|Title||Sender ID: Authenticating E-Mail
|Author||J. Lyon, M. Wong
Network Working Group J. Lyon
Request for Comments: 4406 Microsoft Corp.
Category: Experimental M. Wong
Sender ID: Authenticating E-Mail
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.
Internet mail suffers from the fact that much unwanted mail is sent
using spoofed addresses -- "spoofed" in this case means that the
address is used without the permission of the domain owner. This
document describes a family of tests by which SMTP servers can
determine whether an e-mail address in a received message was used
with the permission of the owner of the domain contained in that
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................4
2. Problem Statement ...............................................4
3. SPF 2.0 Records .................................................5
3.1. Version and Scope ..........................................5
3.1.1. Minor Version .......................................6
3.2. Multiple Records ...........................................6
3.3. Positional Modifiers .......................................7
3.4. Compatibility ..............................................8
4. Decision Model ..................................................8
4.1. Arguments ..................................................9
4.2. Results ....................................................9
4.3. Record Lookup ..............................................9
4.4. Record Selection ...........................................9
5. Actions Based on the Decision ..................................10
5.1. Neutral, None, SoftFail, or PermError .....................11
5.2. Pass ......................................................11
5.3. Fail ......................................................11
5.4. TempError .................................................11
6. Security Considerations ........................................11
6.1. DNS Attacks ...............................................12
6.2. TCP Attacks ...............................................12
6.3. Forged Sender Attacks .....................................12
6.4. Address Space Hijacking ...................................12
6.5. Malicious DNS Attacks on Third Parties ....................13
7. Implementation Guidance ........................................13
7.1. Simple E-Mailers ..........................................14
7.2. E-Mail Forwarders .........................................14
7.3. Mailing List Servers ......................................15
7.4. Third-Party Mailers .......................................15
7.5. MUA Implementers ..........................................15
8. Acknowledgements ...............................................16
9. References .....................................................17
9.1. Normative References ......................................17
9.2. Informative References ....................................17
Today, a huge majority of unwanted e-mail contains headers that lie
about the origin of the mail. This is true of most spam and
substantially all of the virus e-mail that is sent.
This document describes a mechanism such that receiving Mail Transfer
Agents (MTAs), Mail Delivery Agents (MDAs), and/or Mail User Agents
(MUAs) can recognize mail in the above category and take appropriate
action. For example, an MTA might refuse to accept a message, an MDA
might discard a message rather than placing it into a mailbox, and an
MUA might render that message in some distinctive fashion.
In order to avoid further fragmentation of the Internet e-mail
system, it is desirable that the Internet community as a whole come
to a consensus as to what mail senders should do to make their mail
appear non-spoofed, and how mail receivers should determine whether
mail is spoofed. On the other hand, it is not necessary to reach a
consensus regarding the actions that various parties take once a
message has been determined to be spoofed. This can be done
unilaterally -- one agent might decide to discard a spoofed message
whereas another decides to add a disclaimer.
This document defines a pair of closely-related tests. One validates
a message's Purported Responsible Address (PRA) as defined in
[RFC4407]. The other validates a message's Reverse-Path (also known
as MAIL-FROM address) as defined in [RFC4408].
An e-mail sender complying with this specification SHOULD publish
information for both tests, and SHOULD arrange that any mail that is
sent will pass both tests. An e-mail receiver complying with this
specification SHOULD perform at least one of these tests.
1.1. Conventions Used in This Document
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
2. Problem Statement
Briefly stated, the mechanisms of this document allow one to answer
the following question:
When a message is transferred via SMTP between two unrelated
parties, does the SMTP client host have permission to send mail on
behalf of a mailbox referenced by the message?
As seen from the question, this mechanism applies to unrelated
parties: It is useful at the point where a message passes across the
Internet from one organization to another. It is beyond the scope of
this document to describe authentication mechanisms that can be
deployed within an organization.
The PRA version of the test seeks to authenticate the mailbox
associated with the most recent introduction of a message into the
mail delivery system. In simple cases, this is who the mail is from.
However, in the case of a third-party mailer, a forwarder, or a
mailing list server, the address being authenticated is that of the
third party, the forwarder, or the mailing list.
On the other hand, the MAIL-FROM version of the test seeks to
authenticate the mailbox that would receive Delivery Status
Notifications (DSNs, or bounces) for the message. In simple cases,
this too is who the mail is from. However, third-party mailers,
forwarders, and mailing list servers MUST specify an address under
their control, and SHOULD arrange that DSNs received at this address
are forwarded to the original bounce address.
In both cases, the domain associated with an e-mail address is what
is authenticated; no attempt is made to authenticate the local-part.
A domain owner gets to determine which SMTP clients speak on behalf
of addresses within the domain; a responsible domain owner should not
authorize SMTP clients that will lie about local parts.
In the long run, once the domain of the sender is authenticated, it
will be possible to use that domain as part of a mechanism to
determine the likelihood that a given message is spam, using, for
example, reputation and accreditation services. (These services are
not the subject of the present mechanism, but it should enable them.)
3. SPF 2.0 Records
Domains declare which hosts are and are not authorized to transmit
e-mail messages on their behalf by publishing Sender Policy Framework
(SPF) records in the Domain Name System. [RFC4408] defines a format
for these records identified by the version prefix "v=spf1". This
section defines an amended format, identified by the version prefix
"spf2.0", that allows sending domains to explicitly specify how their
records should be interpreted, and provides for additional
extensibility. Sending domains MAY publish either or both formats.
Since the two formats are identical in most respects, the following
subsections define the "spf2.0" format relative to [RFC4408].
3.1. Version and Scope
Under Sender ID, receiving domains may perform a check of either the
PRA identity or the MAIL-FROM identity. Sending domains therefore
require a method for declaring whether their published list of
authorized outbound e-mail servers can be used for the PRA check, the
MAIL-FROM check, or both.
This section replaces the definition of the version identifier in
Section 4.5 of [RFC4408] and adds the concept of SPF record scopes.
SPF records begin with a version identifier and may also include a
record = version terms *SP
version = "v=spf1" | ( "spf2." ver-minor scope)
ver-minor = 1*DIGIT
scope = "/" scope-id *( "," scope-id )
scope-id = "mfrom" / "pra" / name
For example, the SPF record
spf2.0/mfrom,pra +mx +ip4:192.168.0.100 -all
defines an SPF record that can be used for either MAIL FROM or PRA
This document only defines the existence of two scopes: "mfrom" and
"pra". The details of these two scopes are defined in other
documents: "mfrom" is defined in [RFC4408]; "pra" is defined in
Other scopes may be defined by future documents only. There is no
registry for scopes. A scope definition must define what it
identifies as the sending mailbox for a message, how to extract that
information from a message, how to determine the initial arguments
for the check_host() function, and what the compliant responses to
the result are. This ensures that domains with published records and
mail receiver agree on the semantics of the scope.
A compliant domain SHOULD publish authorizations for every defined
3.1.1. Minor Version
All published records that use the "spf2" version identifier MUST
start with "spf2.0". This document only specifies records with a
minor version of "0".
Future versions of this document may define other minor versions to
3.2. Multiple Records
A domain MAY publish multiple SPF 2.0 records, provided that each
scope appears in at most one SPF 2.0 record. In addition, a domain
MAY also publish an SPF record that uses the "v=spf1" version
identifier defined in [RFC4408]. The selection rules in Section 4.4
define the precedence of these records.
3.3. Positional Modifiers
This section replaces Section 4.6.3 of [RFC4408] and adds the concept
of positional modifiers.
Modifiers are key/value pairs that affect the evaluation of the
Modifiers are either global or positional:
Global modifiers MAY appear anywhere in the record, but SHOULD
appear at the end, after all mechanisms and positional modifiers.
Positional modifiers apply only to the mechanism they follow. It
is a syntax error for a positional modifier to appear before the
Modifiers of either type are also either singular or multiple:
Singular modifiers may appear only once in the record if they are
global, or once after each mechanism if they are positional.
Multiple modifiers may appear multiple times in the record if they
are global, or multiple times after each mechanism if they are
A modifier is not allowed to be defined as both global and
The modifiers "redirect" and "exp" described in Section 6 of
[RFC4408] are global and singular.
Ordering of modifiers does not matter, except that:
1. positional modifiers must appear after the mechanism they affect
and before any subsequent mechanisms; and
2. when a multiple modifier appears more than one time, the ordering
of the appearances may be significant to the modifier.
Other than these constraints, implementations MUST treat different
orders of modifiers the same. An intended side effect of these rules
is that modifiers cannot be defined that modify other modifiers.
These rules allow an implementation to correctly pre-parse a record.
Furthermore, they are crafted to allow the parsing algorithm to be
stable, even when new modifiers are introduced.
Modifiers that are unrecognized MUST be ignored. This allows older
implementations to handle records with modifiers that were defined
after they were written.
Domain administrators complying with this specification are required
to publish information in DNS regarding their authorized outbound
e-mail servers. [RFC4408] describes a format for this information
identified by the version prefix "v=spf1". Many domains have
published information in DNS using this format. In order to provide
compatibility for these domains, Sender ID implementations SHOULD
interpret the version prefix "v=spf1" as equivalent to
"spf2.0/mfrom,pra", provided no record starting with "spf2.0" exists.
Administrators who have already published "v=spf1" records SHOULD
review these records to determine whether they are also valid for use
with PRA checks. If the information in a "v=spf1" record is not
correct for a PRA check, administrators SHOULD publish either an
"spf2.0/pra" record with correct information or an "spf2.0/pra ?all"
record indicating that the result of a PRA check is explicitly
4. Decision Model
Sender ID enables receiving e-mail systems to answer the following
Given an e-mail message, and given an IP address from which it has
been (or will be) received, is the SMTP client at that IP address
authorized to send that e-mail message?
This question will usually be asked by an SMTP server as part of
deciding whether to accept an incoming mail message. However, this
question could also be asked later by a different party. An MUA, for
example, could use the result of this question to determine how to
file or present a message.
There are three steps to answering this question:
1. From an e-mail message, extract the address to verify. The PRA
variant of this test does so as specified in [RFC4407], or
alternatively, using the submitter address as specified in
[RFC4405]. The MAIL FROM variant of this test does so as
specified in [RFC4408].
2. Extract the domain part of the address determined in step 1.
3. Call the check_host() function defined in [RFC4408] and modified
by the following subsections.
If the Sender ID check is being performed by an MTA as part of
receiving an e-mail message, and it cannot determine an address in
step 1 above (because the message or address is malformed), then the
message SHOULD be rejected with error "550 5.7.1 Missing Purported
Responsible Address" or error "550 5.7.1 Missing Reverse-Path
Sender ID modifies the check_host() function by the addition of a
scope parameter. Thus, for Sender ID the check_host() function is
called passing the following parameters:
a. A scope of "pra" (for the PRA variant of the test), or "mfrom"
(for the MAIL FROM variant of the test).
b. The IP address (either IPv4 or IPv6) from which the message is
being or has been received.
c. The domain from step 2 above.
d. The address from step 1 above.
The result of the check_host() function is one of the values
"Neutral", "Pass", "Fail", "SoftFail", "None", "TempError", or
"PermError". Section 5 describes how these results are used by MTAs
receiving messages. This specification imposes no requirements on
parties performing this test in other environments.
4.3. Record Lookup
SPF records are looked up in DNS in accordance with Section 4.4 of
When performing the PRA version of the test, if the DNS query returns
"non-existent domain" (RCODE 3), then check_host() exits immediately
with the result "Fail".
4.4. Record Selection
This section replaces the record selection steps described in Section
4.5 of [RFC4408].
Starting with the set of records that were returned by the lookup,
record selection proceeds in these steps:
1. If any records of type SPF are in the set, then all records of
type TXT are discarded.
2. Records that do not begin with proper version and scope sections
are discarded. The version section for "spf2" records contains a
ver-minor field that is for backward-compatible future extensions.
This field must be well formed for a record to be retained, but is
3. Records that use the "spf2" version identifier and do not have a
scope-id that matches <scope> are discarded. Note that this is a
complete string match on the scope-id tokens: If <scope> is "pra",
then the record starting "spf2.0/mfrom,prattle,fubar" would be
discarded, but a record starting "spf2.0/mfrom,pra,fubar" would be
4. If the lookup returned two records, one containing the "v=spf1"
version identifier and the other containing the "spf2" version
identifier, the "spf2" version takes precedence for the desired
scope-id. If the "spf2" record does not contain the desired
scope-id, then the "v=spf1" record is selected.
5. If an "spf2" record does not contain the desired scope-id and
there is no "v=spf1" record for the domain, then no record is
After the above steps, there should be one record remaining and
evaluation can proceed. If there are two or more records remaining,
then check_host() exits immediately with the error "PermError".
If there are no matching records remaining after the initial DNS
query or any subsequent optional DNS queries, then check_host() exits
immediately with the result "None".
5. Actions Based on the Decision
When the Sender ID test is used by an SMTP server as part of
receiving a message, the server should take the actions described by
The check_host() function returns one of the following results. See
[RFC4408] for the meaning of these results.
5.1. Neutral, None, SoftFail, or PermError
An SMTP server receiving one of these results SHOULD NOT reject the
message for this reason alone, but MAY subject the message to
heightened scrutiny by other measures, and MAY reject the message as
a result of this heightened scrutiny.
Such additional security measures MAY take into account that a
message for which the result is "SoftFail" is less likely to be
authentic than a message for which the result is "Neutral".
An SMTP server receiving this result SHOULD treat the message as
authentic. It may accept or reject the message depending on other
When performing the Sender ID test during an SMTP transaction, an MTA
that chooses to reject a message receiving this result SHOULD reject
the message with a "550 5.7.1 Sender ID (xxx) yyy - zzz" SMTP error,
where "xxx" is replaced with "PRA" or "MAIL FROM", "yyy" is replaced
with the additional reason returned by the check_host() function, and
"zzz" is replaced with the explanation string returned by the
When performing the Sender ID test after accepting an e-mail message
for delivery, an MTA that chooses to reject a message receiving this
result SHOULD NOT deliver the message. Instead, it should create a
DSN message, consistent with the usual rules for DSN messages.
An SMTP server receiving this result MAY reject the message with a
"450 4.4.3 Sender ID check is temporarily unavailable" error code.
Alternatively, an SMTP server receiving this result MAY accept a
message and optionally subject it to heightened scrutiny by other
6. Security Considerations
This entire document describes a new mechanism for mitigating spoofed
e-mail, which is today a pervasive security problem in the Internet.
Assuming that this mechanism is widely deployed, the following
sections describe counter attacks that could be used to defeat this
6.1. DNS Attacks
The new mechanism is entirely dependent on DNS lookups, and is
therefore only as secure as DNS. An attacker bent on spoofing
messages could attempt to get his messages accepted by sending forged
answers to DNS queries.
An MTA could largely defeat such an attack by using a properly
paranoid DNS resolver. DNS Security (DNSSEC) may ultimately provide
a way to completely neutralize this class of attacks.
6.2. TCP Attacks
This mechanism is designed to be used in conjunction with SMTP over
TCP. A sufficiently resourceful attacker might be able to send TCP
packets with forged from-addresses, and thus execute an entire SMTP
session that appears to come from somewhere other than its true
Such an attack requires guessing what TCP sequence numbers an SMTP
server will use. It also requires transmitting completely in the
blind -- the attack will be unable to hear any of the server's side
of the conversation.
Attacks of this sort can be ameliorated if IP gateways refuse to
forward packets when the source address is clearly bogus.
6.3. Forged Sender Attacks
This mechanism chooses an address to validate either from one of a
number of message headers or from the RFC 2821 MAIL command, and then
uses that address for validation. A message with a true Resent-From
header or Return-Path, but a forged From header, will be accepted.
Since many MUAs do not display all of the headers of received
messages, the message will appear to be forged when displayed.
In order to neutralize this attack, MUAs will need to start
displaying at least the address that was verified. In addition, MTAs
could subject messages to heightened scrutiny when the validated
address differs from the From header.
6.4. Address Space Hijacking
This mechanism assumes the integrity of IP address space for
determining whether a given client is authorized to send messages
from a given PRA. In addition to the TCP attack given in Section
6.2, a sufficiently resourceful attacker might be able to alter the
IP routing structure to permit two-way communication using a
specified IP address. It would then be possible to execute an SMTP
session that appears to come from an authorized address, without the
need to guess TCP sequence numbers or transmit in the blind.
Such an attack might occur if the attacker obtained access to a
router that participates in external BGP routing. Such a router
could advertise a more specific route to a rogue SMTP client,
temporarily overriding the legitimate owner of the address.
6.5. Malicious DNS Attacks on Third Parties
There is class of attacks in which an attacker A can entice a
participant P to send a malicious message to a victim V.
These attacks are undertaken by A citing the address of V in the SMTP
MAIL FROM request and then by causing P to generate (or invoke the
generation of) a Delivery Status Notification 'bounce' message
(RFC3464), which is sent to the victim V.
The attacker relies upon it being common practice to copy the
original message into the 'bounce' report, thereby causing the malice
to be sent onward to V.
This mode of attack has the advantages (to the attacker) of
obfuscating the location of the host from which the attack was
mounted, and of possibly damaging the reputation of P by making it
appear that P originated or was an active participant in the sending
of the malicious message.
In current practice, A causes P to cause the 'bounce' by addressing
the original message to a nonexistent recipient.
Sender ID enables a new variant of this attack.
In this variant, the attacker A sends a message whose PRA (Section 4)
is selected by the attacker to be such that, when P undertakes the
Sender ID test, a Fail will result (Section 5.3).
The message will be rejected (as the attacker intended) and a
malicious 'bounce' message may be generated and sent to the victim V.
7. Implementation Guidance
This section describes the actions that certain members of the
Internet e-mail ecosystem must take to be compliant with this
7.1. Simple E-Mailers
A domain that injects original e-mail into the Internet, using its
own name in From headers, need do nothing to be compliant. However,
such domains SHOULD publish records in DNS as defined by [RFC4408]
and this specification.
In the majority of cases, the domain's published information will be
the same for both the PRA and MAIL FROM variants of this test. In
this case, domains SHOULD publish their information using an SPF
record with the prefix "v=spf1". Doing so will render their
published information usable by the older SPF protocol, too. (See
[RFC4408] for information on the SPF protocol.)
7.2. E-Mail Forwarders
In order to pass the PRA variant of the test, a program that forwards
received mail to other addresses MUST add an appropriate header that
contains an e-mail address that it is authorized to use. Such
programs SHOULD use the Resent-From header for this purpose.
In order to pass the MAIL FROM variant of the test, a program that
forwards received mail to other addresses MUST alter the MAIL FROM
address to an address under its control. Should that address
eventually receive a DSN relating to the original message, that DSN
SHOULD be forwarded to the original MAIL FROM address. However, if
this altered address receives any messages other than DSNs related to
the original message, these messages MUST NOT be forwarded to the
original MAIL FROM address; they SHOULD be refused during an SMTP
In addition, e-mail forwarders SHOULD publish Sender ID records for
their domains, and SHOULD use MTAs for which the Sender ID check
yields a "pass" result.
Some of today's forwarders already add an appropriate header
(although many of them use Sender rather than Resent-From.) Most of
them do not perform the address-rewriting specified above.
Note that an e-mail forwarder might receive a single message for two
or more recipients, each of whom requests forwarding to a new
address. In this case, the forwarder's MTA SHOULD transmit the
message to each new recipient individually, with each copy of the
message containing a different newly inserted Resent-From header
7.3. Mailing List Servers
In order to pass the PRA variant of the test, a mailing list server
MUST add an appropriate header that contains an e-mail address that
it is authorized to use. Such programs SHOULD use the Resent-From
header for this purpose.
In order to pass the MAIL FROM variant of the test, a mailing list
server MUST alter the MAIL FROM address to an address under its
In addition, mailing list servers SHOULD publish Sender ID records
for their domains, and SHOULD use MTAs for which the Sender ID check
yields a "pass" result.
Most of today's mailing list software already adds an appropriate
header (although most of them use Sender rather than Resent-From),
and most of them already alter the MAIL FROM address.
7.4. Third-Party Mailers
In order to pass the PRA variant of this test, a program that sends
mail on behalf of another user MUST add an appropriate header that
contains an e-mail address that it is authorized to use. Such
programs SHOULD use the Sender header for this purpose.
In order to pass the MAIL FROM variant of this test, a program that
sends mail on behalf of another user MUST use a MAIL FROM address
that is under its control. Defining what the program does with any
mail received at that address is beyond the scope of this document.
In addition, third-party mailers, servers SHOULD publish Sender ID
records for their domains, and SHOULD use MTAs for which the Sender
ID check yields a "pass" result.
Many, but not all, of today's third-party mailers are already
compliant with the PRA variant of the test. The extent to which
mailers are already compliant with the MAIL FROM variant of this test
7.5. MUA Implementers
When displaying a received message, an MUA SHOULD display the
purported responsible address as defined by this document whenever
that address differs from the RFC 2822 From address. This display
SHOULD be in addition to the RFC 2822 From address.
When a received message contains multiple headers that might be used
for the purported responsible address determination, an MUA should
consider displaying all of them. That is, if a message contains
several Resent-From's, a Sender, and a From, an MUA should consider
displaying all of them.
Sender ID also does not validate the display name that may be
transmitted along with an e-mail address. The display name is also
vulnerable to spoofing and other forms of attacks. In order to
reduce the occurrence and effectiveness of such attacks, MUA
implementers should consider methods to safeguard the display name.
This could include the following:
* Not presenting the display name to the user at all, or not
presenting the display name unless the corresponding e-mail address
is listed in the user's address book.
* Treating as suspicious any e-mail where the display name is itself
in the form of an e-mail address, especially when it differs from
the actual e-mail address in the header.
* Making it clear to users that the e-mail address has been checked
rather than the display name.
This design is based on earlier work published in 2003 in [RMX] and
[DMP] drafts (by Hadmut Danisch and Gordon Fecyk, respectively). The
idea of using a DNS record to check the legitimacy of an e-mail
address traces its ancestry to "Repudiating Mail From" draft by Paul
Vixie [Vixie] (based on suggestion by Jim Miller) and to "Domain-
Authorized SMTP Mail" draft by David Green [Green], who first
introduced this idea on the namedroppers mailing list in 2002.
The current document borrows heavily from each of the above, as well
as earlier versions of [RFC4408] and [CallerID], and incorporates
ideas proposed by many members of the MARID working group. The
contributions of each of the above are gratefully acknowledged.
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4405] Allman E. and H. Katz, "SMTP Service Extension for
Indicating the Responsible Submitter of an E-Mail
Message", RFC 4405, April 2006.
[RFC4407] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, April 2006.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail", RFC 4408,
9.2. Informative References
[CallerID] Microsoft Corporation, Caller ID for E-Mail Technical
[DMP] Fecyk, G., "Designated Mailers Protocol",
[Green] David Green, "Mail-Transmitter RR",
msg00656.html, June 2002.
[RMX] H. Danisch, "The RMX DNS RR and method for lightweight
SMTP sender authorization",
[Vixie] Paul Vixie, "Repudiating Mail From",
msg00658.html, June 2002.
One Microsoft Way
Redmond, WA 98052
Meng Weng Wong
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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