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Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6854.
Author Barry Leiba
Last updated 2015-10-14 (Latest revision 2012-12-05)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 6854 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Pete Resnick
Send notices to (None)
EAI Working Group                                               B. Leiba
Internet-Draft                                       Huawei Technologies
Updates: 5322 (if approved)                             December 5, 2012
Intended status: Standards Track
Expires: June 8, 2013

 Update to Internet Message Format to Allow Group Syntax in the "From:"
                      and "Sender:" Header Fields


   The Internet Message Format (RFC 5322) allows "group" syntax in some
   email header fields, such as "To:" and "CC:", but not in "From:" nor
   "Sender:".  This document updates RFC 5322 to relax that restriction,
   allowing group syntax in those latter fields, as well as in "Resent-
   From:" and "Resent-Sender:", in certain situations.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on June 8, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.    Notational Conventions . . . . . . . . . . . . . . . . . .  3
   1.1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  4
   1.1.2.  Syntactic Notation . . . . . . . . . . . . . . . . . . . .  4

   2.      Allowing Group Syntax in "From:" and "Sender:" . . . . . .  4
   2.1.    Replacement of RFC 5322, Section 3.6.2. Originator
           Fields . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2.    Update to RFC 5322, Section 3.6.6. Resent Fields . . . . .  6

   3.      Applicability Statement  . . . . . . . . . . . . . . . . .  6

   4.      Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6

   5.      Security Considerations  . . . . . . . . . . . . . . . . .  7

   6.      IANA Considerations  . . . . . . . . . . . . . . . . . . .  8

   7.      References . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.1.    Normative References . . . . . . . . . . . . . . . . . . .  9
   7.2.    Informative References . . . . . . . . . . . . . . . . . . 10

           Author's Address . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   The Internet Message Format, as far back as RFC 822 [RFC0822], has
   always required a usable address to appear in the "From:" header
   field of messages in order to allow replies to be sent.  To this end,
   the syntax of messages, up to and including the current specification
   [RFC5322], has required the use of the mailbox address form in the
   originator ("From:" and "Sender:") fields of messages and has
   specifically forbidden the use of the group address form, which
   permits an empty list of addresses (that is, an address list with no
   address included that might be used for a reply).

   However, the use cases for the "From:" field have evolved.  There are
   numerous instances of automated systems that wish to send email but
   cannot handle replies, and a "From:" field with no usable addresses
   would be extremely useful for that purpose.  More recently, work with
   internationalized email addresses [RFC6530] creates the real need to
   take a message with an internationalized email address and hand it to
   an older client that would have no ability to reply to such an
   address but might still wish to display the contents of the message.
   The group construct provides an existing syntax for unusable
   addresses (using the empty list of addresses) and also allows for a
   text label that describes the originator.  For example:

      From: Automated System:;

   A review of many current email programs finds that all reviewed
   clients will properly display a message with group syntax in the
   "From:" field.  At worst, such programs generate an error message
   when an attempt is made to reply to such a message.  No other
   interoperability problems have been discovered.

   This document therefore updates the Internet Message Format
   specification [RFC5322] to relax that restriction, allowing group
   syntax to be used in the originator ("From:" and "Sender:") fields,
   as well as in their corresponding resent ("Resent-From:" and "Resent-
   Sender:") fields.  This change permits empty groups, as described
   above, and also permits named groups of mailboxes (groups with non-
   empty lists of addresses; see Section 4).  Nevertheless, this
   document recommends against the general use of group syntax in these
   fields at this time (see Section 3).

1.1.  Notational Conventions

   The notational conventions here are the same as those in RFC 5322,
   and the following two subsections are copied directly from that

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1.1.1.  Requirements Notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
   NOT", and "MAY" appear capitalized, they are being used to indicate
   particular requirements of this specification.  A discussion of the
   meanings of these terms appears in [RFC2119].

1.1.2.  Syntactic Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation for the formal definitions of the syntax of
   messages.  Characters will be specified either by a decimal value
   (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
   a case-insensitive literal value enclosed in quotation marks (e.g.,
   "A" for either uppercase or lowercase A).

2.  Allowing Group Syntax in "From:" and "Sender:"

   Section 3.6.2 of RFC 5322 defines the "From:" header field as
   containing a <mailbox-list> syntax element.  This specification
   changes that definition to use the <address-list> syntax element, as
   is used in other fields, such as "To:", "CC:", and "Reply-To:".  This
   specification also changes the definition of the "Sender:" header
   field from the <mailbox> syntax element to the <address> syntax
   element.  While the <address> element includes the <mailbox> element
   already, we have chosen to specify both in the updated syntax as a
   way of highlighting the limited use intended for the change (see
   Section 3).

   Section 2.1 below is a full replacement for Section 3.6.2 of RFC
   5322, containing the new syntax as well as a new description of the
   semantics for the "From:" and "Sender:" fields.  Section 2.2 below is
   a replacement of only the ABNF syntax for the "Resent-From:" and
   "Resent-Sender:" fields in section 3.6.6 of RFC 5322; the rest of the
   syntax as well as the descriptive text of section 3.6.6 of RFC 5322
   remains unchanged.

   [The text in the following section is not consistent within itself
   nor with the rest of this document in how it refers to message header
   fields, sometimes putting the field name in quotation marks and
   sometimes not, sometimes capitalizing the field name and sometimes
   not, and sometimes including the final colon and sometimes not.
   Because minimizing changes to the original text is more important, in
   this case, than attaining consistency, the text in Section 2.1, as
   well as that in Section 1.1.1 and Section 1.1.2 above, is left as it
   was in RFC 5322.]

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   [[RFC Editor (please remove this paragraph before publication):
   Please, therefore, hold back edits to Section 1.1.1, Section 1.1.2,
   and Section 2.1.  If you think there are editorial changes that you
   must make, let's please discuss them explicitly during AUTH48.]]

2.1.  Replacement of RFC 5322, Section 3.6.2. Originator Fields

   The originator fields of a message consist of the from field, the
   sender field (when applicable), and optionally the reply-to field.
   The from field consists of the field name "From" and a comma-
   separated list of one or more addresses (either mailbox or group
   syntax).  If the from field contains more than one mailbox
   specification (including all mailboxes included in any groups), then
   the sender field, containing the field name "Sender" and a single
   address, MUST appear in the message.  The from field and the sender
   field SHOULD NOT use group syntax; rather, the from field SHOULD use
   only the mailbox-list syntax and the sender field SHOULD use only
   mailbox syntax (see Section 3).  If the sender field uses group
   syntax, the group MUST NOT contain more than one mailbox.  In either
   case, an optional reply-to field MAY also be included, which contains
   the field name "Reply-To" and a comma-separated list of one or more

   from = "From:" (mailbox-list / address-list) CRLF

   sender = "Sender:" (mailbox / address) CRLF

   reply-to = "Reply-To:" address-list CRLF

   The originator fields indicate the mailbox(es) of the source of the
   message.  The "From:" field specifies the author(s) of the message,
   that is, the mailbox(es) of the person(s) or system(s) responsible
   for the writing of the message.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated
   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD

      Note: The transmitter information is always present.  The absence
      of the "Sender:" field is sometimes mistakenly taken to mean that
      the agent responsible for transmission of the message has not been
      specified.  This absence merely means that the transmitter is
      identical to the author and is therefore not redundantly placed
      into the "Sender:" field.

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   The originator fields also provide the information required when
   replying to a message.  When the "Reply-To:" field is present, it
   indicates the address(es) to which the author of the message suggests
   that replies be sent.  In the absence of the "Reply-To:" field,
   replies SHOULD by default be sent to the mailbox(es) specified in the
   "From:" field unless otherwise specified by the person composing the

   In all cases, the "From:" field SHOULD NOT contain any mailbox that
   does not belong to the author(s) of the message.  See also [RFC5322]
   Section 3.6.3 for more information on forming the destination
   addresses for a reply.

2.2.  Update to RFC 5322, Section 3.6.6. Resent Fields

   This updates RFC 5322, Section 3.6.6, to allow groups (via the
   address-list ABNF production) in the "Resent-From:" and "Resent-
   Sender:" fields, to parallel the change to "From:" and "Sender:"
   above.  The ABNF for those fields is changed as follows:

   resent-from = "Resent-From:" (mailbox-list / address-list) CRLF

   resent-sender = "Resent-Sender:" (mailbox / address) CRLF

3.  Applicability Statement

   Mailbox syntax is the normal use in the "From:" and "Sender:" header
   fields; the address syntax defined in Section 2.1, which allows the
   specification of a group, is only for Limited Use (see [RFC2026],
   Section 3.3, item (d)) for the reasons described below.

   Very many Internet email procedures and software assume that the
   addresses in "From:" and "Sender:" fields can be replied to and are
   suitable for use in mail organizing and filtering.  The use of groups
   instead of mailboxes can disrupt those uses.  Consequently, while
   this specification legitimizes the use of groups, it does so only to
   enable circumstances when that use is necessary, and because the use
   of this mechanism is new it is important that its use be limited to
   those circumstances and that it be used with caution.  In particular,
   user agents SHOULD NOT permit the use of groups in those fields in
   outgoing messages.

4.  Examples

   First, consider an email message that is sent by an automated nightly
   monitor program, to which replies should not be sent.  Such messages

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   commonly include a valid, replyable address that will discard any
   replies that are sent to it, but recipients who do reply might be
   unaware that their replies will be discarded.  If the message is
   instead presented this way, the recipients' email clients will not
   allow them to reply in the first place:

      From: Nightly Monitor Robot:;

   Second, consider an email message that is meant to be "from" the two
   managing partners of a business, Ben, and Carol, and that is sent by
   their assistant, Dave.  That could always have been presented this


   This change allows that to be represented this way:

      From: Managing,;

5.  Security Considerations

   See the Internet Message Format specification [RFC5322] for general
   discussion of security considerations related to the formatting of
   email messages.

   The "From:" address is special, in that most user agents display that
   address, or the "friendly" text associated with it, to the end user,
   and label that so as to identify it as the origin of the message (as
   implied in Section 3.6.2 of RFC 5322).  Group syntax in the "From:"
   header field can be used to hide the identity of the message
   originator.  It is as easy to use a fabricated "From:" address to
   accomplish the same thing, so allowing groups there does not
   exacerbate the security problem.

   Some protocols attempt to validate the originator address by matching
   the "From:" address to a particular verified domain (see Author
   Domain Signing Practices (ADSP) [RFC5617] for one such protocol).
   Such protocols will not be applicable to messages that lack an actual
   email address (whether real or fake) in the "From:" field.  Local
   policy will determine how such messages are handled, and senders,
   therefore, need to be aware that using groups in the "From:" might
   adversely affect deliverability of the message.

   Because groups have previously not been allowed in the "From:" and
   "Sender:" header fields, it is possible that some implementations

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   that conform to RFC 5322 might not be prepared to handle that syntax,
   and, indeed, might not even recognize that group syntax is being
   used.  Of those implementations, some subset might, when presented
   with group syntax in those header fields, behave in a way that is
   exploitable by an attacker.  It is deemed unlikely that this will be
   a serious problem in practice: address field parsing is generally an
   integral component of implementations, and address field parsers are
   required to understand group syntax.  In addition, if any
   implementations should be exploitable through this mechanism, it is
   already possible for attackers to do it by violating RFC 5322, and
   other RFC 5322 violations are commonly used by malefactors.

6.  IANA Considerations

   IANA is asked to update the Permanent Message Header Field Names
   registry ( )
   as follows:

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   |  From          |  mail  |  standard  |  [RFC5322]               |

   |  Sender        |  mail  |  standard  |  [RFC5322]               |

   |  Resent-From   |  mail  |  standard  |  [RFC5322]               |

   |  Resent-Sender |  mail  |  standard  |  [RFC5322]               |

   |  From          |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Sender        |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Resent-From   |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Resent-Sender |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

7.  References

7.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

7.2.  Informative References

   [RFC0822]  Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC5617]  Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, August 2009.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

Author's Address

   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648

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