Network Working Group J. Lyon
Request for Comments: 4407 Microsoft Corp.
Category: Experimental April 2006 Purported Responsible Address in E-Mail Messages
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
The following documents (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
are published simultaneously as Experimental RFCs, although there is
no general technical consensus and efforts to reconcile the two
approaches have failed. As such, these documents have not received
full IETF review and are published "AS-IS" to document the different
approaches as they were considered in the MARID working group.
The IESG takes no position about which approach is to be preferred
and cautions the reader that there are serious open issues for each
approach and concerns about using them in tandem. The IESG believes
that documenting the different approaches does less harm than not
Note that the Sender ID experiment may use DNS records that may have
been created for the current SPF experiment or earlier versions in
this set of experiments. Depending on the content of the record,
this may mean that Sender-ID heuristics would be applied incorrectly
to a message. Depending on the actions associated by the recipient
with those heuristics, the message may not be delivered or may be
discarded on receipt.
Participants relying on Sender ID experiment DNS records are warned
that they may lose valid messages in this set of circumstances.
Participants publishing SPF experiment DNS records should consider
the advice given in section 3.4 of RFC 4406 and may wish to publish
both v=spf1 and spf2.0 records to avoid the conflict.
Participants in the Sender-ID experiment need to be aware that the
way Resent-* header fields are used will result in failure to receive
legitimate email when interacting with standards-compliant systems
(specifically automatic forwarders which comply with the standards by
not adding Resent-* headers, and systems which comply with RFC 822
but have not yet implemented RFC 2822 Resent-* semantics). It would
be inappropriate to advance Sender-ID on the standards track without
resolving this interoperability problem.
The community is invited to observe the success or failure of the two
approaches during the two years following publication, in order that
a community consensus can be reached in the future.
This document defines an algorithm by which, given an e-mail message,
one can extract the identity of the party that appears to have most
proximately caused that message to be delivered. This identity is
called the Purported Responsible Address (PRA).
Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. Determining the Purported Responsible Address ...................33. Security Considerations .........................................54. Acknowledgements ................................................55. References ......................................................55.1. Normative References .......................................55.2. Informative References .....................................51. Introduction
Most e-mail flows relatively directly from a sender to a recipient,
with a small number of Mail Transfer Agents (MTAs) in between. Some
messages, however, are resent by forwarding agents, mailing list
servers, and other such software. These messages effectively result
in two or more mail transactions: one from the sender to the
forwarding agent, and another from the agent to the destination.
In some cases, messages travel through more than one of these agents.
This can occur, for example, when one mailing list is subscribed to
another, or when the address subscribed to a mailing list is a
Further complicating the situation, in some cases the party that
introduces a message is not the author of the message. For example,
many news web sites have a "Mail this article" function that the
public can use to e-mail a copy of the article to a friend. In this
case, the mail is "from" the person who pressed the button, but is
physically sent by the operator of the web site.
This document defines a new identity associated with an e-mail
message, called the Purported Responsible Address (PRA), which is
determined by inspecting the header of the message. The PRA is
designed to be the entity that (according to the header) most
recently caused the message to be delivered.
Note that the results of this algorithm are only as truthful as the
headers contained in the message; if a message contains fraudulent or
incorrect headers, this algorithm will yield an incorrect result.
For this reason, the result of the algorithm is called the "Purported
Responsible Address" -- "purported" because it tells you what a
message claims about where it came from, but not necessarily where it
actually came from.
This document does not prescribe any particular uses for the
Purported Responsible Address. However, [RFC4406] describes a method
of determining whether a particular MTA is authorized to send mail on
behalf of the domain contained in the PRA.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Determining the Purported Responsible Address
The PRA of a message is determined by the following algorithm:
1. Select the first non-empty Resent-Sender header in the message.
If no such header is found, continue with step 2. If it is
preceded by a non-empty Resent-From header and one or more
Received or Return-Path headers occur after said Resent-From
header and before the Resent-Sender header, continue with step 2.
Otherwise, proceed to step 5.
2. Select the first non-empty Resent-From header in the message. If
a Resent-From header is found, proceed to step 5. Otherwise,
continue with step 3.
3. Select all the non-empty Sender headers in the message. If there
are no such headers, continue with step 4. If there is exactly
one such header, proceed to step 5. If there is more than one
such header, proceed to step 6.
4. Select all the non-empty From headers in the message. If there is
exactly one such header, continue with step 5. Otherwise, proceed
to step 6.
5. A previous step has selected a single header from the message. If
that header is malformed (e.g., it appears to contain multiple
mailboxes, or the single mailbox is hopelessly malformed, or the
single mailbox does not contain a domain name), continue with step
6. Otherwise, return that single mailbox as the Purported
6. The message is ill-formed, and it is impossible to determine a
Purported Responsible Address.
For the purposes of this algorithm, a header field is "non-empty" if
and only if it contains any non-whitespace characters. Header fields
that are otherwise relevant but contain only whitespace are ignored
and treated as if they were not present.
Note that steps 1 and 2 above extract the Resent-Sender or Resent-
From header from the first resent block (as defined by section 3.6.6
of [RFC2822]) if any. Steps 3 and 4 above extract the Sender or From
header if there are no resent blocks.
Note that what constitutes a hopelessly malformed header or a
hopelessly malformed mailbox in step 5 above is a matter for local
policy. Such local policy will never cause two implementations to
return different PRAs. However, it may cause one implementation to
return a PRA where another implementation does not. This will occur
only when dealing with a message containing headers of questionable
Although the algorithm specifies how messages that are not in strict
conformance with the provisions of RFC 2822 should be treated for the
purposes of determining the PRA, this should not be taken as
requiring or recommending that any systems accept such messages when
they otherwise would not have done so. However, if a liberal
implementation accepts such messages and desires to know their PRAs,
it MUST use the algorithm specified here.
Where messages conform to RFC 822 rather than RFC 2822, it is
possible for the algorithm to give unexpected results. An RFC822
message should not normally contain more than one set of resent
headers; however, the placement of those headers is not specified,
nor are they required to be contiguous. It is therefore possible
that the Resent-From header will be selected even though a Resent-
Sender header is present. Such cases are expected to be rare or
non-existent in practice.
3. Security Considerations
The PRA, as described by this document, is extracted from message
headers that have historically not been verified. Thus, anyone using
the PRA for any purpose MUST be aware that the headers from which it
is derived might be fraudulent, malicious, malformed, and/or
incorrect. [RFC4406] describes one mechanism for validating the PRA.
A message's PRA will often be extracted from a header field that is
not normally displayed by existing mail user agent software. If the
PRA is used as part of a mechanism to authenticate the message's
origin, the message SHOULD NOT be displayed with an indication of its
authenticity (positive or negative) without the PRA header field also
The PRA concept was first published in [CallerID]. It has been
refined using valuable suggestions from members of the MARID working
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
5.2. Informative References
[CallerID] Microsoft Corporation, Caller ID for E-Mail Technical
senderid/resources.mspx[RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.
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