you
might not be able to send
to. Because of this, some
mail software put the POP or
IMAP account into an X-
sender header field instead
of a Sender header field, to
indicate that you may not be
able to send e-mail to this
address. See also "X-X-
Sender".

Another use of" X-Sender:"
is that some e-mail
software, which wants to
insert a "Sender:" header,
will first change an
existing "Sender:" header to
"X-Sender". This use is
actually often the same as
that described in the
previous paragraph, since
the new "Sender:" is added
because it is better
authenticated than the old
value.

Even though some systems put  X-X-Sender:   Non-standard
the POP or IMAP account name
into the "X-Sender:" instead
of the Sender header field,
some mail software tries to
send to the "X-Sender:" too.
To stop this, some systems
have begun to use "X-X-
Sender:" to indicate an
authentication of the sender
which might not be useable
to send e-mail to. See also
"Originator-Info:"

When a message is sent both   Posted-To:    Non-standard
to netnews and e-mail, this
header is used in the e-mail
version of the message to
indicate which newsgroup it
was sent to. This header
thus contains the same
information as the
"Newsgroups:" header in the
netnews version of the
message.

E-mail address of             X-Admin:      Non-standard
administrator of a server,
through which this message
was submitted.


**** 3.5 Response control

Indicates whether the         Content-      RFC 2156, not
content of a message is to    Return:       for general
be returned with non-                       usage.
delivery notifications.

For future options on         Disposition-  RFC 2298
disposition notifications.    Notificatio
                               n-Options:


Indicate that the sender      Disposition-  RFC 2298
wants a dispoisition          Notificatio
notification when this        n-To:
message is received (read,
processed, etc.) by its
receipents.

Address to which              Errors-To:,   Non-standard,
notifications are to be sent  Return-       discouraged,
and a request to get          Receipt-      some of them
delivery notifications.       To:,          widely used.
Internet standards            Read-
recommend, however, the use   Receipt-
of MAIL FROM and Return-      To:, X-
Path, not Errors-To, for      Confirm-
where delivery notifications  reading-
are to be sent.               to:, Return-
                               Receipt-
                               Requested,
                               Register-
                               Mail-Reply-
                               Requested-
                               By:

Used in Usenet News to        Followup-     RFC 1036:
indicate that future          To:           2.2.3, not
discussions (=follow-up) on                 standardized
an article should go to a                   for use in e-
different set of newsgroups                 mail.
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 field
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 field
does not say anything about
where follow-up in e-mail is
to be sent.

The value of this header
field should be one or more
newsgroup names.

The special value "poster"
as in "Followup-To: poster"
means that replies are to be
sent as e-mail to the author
only.

Whether a delivery report is  Generate-     RFC 2156, not
wanted at successful          Delivery-     for general
delivery. Default is not to   Report:       usage.
generate such a report.

Original Recipient            Original-     RFC 2298
information for inclusion in  Recipient
disposition notifications.

Whether non-delivery report   Prevent-      RFC 2156, not
is wanted at delivery error.  NonDelivery-  for general
Default is to want such a     Report:       usage.
report.

This header field is meant    Reply-To:     RFC 822: 4.4.3,
to indicate where the sender                RFC 1036: 2.2.1
wants replies to go.                        controversial.
Unfortunately, this is
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    Reply-To2
header field 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 field. These
practices are controversial.
The personal opinion of the
author of this RFC is that
this header field should be
avoided except in special
cases, but this is a
personal opinion not shared
by all specialists in the
area.

Indicates where to send       Abuse-        non-standard
complains if you get a        Reports-
message which you think is    To:, X-
against the laws or rules.    Complaints-
                               To:, X-
                               Report-
                               Abuse-To:

Used in netnews articles to   Mail-Copies-  non-standard,
indicate that followup        To:           but commonly
(=replies) should be sent to                supported by
the indicated e-mail                        newsreaders
address.

Possible future change of     X400-         non-standard
name for "Content-Return:"    Content-
                               Return:


**** 3.6 Message identification and referral header fields

Reference to specially        Article-      son-of-RFC1036
important articles for a      Names:        [21], non-
particular Usenet Newsgroup.                standard

Only in Usenet News, similar  Article-      son-of-RFC1036
to "Supersedes:" but does     Updates:      [21], non-
not cause the referenced                    standard
article to be physically
deleted.

Used in addition to Content-  Content-      Work in
Location if this content      Alias:        progress
part can be retrieved
through more than one URI.
Only one of them is allowed
in the Content-Location, the
other can be specified in
Content-Alias.

Base to be used for           Content-      RFC 2110
resolving relative URIs       Base:
within this content part.

Unique ID of one body part    Content-ID:   RFC 2045: 7.
of the content of a message.

URI with which the content    Content-      RFC 2110
of this content part might    Location:
be retrievable.

Used by some automatic        Delivered-    non-standard
services (mainly MLMs and     To:
autoresponders) for the       or
purpose of loop detection.    X-Loop:
The service adds the
Delivered-To header to
outgoing messages, with its
e-mail address as a value,
and discards incoming
messages which already have
it.

Reference to message which    In-Reply-     RFC 822: 4.6.2.
this message is a reply to.   To:

Unique ID of this message.    Message-ID:   RFC 822: 4.6.1
                                             RFC 1036:
                                             2.1.5.

Reference to previous         Obsoletes:    RFC 2156, not
message being corrected and                 for general
replaced. Compare to                        usage.
"Supersedes:" below. This
field may in the future be
replaced with "Supersedes:".

In e-mail: reference to       References:   RFC 822: 4.6.3
other related messages, in                  RFC 1036:
Usenet News: reference to                   2.1.5.
replied-to-articles.

Still another name for        Replaces:     non-standard,
similar functionality as for                proposed in
"Obsoletes:" and                            IETF USEFOR
"Supersedes:". This may                     working group
become the most recommended
header in the future, but is
still under discussion in
IETF standards development
work.

References to other related   See-Also:     Son-of-RFC1036
articles in Usenet News.                    [21], non-
                                             standard

Commonly used in Usenet News  Supersedes:   son-of-RFC1036
in similar ways to the                      [21], non-
"Obsoletes" header field                    standard
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.

Mailbox of the person who     Translated-   non-standard
made the translation.         By:

Reference to the Message-ID   Translation-  non-standard
of a message, which the       Of:
current message is a
translation of.

Unique identifier for a       X-UIDL:       non-standard
message, local to a
particular local mailbox
store. The UIDL identifier
is defined in the POP3
standard, but not the "X-
UIDL:" header.

Similar usage as "X-URL".     X-URI:        Non-standard
The URI can be either a URL
or a URN. URNs are meant to
become more persistent
references to resources than
URLs.

Sometimes used with the same  X-URL:        Non-standard
meaning as "Content-
Location:", sometimes to
indicate the web home page
of the sender or of his
organisation.

The UID, as defined in the    X-IMAP:       Non-standard
IMAP standard. Only used in
internal mailbox storage in
some mail systems, should
never be visible to a user.


**** 3.7 Other textual header fields

Comments on a message.        Comments:     RFC 822: 4.7.2.

Description of a particular   Content-      RFC 2045: 8.
body part of a message, for   Description
example a caption for an      :
image body part.

A text string which           Content-      RFC 2156, not
identifies the content of a   Identifier:   for general
message.                                    usage.

Search keys for data base     Keywords:     RFC 822: 4.7.1
retrieval.                                  RFC 1036:
                                             2.2.9.

See Organization above.       Organisatio   Non-standard.
                               n:

Organization to which the     Organizatio   RFC 1036:
sender of this article        n:            2.2.8, not
belongs.                                    standardized
                                             for use in e-
                                             mail.

Title, heading, subject.      Subject:      RFC 822: 4.7.1
Often used as thread                        RFC 1036:
indicator for messages                      2.1.4.
replying to or commenting on
other messages.

Short text describing a       Summary:      RFC 1036:
longer article. Warning:                    2.2.10, not
Some mail systems will not                  standardized
display this text to the                    for use in e-
recipient. Because of this,                 mail,
do not use this header field                discouraged.
for text which you want to
ensure that the recipient
gets.


**** 3.8 Header fields containing dates and times

In Internet, the date when a  Date:         RFC 822: 5.1,
message was written, in                     RFC 1123:
X.400, the time a message                   5.2.14
was submitted. Some Internet                RFC 1036:
mail systems also use the                   2.1.2.
date when the message was
submitted.

The time when a message was   Delivery-     RFC 2156, not
delivered to its recipient.   Date:         for general
                                             usage.

A suggested expiration date.  Expires:      RFC 1036:
Can be used both to limit                   2.2.4, not
the time of an article which                standardized
is not meaningful after a                   for use in e-
certain date, and to extend                 mail.
the storage of important
articles.

Time at which a message       Expiry-       RFC 2156, not
loses its validity. This      Date:         for general
field may in the future be                  usage.
replaced by "Expires:".

Latest time at which a reply  Reply-By:     RFC 2156, not
is requested (not demanded).                for general
                                             usage.

Time when this message was    X-OriginalA   Non-standard
delivered into the message    rrivalTime:
transport system (usually
the same time as in the last
"Received:" header)


**** 3.9 Quality information

A hint from the originator    Importance:   RFC 2156 and
to the recipients about how                 RFC 2421,
important a message is.                     proposed
Values: High, normal or low.
Not used to control
transmission speed.

Body parts are missing.       Incomplete-   RFC 2156, not
                               Copy:         for general
                                             usage.

Ratings label to control      PICS-Label:   REC-PICS-
selection (filtering) of                    labels, W3C
messages according to the                   document [23].
PICS protocol.

Sometimes used as a priority  Precedence:   Non-standard,
value which can influence                   controversial,
transmission speed and                      widely used.
delivery. 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.

Can be "normal", "urgent" or  Priority:     RFC 2156, not
"non-urgent" and can                        for general
influence transmission speed                usage.
and delivery.

How sensitive it is to        Sensitivity   RFC 2156 and
disclose this message to      :             RFC 2421,
other people than the                       proposed
specified recipients.
Values: Personal, private,
company confidential. The
absence of this header field
in messages gatewayed from
X.400 indicates that the
message is not sensitive.

Yet another priority          X-MSMail-     Non-standard
indication.                   Priority:

Values: 1 (Highest), 2        X-Priority:   Non-standard
(High), 3 (Normal), 4 (Low),                [24]
5 (Lowest). 3 (Normal) is
default if the field is
omitted.


**** 3.10 Language information

Can include a code for the    Content-      RFC 1766,
natural language used in a    Language:     proposed
message, e.g. "en" for                      standard.
English.

Can include a code for the    Language:     RFC 2156, not
natural language used in a                  for general
message, e.g. "en" for                      usage.
English.


**** 3.11 Size information

Inserted by certain mailers   Content-      Non-standard,
to indicate the size in       Length:       discouraged.
bytes of the message text.
This is part of a format
some mailers use when
showing a message to its
users, and this header field
should not be used when
sending a message through
the net. The use of this
header field 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.


**** 3.12 Conversion control

Information on where an       Content-      Non-standard
alternative variant of this   Alternative   [27].
document might be found.      :

Non-standard variant of       Content-      Non-standard.
Conversion: with the same     Conversion:
values.

The body of this message may  Conversion:   RFC 2156, not
not be converted from one                   for general
character set to another.                   usage.
Values: Prohibited and
allowed.

The body of this message may  Conversion-   RFC 2156, not
not be converted from one     With-Loss:    for general
character set to another if                 usage.
information will be lost.
Values: Prohibited and
allowed.


**** 3.13 Encoding information

Type information of the       Content-      non-standard
content in some class         Class:
hierarchy. Class hierarchies
are commonly used to
classify data structures in
software development.

Can give more detailed        Content-      Proposed
information about the         Features:     Standard, RFC
Content-Type. Example:                      2912

Content-features:
   (& (Type="image/tiff")
   (color=Binary)
   (image-file-structure=TIFF-
S)
   (dpi=200)
   (dpi-xyratio=200/100)
   (paper-size=A4)
   (image-coding=MH) (MRC-
mode=0)
   (ua-media=stationery) )

This header is meant to be
used when you can choose
between different versions of
a resource, such as when
using multipart/atlernative.

Information from the SGML     Content-      non-standard
entity declaration            SGML-
corresponding to the entity   Entity:
contained in the body of the
body part.

Coding method used in a MIME  Content-      RFC 2045: 6.
message body.                 Transfer-
                               Encoding:

Format of content (character  Content-      RFC 1049,
set etc.) Note that the       Type:         RFC 1123:
values for this header field                5.2.13,
are defined in different                    RFC 1766: 4.1
ways in RFC 1049 and in MIME                RFC 2045: 5.
(RFC 2045), look for the
"MIME-version" header field
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 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.

Used in several different     Encoding:     RFC 1154,
ways by different mail                      RFC 1505,
systems. Some use it for a                  experimental.
kind of content-type
information, some for
encoding and length
information, some for a kind
of boundary information,
some in other ways.

Only used with the value      Message-      RFC 2156, not
"Delivery Report" to          Type:         for general
indicates that this is a                    usage.
delivery report gatewayed
from X.400.

Information about conversion  X-MIME-       non-standard
of this message on the path   Autoconverte
from sender to recipient,     d:
like conversion between MIME
encoding formats. Note: Auto-
conversion may invalidate
digital seals and
signatures.


**** 3.14 Resent-header fields

When manually forwarding a    Resent-       RFC 822: C.3.3.
message, header fields        Reply-To:,
referring to the forwarding,  Resent-
not to the original message.  From:,
Note: MIME specifies another  Resent-
way of resending messages,    Sender:,
using the "Message" Content-  Resent-
Type.                         From:,
                               Resent-
                               Date:,
                               Resent-To:,
                               Resent-cc:,
                               Resent-
                               bcc:,
                               Resent-
                               Message-ID:


**** 3.15 Security and reliability

Checksum of content to        Content-      RFC 1864,
ensure that it has not been   MD5:          proposed
modified.                                   standard.

Used in Usenet News to store  Xref:         RFC 1036:
information to avoid showing                2.2.13, only in
a reader the same article                   Usenet News,
twice if it was sent to more                not in e-mail.
than one newsgroup. Only for
local usage within one
Usenet News server, should
not be sent between servers.


**** 3.16 Mailing list control

Contains URL to use to        List-         RFC 2369 [26]
browse the archives of the    Archive:
mailing list from which this
message was relayed.

URL to use to get a           List-         Non-standard
subscription to the digest    Digest:
version of the mailing list
from which this message was
relayed.

Contains URL to use to get a  List-Help:    RFC 2369 [26]
information about the
mailing list from which this
message was relayed.

Stores an identification of   List-ID:      RFC 2919 [27].
the mailing list, through
which this message was
distributed.

Non-standard precursors to    Mailing-      Non-standard
List-ID and List-Post.        List:, X-
                               Mailing-
                               List:

Contains URL to send e-mail   List-Owner:   RFC 2369 [26]
to the owner of the mailing
list from which this message
was relayed.

Contains URL to use to send   List-Post:    RFC 2369 [26]
contributions to the mailing
list from which this message
was relayed.

Information about the         List-         Non-standard,
software used in a mailing    Software:     has been
list expander through which                 considered for
this message has passed.                    inclusion in
                                             [26].
Contains URL to use to get a  List-         RFC 2369 [26]
subscription to the mailing   Subscribe:
list from which this message
was relayed.

Contains URL to use to        List-         RFC 2369 [26]
unsubscribe the mailing list  Unsubscribe
from which this message was   :
relayed.

Contains URL where            List-URL:     Non-standard
information of various kinds
about the mailing list from
which this message was
relayed.

Information about the server  X-            Non-standard.
and software used in a        Listserver:   Recommended to
mailing list expander         , X-List-     use "List-
through which this message    Host:         Software"
has passed. Warning:                        instead.
"Listserv" is a trademark
and should not be used for
other than the "Listserv"
product. Use, instead the
"List-Software" header
field.


**** 3.17 Miscellaneous

Has been automatically        Autoforward   RFC 2156, not
forwarded.                    ed:           for general
                                             usage.

Can be used in Internet mail  Discarded-    RFC 2156, not
to indicate X.400 IPM         X400-IPMS-    for general
extensions which could not    Extensions:   usage.
be mapped to Internet mail
format.

Can be used in Internet mail  Discarded-    RFC 2156, not
to indicate X.400 MTS         X400-MTS-     for general
extensions which could not    Extensions:   usage.
be mapped to Internet mail         
format.

Name of file in which a copy  Fcc:          Non-standard.
of this message is stored.

Speech act categoriztion of   Speech-Act:   Non-standard
a message, examples of
speeach acts are Question,
Idea, More, Promise, Sad,
Happy, Angry, summary,
Decision
This field is used by some    Status:       Non-standard,
mail delivery systems to                    should never
indicate the status of                      appear in mail
delivery for this message                   in transit.
when stored. Common 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 field.

Combinations of these
characters can occur, such
as "Status: OR" to indicate
that a message is downloaded
but not deleted.

Do not archive this message   X-No-         Non-standard
in publicly available         Archive:
archives.                     Yes



                           4. Acknowledgments

Harald Tveit Alvestrand, Neil Carpenter, William C.
Carpenter, Rob Chandhok, Ned Freed, Olle Järnefors, Jukka
Korpela, Usi Paz, Martin Platt, Keith Moore, Robert A.
Rosenberg, Mark Symons, Nick Smith Michael C. Tiernan and
several other people have helped me with compiling this
list. I especially thank Ned Freed and Olle Järnefors 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 [13].


                  Copyright and disclaimer

The IETF takes no position regarding the validity
or scope of any intellectual property 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; neither does it represent that it has
made any effort to identify any such rights.
Information on the IETF's procedures with respect
to rights in standards-track and standards-
related documentation can be found in BCP-11.
Copies of claims of rights made available for
publication 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 implementors or
users of this specification can be obtained from
the IETF Secretariat."

The IETF invites any interested party to bring to
its attention any copyrights, patents or patent
applications, or other proprietary rights which
may cover technology that may be required to
practice this standard. Please address the
information to the IETF Executive Director.

Copyright (C) The Internet Society (date). All
Rights Reserved.

This document and translations of it may be
copied and furnished to others, and derivative
works that comment on or otherwise explain it or
assist in its implmentation may be prepared,
copied, published and distributed, in whole or in
part, without restriction of any kind, provided
that the above copyright notice and this
paragraph are included on all such copies and
derivative works. However, this document itself
may not be modified in any way, such as by
removing the copyright notice or references to
the Internet Society or other Internet
organizations, except as needed for the purpose
of developing Internet standards in which case
the procedures for copyrights defined in the
Internet Standards process must be followed, or
as required to translate it into languages other
than English.

The limited permissions granted above are
perpetual and will not be revoked by the Internet
Society or its successors or assigns.


                             5. References

Ref.   Author, title                            IETF
                                                 status
----   -------------------------------------    (Dec 2000)
-      --------                                 ----------
                                                 -
[1]    J. Klensin: "Simple Mail Transfer        Proposed
        Protocol", RFC 2821, April 2001.         Standard

[2]    P. Resnick: "Internet Message Format"    Proposed
        STD 11, RFC 2822, April 2001.            Standard

[3]    M.R. Horton, R. Adams: "Standard for     Not an
        interchange of USENET messages", RFC     offi-cial
        1036, December 1987.                     IETF
                                                 standard,
                                                 but in
                                                 reality a
                                                 de-facto
                                                 standard
                                                 for Usenet
                                                 News

[4]    M. Sirbu: "A Content-Type header         Historic
        field header field for internet
        messages", RFC 1049, March 1988.

[5]    R. Braden (editor): "Requirements for    Standard,
        Internet Hosts -- Application and        Required
        Support", STD-3, RFC 1123, October
        1989.

[6]    D. Robinson, R. Ullman: "Encoding        Non-
        Header field for Internet Messages",     standard
        RFC 1505, August 1993.

[7]    S. Hardcastle-Kille: "Mapping between    Proposed
        X.400(1988) / ISO 10021 and RFC 822",    standard,
        RFC 2156 January 1998.                   elective

[8]    H. Alvestrand & J. Romaguera: "Rules     Proposed
        for Downgrading Messages from            standard,
        X.400/88 to X.400/84 When MIME           elective
        Content-Types are Present in the
        Messages", RFC 1496, August 1993.

[9]    A. Costanzo: "Encoding Header field      Non-
        Header field for Internet Messages",     standard
        RFC 1154, April 1990.

[10]   A. Costanzo, D. Robinson: "Encoding      Experiment
        Header field Header field for            al
        Internet Messages", RFC 1505, August
        1993.

[11]   N. Freed & N. Borenstein: "MIME          Draft
        (Multipurpose Internet Mail              Standard,
        Extensions) Part One: Format of          elective
        Internet Message Bodies. RFC 2045.
        November 1996.

[12]   H. Alvestrand: "Tags for the             Best
        Identification of Languages", RFC        Current
        3066, January 2001.                      Practice,
                                                 elective

[13]   J. Palme: "Electronic Mail", Artech      Non-
        House publishers, London-Boston          standard
        January 1995.

[14]   R. Troost, S. Dorner: "Communicating     Experimental
        Presentation Information in Internet
        Messages: The Content-Disposition
        Header field", RFC 2183, June 1995.

[15]   B. Kantor, P. Lapsley, "Network News     Proposed
        Transfer Protocol: "A Proposed           standard
        Standard for the Stream-Based
        Transmission of News", RFC 977,
        January 1986.
[16]   1848  PS   S. Crocker, N. Freed, J.      Proposed
        Galvin, S. Murphy, "MIME Object          standard
        Security Services", RFC 1848, March
        1995.

[17]   J. Myers, M. Rose: The Content-MD5       Draft
        Header field Header field, RFC 1864,     standard
        October 1995.

[18]   M. Horton, UUCP mail interchange         Not an
        format standard, RFC 976, Januari        offi-cial
        1986.                                    IETF
                                                 standard,
                                                 but in
                                                 reality a
                                                 de-facto
                                                 standard
                                                 for Usenet
                                                 News

[19]   R. Fielding, J. Gettys, J. Mogul, H.     Draft
        Frystyk, L. Masinter, P. Leach, T.       standard
        Berners-Lee: Hypertext Transfer
        Protocol -- HTTP/1.1, June 1999.
[20]   G. Vaudreuil: Voice Profile for          Proposed
        Internet Mail, RFC 2421 Feburary
        1998.

[21]   H. Spencer: News Article Format and      Not even an
        Transmission, June 1994,                 RFC, but
        FTP://zoo.toronto.edu/pub/news.ps.Z      still
        FTP://zoo.toronto.edu/pub/news.txt.Z     widely used
                                                 and partly
        This document is often referenced        almost a de-
        under the name "son-of-RFC1036".         facto
                                                 standard
                                                 for Usenet
                                                 News

[23]   PICS Label Distribution Label Syntax     Other
        and Communication Protocols, World       standard
        Wide Web Consortium, October 1996.

[24]   Eudora Pro Macintosh User Manual,        Non-
        Qualcomm Inc., 1988-1995.                standard

[25]   C. Newman: Originator-Info Message       Non-
        Header field. work in progress, July     standard
        1997.

[26]   Grant Neufeld and Joshua D. Baer: The    Proposed
        Use of URLs as Meta-Syntax for Core      standard
        Mail List Commands and their
        Transport through Message Header
        fields, RFC 2369, July 1998.

[27]   G. Klyne (ed.): Content Negotiation      Non-
        for Facsimile Using Internet Mail,       standard
        Work in progress, March 2000.

[27]   R. Chandhok, G. Wenger: List-IDE: A      Proposed
        Structured Field and Namespace for       standard
        the Identification if Mailing Lists,
        RFC 2919, March 2001.

[28]   Jukka "Yucca" Korpela: Quick             Non-
        reference to Internet message            standard
        headers,
        http://www.cs.tut.fi/~jkorpela/header
        s.html, October 2001.


                           6. Author's address

Jacob Palme                Phone: +46-8-16 16 67
Stockholm University/KTH   Fax: +46-8-783 08 29
Electrum 230               E-mail: jpalme@dsv.su.se
S-164 40 Kista, Sweden


                              Appendix A:
Header fields sorted by Internet RFC document in which
they appear.

RFC 822
-------

bcc
cc
Comments
Date
From
In-Reply-To
Keywords
Message-ID
Received
References
Reply-To
Resent-
Resent-bcc
Resent-cc
Resent-Date
Resent-From
Resent-From
Resent-Message-ID
Resent-Reply-To
Resent-Sender
Resent-To
Return-Path
Sender
Subject
To

RFC 976
-------

"From " (followed by space, not colon (:")

RFC 1049
--------

Content-Type

RFC 1036
--------

Approved
Control
Distribution
Expires
Followup-To
Lines
Newsgroups
Organization
Path
Summary
Xref

RFC 1123
--------

Content-Type

RFC 2156
--------

Alternate-recipient
Auto-forwarded see Autoforwarded
Autoforwarded
Content-Identifier
Content-Return
Conversion
Conversion-With-Loss
Delivery-Date
Discarded-X400-IPMS-Extensions
Discarded-X400-MTS-Extensions
Disclose-Recipients
DL-Expansion-History
Expiry-Date
Generate-Delivery-Report
Importance
Incomplete-Copy
Language
Message-Type
Obsoletes
Original-Encoded-Information-Types
Prevent-NonDelivery-Report
Priority
Reply-By
Sensitivity

RFC 1505
--------

Encoding

RFC 1766
--------

Content-Language

RFC 2183
--------

Content-Disposition

RFC 1864
--------

Content-MD5

RFC 2045
--------

Content-Description
Content-ID
Content-Transfer-Encoding
Content-Type
MIME-Version

RFC 2110
--------

Content-Base
Content-Location

RFC 2298
--------

Disposition-Notification-To
Disposition-Notification-Options
Original-Recipient

RFC 2369
--------

List-Archive
List-Help
List-Owner
List-Post
List-Software
List-Subscribe
List-Unsubscribe

RFC 2421
--------

Importance
Sensitivity

son-of-RFC1036 [21]
-------------------

Also-Control
Article-Names
Article-Updates
See-Also
Supersedes

RFC 2912
--------

Content-Features

RFC 2919:
--------

List-ID

World Wide Web Consortium (W3C) Recommendations
-----------------------------------------------

Pics-Label

Not Internet standard (as of May 2001)
-------------------------------------------

"From " (not followed by ":")
Abouse-Reports-To
Apparently-To
Approved-By
Content-Alias
Content-Alternative
Content-Class
Content-Conversion
Content-Length
Content-SGML-Entity
Delivered-To
Encoding
Errors-To
Fax
Fcc
For-Approval
For-Comment
For-Handling
List-Digest
List-URL
Mailing-List
Mail-Copies-To
Mail-System-Version
Mailer
Message-Context
NNTP-Posting-Host
Organisation
Originating-Client
Originator
Originator-Info
Phone
Posted-To
Precedence
Registered-Mail-Reply-Requested-By
Replaces
Return-Receipt-Requested
Return-Receipt-To
Read-Receipt-To
Speech-Act
Status
Supersedes
Telefax
Translated-By
Translation-Of
User-Agent
X-Admin
X-Confirm-Reading-To
X-Complaints-To
X-Envelope-From
X-Envelope-To
X-Face
X-IMAP
X-Loop
X-List-Host
X-Listserver
X-Mailer
X-Mailing-List
X-MIME-Autoconverted
X-MIMEOLE
X-MSMail-Priority
X-Newsreader
X-No-Archive
X-OriginalArrivalTime
X-Priority
X-RCPT-TO
X-Report-Abuse-To
X-Sender
X-UIDL
X-URI
X-URL
X-X-Sender
X400-Content-Return

                    Appendix B: Alphabetical index

Sectio Header field
n      ------------
------
-

3.5    Abuse-Reports-To
3.3    Also-Control
3.3    Alternate-Recipient
3.4    Apparently-To
3.4    Approved
3.4    Approved-By
3.6    Article-Names
3.6    Article-Updates
        Auto-Forwarded see Autoforwarded
3.17   Autoforwarded
3.4    bcc
3.4    cc
        Client, see Originating-Client
        Comment, see For-Comment
3.7    Comments
3.6    Content-Alias
3.12   Content-Alternative
3.6    Content-Base
3.13   Content-Class
3.12   Content-Conversion
3.7    Content-Description
3.3    Content-Disposition
3.13   Content-Features
3.6    Content-ID
3.7    Content-Identifier
3.10   Content-Language see also Language
3.11   Content-Length
3.6    Content-Location
3.15   Content-MD5
3.4    Content-Return
3.13   Content-SGML-Entity
3.13   Content-Transfer-Encoding
3.13   Content-Type
3.3    Control
3.12   Conversion
3.12   Conversion-With-Loss
        Copy, see Incomplete-Copy
3.8    Date, see also Delivery-Date, Received, Expires,
        Expiry-Date
3.6    Delivered-To
3.8    Delivery-Date
        Delivery-Report, see Generate-Delivery-Report,
        Prevent-Delivery-Report, Non-Delivery-Report,
        Content-Type
        Description, see Content-Description
3.17   Discarded-X400-IPMS-Extensions
3.17   Discarded-X400-MTS-Extensions
3.3    Disclose-Recipients
        Disposition, see also Content-Disposition
3.5    Disposition-Notification-Options
3.5    Disposition-Notification-To
3.4    Distribution
3.2    DL-Expansion-History
3.13   Encoding see also Content-Transfer-Encoding
3.4    Errors-To
3.8    Expires
3.8    Expiry-Date
        Extension see Discarded-X400-IPMS-Extensions,
        Discarded-X400-MTS-Extensions
3.4    Fax see also Telefax
3.17   Fcc
3.4    Followup-To
3.4    For-Approval
3.4    For-Comment
3.4    For-Handling
        Forwarded, see Autoforwarded
3.4    From (not followed by (":" or preceded by ">")
3.4    From (followed by ":")
3.4    Generate-Delivery-Report
        Handling, see For-Handling
        History, see DL-Expansion-History
        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
        Label, see PICS-Label
3.10   Language see also Content-Language
        Length see Content-Length
3.11   Lines
3.16   List-Archive
3.16   List-Digest
3.16   List-Help
3.16   List-ID
3.16   List-Owner
3.16   List-Post
3.16   List-Software
3.16   List-Subscribe
3.16   List-URL
3.16   List-Unsubscribe
        Loss, see Conversion-With-Loss
3.16   Mailing-List, see also X-Mailing-List
3.5    Mail-Copies-To
3.4    Mail-System-Version see also X-mailer
3.4    Mailer
        MD5 see Content-MD5
3.3    Message-Context
3.6    Message-ID
3.13   Message-Type
3.3    MIME-Version
3.4    Newsgroups
        Newsreader, see X-Newsreader
3.3    NNTP-Posting-Host
3.6    Obsoletes
3.7    Organisation
3.7    Organization
3.3    Original-Encoded-Information-Types
3.6    Original-Recipient
3.4    Originating-Client
3.4    Originator
3.4    Originator-Info see also Sender
3.2    Path
3.4    Phone
3.9    PICS-Label
3.4    Posted-To
3.9    Precedence
3.4    Prevent-NonDelivery-Report
3.9    Priority
3.5    Read-Reciept-To
3.2    Received
        Recipient, see To, cc, bcc, Alternate-Recipient,
        Disclose-Recipients
3.6    References
3.5    Registered-Mail-Reply-Requested-By
3.6    Replaces
3.8    Reply-By
3.4    Reply-To, see also In-Reply-To, References
3.14   Resent-
        Return see Content-Return
3.2    Return-Path
3.5    Return-Receipt-Requested
3.5    Return-Receipt-To
3.6    See-Also
3.4    Sender
3.9    Sensitivity
3.17   Speech-Act
3.17   Status
3.7    Subject
3.7    Summary
3.6    Supersedes
3.4    Telefax see also Fax
3.4    To
        Transfer-Encoding see Content-Transfer-Encoding
3.6    Translated-By
3.6    Translation-Of
        Type see Content-Type, Message-Type, Original-
        Encoded-Information-Types
3.4    User-Agent
        Version, see MIME-Version, X-Mailer
3.4    X-Admin
3.4    X-Complaints-To
3.5    X-Confirm-Reading-To
3.4    X-Envelope-From
3.4    X-Envelope-To
3.4    X-Face
3.6    X-IMAP
3.16   X-List-Host
3.16   X-Listserver
3.6    X-Loop
3.16   X-Mailing-List, see also Mailing-List
3.4    X-Mailer see also Mail-System-Version
3.13   X-MIME-Autoconverted
3.4    X-MimeOLE
3.9    X-MSMail-Priority
3.4    X-Newsreader
3.17   X-No-Archive
3.8    X-OriginalArrivaltime
3.9    X-Priority
3.4    X-Report-Abuse-To
3.4    X-RCPT-TO
3.4    X-Sender see also Originator-Info
3.6    X-UIDL
3.6    X-URI
3.6    X-URL see also Content-Location
3.4    X-X-Sender see also Originator-Info
3.4    X400-Content-Return
3.15   Xref

