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Versions: 00 01 02 RFC 4408

Network Working Group                                            M. Wong
Internet-Draft                                                W. Schlitt
Expires: June 30, 2005                                 December 30, 2004

     Sender Policy Framework: Authorizing Use of Domains in E-MAIL

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   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 are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on June 30, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).


   E-mail on the Internet can be forged in a number of ways.  In
   particular, existing protocols place no restriction in what a sending
   host can use as the reverse-path of a message.  This document
   describes a protocol whereby a domain can explicitly authorize the
   hosts that are allowed to use its domain name in a reverse-path, and
   a way for receiving hosts to check such authorization.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Protocol Status  . . . . . . . . . . . . . . . . . . . . .  4
     1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1   The HELO Identity  . . . . . . . . . . . . . . . . . . . .  6
     2.2   The MAIL FROM Identity . . . . . . . . . . . . . . . . . .  6
     2.3   Publishing Authorization . . . . . . . . . . . . . . . . .  6
     2.4   Checking Authorization . . . . . . . . . . . . . . . . . .  7
     2.5   Interpreting the Result  . . . . . . . . . . . . . . . . .  8
       2.5.1   None . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.5.2   Neutral  . . . . . . . . . . . . . . . . . . . . . . .  8
       2.5.3   Pass . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.5.4   Fail . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.5.5   SoftFail . . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.6   TempError  . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.7   PermError  . . . . . . . . . . . . . . . . . . . . . .  9
   3.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1   Publishing . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.1   DNS Resource Record Types  . . . . . . . . . . . . . . 10
       3.1.2   Multiple Records . . . . . . . . . . . . . . . . . . . 11
       3.1.3   Multiple Strings . . . . . . . . . . . . . . . . . . . 11
       3.1.4   Record Size  . . . . . . . . . . . . . . . . . . . . . 11
       3.1.5   Wildcard Records . . . . . . . . . . . . . . . . . . . 12
   4.  The check_host() Function  . . . . . . . . . . . . . . . . . . 13
     4.1   Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.2   Results  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.3   Initial Processing . . . . . . . . . . . . . . . . . . . . 13
     4.4   Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 13
     4.5   Selecting Records  . . . . . . . . . . . . . . . . . . . . 14
     4.6   Record Evaluation  . . . . . . . . . . . . . . . . . . . . 14
       4.6.1   Term Evaluation  . . . . . . . . . . . . . . . . . . . 15
       4.6.2   Mechanisms . . . . . . . . . . . . . . . . . . . . . . 15
       4.6.3   Modifiers  . . . . . . . . . . . . . . . . . . . . . . 16
     4.7   Default Result . . . . . . . . . . . . . . . . . . . . . . 16
     4.8   Domain Specification . . . . . . . . . . . . . . . . . . . 16
   5.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 17
     5.1   "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2   "include"  . . . . . . . . . . . . . . . . . . . . . . . . 18
     5.3   "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.4   "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.5   "ptr"  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.6   "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 21
     5.7   "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 23
     6.1   redirect: Redirected Query . . . . . . . . . . . . . . . . 23
     6.2   exp: Explanation . . . . . . . . . . . . . . . . . . . . . 24

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   7.  Miscellaneous  . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.1   Processing Limits  . . . . . . . . . . . . . . . . . . . . 26
     7.2   The Received-SPF header  . . . . . . . . . . . . . . . . . 27
   8.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     8.1   Macro definitions  . . . . . . . . . . . . . . . . . . . . 30
     8.2   Expansion Examples . . . . . . . . . . . . . . . . . . . . 33
   9.  Implications . . . . . . . . . . . . . . . . . . . . . . . . . 34
     9.1   Sending Domains  . . . . . . . . . . . . . . . . . . . . . 34
     9.2   Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 34
     9.3   Forwarding Services and Aliases  . . . . . . . . . . . . . 34
     9.4   Mail Services  . . . . . . . . . . . . . . . . . . . . . . 35
     9.5   MTA Relays . . . . . . . . . . . . . . . . . . . . . . . . 35
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 37
   11.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 39
   12.   Contributors and Acknowledgements  . . . . . . . . . . . . . 40
   13.   References . . . . . . . . . . . . . . . . . . . . . . . . . 41
   13.1  Normative References . . . . . . . . . . . . . . . . . . . . 41
   13.2  Informative References . . . . . . . . . . . . . . . . . . . 41
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 42
   A.  Collected ABNF . . . . . . . . . . . . . . . . . . . . . . . . 43
   B.  Extended Examples  . . . . . . . . . . . . . . . . . . . . . . 45
     B.1   Simple Examples  . . . . . . . . . . . . . . . . . . . . . 45
     B.2   Multiple Domain Example  . . . . . . . . . . . . . . . . . 46
     B.3   DNSBL Style Example  . . . . . . . . . . . . . . . . . . . 47
       Intellectual Property and Copyright Statements . . . . . . . . 48

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1.  Introduction

   The current e-mail infrastructure has the property that any host
   injecting mail into the mail system can identify itself as any domain
   name it wants.  Hosts can do this at a variety of levels: in
   particular, the session, the envelope, and the mail headers.  While
   this feature is desirable in some circumstances, it is a major
   obstacle to reducing end-user unwanted e-mail (or "spam").
   Furthermore, many domain name holders are understandably concerned
   about the ease with which other entities may make use of their domain
   names, often with intent to impersonate.

   This document defines a protocol by which domain owners may authorize
   hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
   Compliant domain holders publish SPF records about which hosts are
   permitted to use their names, and compliant mail receivers use the
   published SPF records to test the authorization of hosts using a
   given "HELO" or "MAIL FROM" identity during a mail transaction.

   An additional benefit to mail receivers is that when the use of an
   identity is verified, then local policy decisions about the mail can
   be made on the basis of the domain, rather than the host's IP
   address.  This is advantageous because reputation of domain names is
   likely to be more accurate than reputation of host IP addresses.
   Furthermore, if a claimed identity fails verification, then local
   policy can take stronger action against such e-mail, such as
   rejecting it.

1.1  Protocol Status

   SPF has been in development since the Summer of 2003, and has seen
   deployment beyond the developers beginning in December, 2003.  The
   design of SPF slowly evolved until the spring of 2004 and has since
   stabilized.  There have been quite a number of forms of SPF, some
   written up as documents, some submitted as Internet Drafts, and many
   discussed and debated in development forums.

   The goal of this document is to clearly document the protocol defined
   by earlier drafts specifications of SPF as used in existing
   implementations.  This conception of SPF is sometimes called "SPF
   Classic".  It is understood that particular implementations and
   deployments may differ from, and build upon, this work.  It is hoped
   that we have nonetheless captured the common understanding of SPF
   version 1.

1.2  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in [RFC2119].

   This document is concerned with a portion of a mail message commonly
   called "envelope sender", "return path", "reverse path", "bounce
   address", "2821 FROM", or "MAIL FROM".  Since these terms are either
   not well defined, or often used casually, this document defines the
   "MAIL FROM" identity in Section 2.2.  Note that other terms, that may
   superficially look like the common terms, such as "reverse-path", are
   used only with the defined meanings from normative documents.

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2.  Operation

2.1  The HELO Identity

   The "HELO" identity derives from either the SMTP HELO or EHLO command
   (see [RFC2821].) These commands supply the SMTP client (sender) for
   the SMTP session.  Note that requirements for the domain presented in
   the EHLO or HELO command are not always clear to the sending party,
   and SPF client must be prepared for the "HELO" identity to be

   SPF clients MAY check the "HELO" identity by calling the check_host()
   function (Section 4) with the "HELO" identity as the <sender>.  If
   the HELO test returns a "fail", the overall result for the SMTP
   session is "fail", and there is no need to test the "MAIL FROM"

2.2  The MAIL FROM Identity

   The "MAIL FROM" identity derives from the SMTP MAIL command (see
   [RFC2821].) This command supplies the "reverse-path" for a message,
   which generally consists of the sender mailbox, and is the mailbox to
   which notification messages are sent if there are problems delivering
   the message.

   [RFC2821] allows the reverse-path to be null (see Section 4.5.5.) In
   this case, there is no explicit sender mailbox, and such a message
   can be assumed to be a notification message from the mail system
   itself.  When the reverse-path is null, this document defines the
   "MAIL FROM" identity to be the mailbox composed of the localpart
   "postmaster" and the "HELO" identity

   SPF clients MUST check the "MAIL FROM" identity unless HELO testing
   produced a "fail".  SPF clients check the "MAIL FROM" identity by
   calling the check_host() function with the "MAIL FROM" identity as
   the <sender>.

2.3  Publishing Authorization

   An SPF compliant domain MUST publish a valid SPF record as described
   in Section 3.  This record authorizes the use of the domain name in
   the "HELO" and/or "MAIL FROM" identity, by some sending MTAs, and not
   by others.

   It is RECOMMENDED that domains publish SPF records that end in
   "-all", or redirect to other records that do, so that a definitive
   determination of authorization can be made.

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   Domain holders may publish SPF records that explicitly authorize no
   hosts for domain names that shouldn't be used in sender mailboxes.

2.4  Checking Authorization

   A mail receiver can perform an SPF compliant check for each mail
   message it receives.  This check tests the authorization of a client
   host to inject mail with a given "MAIL FROM" identity.  This check
   MAY also be applied to the "HELO" identity.  Typically, such checks
   are done by a receiving MTA, but can be performed elsewhere in the
   mail processing chain so long as the required information is
   available.  Checking other identities against SPF records is NOT
   RECOMMENDED because there are cases that are known to give incorrect

   It is possible that mail receivers will use the SPF check as part of
   a larger set of tests on incoming mail.  The results of other tests
   may influence whether or not a particular SPF check is performed.
   For example, finding the sending host on a local white list may cause
   all other tests to be skipped and all mail from that host to be

   When a mail receiver decides to perform an SPF check, it MUST
   implement and evaluate the check_host() function (Section 4)
   correctly.  While the test as a whole is optional, once it has been
   decided to perform a test it must be performed as specified so that
   the correct semantics are preserved between publisher and receiver.

   To make the test, the mail receiver MUST evaluate the check_host()
   with the arguments set as follows:

   <ip>     - the IP address of the SMTP client that is injecting the
            mail, either IPv4 or IPv6.
   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
   <sender> - the "MAIL FROM" or "HELO" identity.

   Note that the <domain> argument may not be a well formed domain name.
   For example, if the reverse-path was null, then the EHLO or HELO
   domain is used.  In a valid SMTP session, this can be an address
   literal or entirely malformed.  In these cases, check_host() is
   defined in Section 4.3 to return a "None" result.

   Care must be taken to correctly extract the <domain> from the
   <sender> as many MTAs will still accept such things as source routes
   (see [RFC2821] appendix C), the percent hack (see [RFC2162]) and bang
   paths (see [RFC1983]).  These archaic features have been maliciously
   used to bypass security systems.

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   Software SHOULD perform this authorization check during the
   processing of the SMTP transaction that injects the mail.  This
   allows errors to be returned directly to the injecting server by way
   of SMTP replies.  Software can perform the check as early as the MAIL
   command, though it may be easier to delay the check to some later
   stage of the transaction.

   Software can perform the authorization after the corresponding SMTP
   transaction has completed.  There are two problems with this
   approach: 1) It may be difficult to accurately extract all the
   required information such as client IP address and HELO domain name.
   2) If the authorization fails, then generating a non-delivery
   notification to the alleged sender is problematic due to the large
   number of forged emails on the Internet today.  Such an action would
   go against the explicit wishes of the alleged sender.

2.5  Interpreting the Result

   The check_host() function returns one of seven results.  This section
   describes how software that performs the authorization must interpret
   the results.  If the check is being performed during the SMTP mail
   transaction, it also describes how to respond.

2.5.1  None

   A result of None means that no records were published by the domain.
   The checking software cannot ascertain if the client host is
   authorized or not.

2.5.2  Neutral

   The domain owner has explicitly stated that doesn't know whether the
   IP is authorized or not.  A Neutral result MUST be treated exactly
   like the None result.

2.5.3  Pass

   A Pass result means that the client is authorized to inject mail with
   the given identity.  Further policy checks, such as reputation, or
   black and/or white listing, can now proceed with confidence in the

2.5.4  Fail

   A Fail result is an explicit statement that the client is not
   authorized to use the domain in the given identity.  The checking
   software can choose to mark the mail based on this, or to reject the
   mail outright.

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   If the checking software chooses to reject the mail during the SMTP
   transaction, then it SHOULD use an SMTP reply code of 550 (see
   [RFC2821]) and, if supported, the 5.7.1 DSN code (see [RFC2034]), in
   addition to an appropriate message.  The check_host() function may
   return either a default explanation string, or one from the domain
   that published the SPF records (see Section 6.2).  If the information
   doesn't originate with the checking software, it should be made clear
   that text is not trusted.  For example:

       550-5.7.1 SPF MAIL FROM check failed:
       550-5.7.1 The domain example.com explains:
       550 5.7.1 Please see http://www.example.com/mailpolicy.html

2.5.5  SoftFail

   A SoftFail result should be treated as somewhere between a Fail and a
   Neutral.  The domain believes the host isn't authorized but isn't
   willing to make that strong of a statement.  Receiving software
   SHOULD NOT reject the message based on this result, but MAY subject
   the message to closer scrutiny.

   Since the domain has discouraged the use of this host, receivers MAY
   try to inform either the sender or the recipient of the e-mail.  As
   examples, the recipient's MUA could highlight the SoftFail status.
   Or the MTA could give the sender a message using a technique called
   "greylisting" where by the MTA can issue an SMTP reply code of 451
   (4.3.0 DSN code) with a note the first time the message was received,
   but accept it the second time.

2.5.6  TempError

   A TempError result means that the SPF client encountered a transient
   error when performing the check.  Checking software can choose to
   accept or temporarily reject the message.  If the message is rejected
   during the SMTP transaction for this reason, the software SHOULD use
   an SMTP reply code of 451 and, if supported, the 4.4.3 DSN code.

2.5.7  PermError

   A PermError result means that the domain's published records couldn't
   be correctly interpreted.  Checking software SHOULD reject the
   message.  If rejecting during SMTP transaction time, it SHOULD use an
   SMTP reply code of 550 and, if supported, the 5.5.2 DSN code.

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3.  SPF Records

   An SPF record declares which hosts are, and are not, authorized to
   use a domain name for the "HELO" or "MAIL FROM" identity.  Loosely,
   the record partitions all hosts into permitted and not-permitted
   sets.  (Though some hosts might fall into neither category.)

   The SPF record is a single string of text.  An example record is:

      v=spf1 +mx a:colo.example.com/28 -all

   This record has a version of "v=spf1" and three directives: "+mx",
   "a:colo.example.com/28" (the + is implied), and "-all".

3.1  Publishing

   Domain owners wishing to be SPF compliant must publish SPF records
   for the hosts that are used in both the MAIL FROM and HELO
   identities.  The SPF records are placed in the DNS tree at the host
   name it pertains to, not a subdomain under it, such as is done with
   SRV records.  This is the same whether TXT RRs or SPF RRs are used.

   The example above in Section 3 might be published easily via this
   lines in a domain zone file:

      example.com.  IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
      smtp-out.example.com.  IN TXT "v=spf1 a -all"

   When publishing via TXT records, beware of other TXT records
   published there for other purposes.  They may cause problems with
   size limits (see Section 3.1.4.)

   An SPF record published at the zone cut for the domain will be used
   as a default for all subdomains within the zone (See Section 4.5.)
   Domain owners SHOULD publish SPF records for hosts used for the HELO
   and MAIL FROM identities instead of using the zone cut default
   because the fallback requires additional DNS lookups.  The zone cut
   default does reduce the need to publish SPF records for non-email
   related hosts, such as www.example.com.

3.1.1  DNS Resource Record Types

   This document defines a new DNS Resource Record (RR) of type SPF,
   type code to be determined.  The format of this type is identical to
   the TXT RR [RFC1035].  For either type, the character content of the
   record is encoded as US-ASCII.

   It is recognized that the current practice (using a TXT record) is

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   not optimal, but it is necessary because there are a number of DNS
   server and resolver implementations in common use that cannot handle
   the new RR type.  The two record type scheme provides a forward path
   to the better solution of using a RR type reserved for this purpose.

   An SPF compliant domain name SHOULD have SPF records of both RR
   types.  A compliant domain name MUST have a record of at least one
   type.  If a domain has records of both types, they MUST have
   identical content.  For example, instead of just publishing one
   record as in Section 3.1 above, it is better to publish:

      example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
      example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"

   An SPF compliant check SHOULD lookup both types.  Lookups can be
   performed serially or in parallel.  If both types of records are
   obtained for a domain, the SPF type MUST be used.

   Example RRs in this document are shown with the TXT record type,
   however they could also be published with both RR types.

3.1.2  Multiple Records

   A domain name MUST NOT have multiple records that would cause an
   authorization check to select more than one record.  See Section 4.5
   for the selection rules.

3.1.3  Multiple Strings

   A text DNS record (either TXT and SPF RR types) can be composed of
   more than one string.  If a published record contains multiple
   strings, then the record MUST be treated as if those strings are
   concatenated together without adding spaces.  For example:

      IN TXT "v=spf1 ....  first" "second string..."

   MUST be treated as equivalent to

      IN TXT "v=spf1 ....  firstsecond string..."

   SPF or TXT records containing multiple strings are useful in order to
   construct longer records which would otherwise exceed the maximum
   length of a string within a TXT or SPF RR record.

3.1.4  Record Size

   The published SPF record for a given domain name SHOULD remain small
   enough that the results of a query for it will fit within 512 octets.

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   This will keep even older DNS implementations from falling over to
   TCP.  Since the answer size is dependent on many things outside the
   scope of this document, it is only possible to give this guideline:
   If the combined length of the DNS name and the text of all the
   records of a given type (TXT or SPF) is under 450 characters, then
   DNS answers should fit in UDP packets.  Note that when computing the
   sizes for queries of the TXT format, one must take into account any
   other TXT records published at the domain name.  Records that are too
   long to fit in a single UDP packet MAY be silently ignored.

3.1.5  Wildcard Records

   Use of wildcard records for publishing is not recommended.  Care must
   be taken if wildcard records are used.  If a domain publishes
   wildcard MX records, it may want to publish wildcard declarations,
   subject to the same requirements and problems.  In particular, the
   declaration must be repeated for any host that has any RR records at
   all, and for subdomains thereof.  For example, the example given in
   [RFC1034], Section 4.3.3, could be extended with:

       X.COM.          MX      10      A.X.COM
       X.COM.          TXT     "v=spf1 a:A.X.COM -all"

       *.X.COM.        MX      10      A.X.COM
       *.X.COM.        TXT     "v=spf1 a:A.X.COM -all"

       A.X.COM.        A
       A.X.COM.        MX      10      A.X.COM
       A.X.COM.        TXT     "v=spf1 a:A.X.COM -all"

       *.A.X.COM.      MX      10      A.X.COM
       *.A.X.COM.      TXT     "v=spf1 a:A.X.COM -all"

   Notice that SPF records must be repeated twice for every name within
   the domain: Once for the name, and once with a wildcard to cover the
   tree under the name.

   Use of wildcards is discouraged in general as they cause every name
   under the domain to exist and queries against arbitrary names will
   never return RCODE 3 (Name Error).

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4.  The check_host() Function

   The check_host() function fetches SPF records, parses them, and
   interprets them to evaluate if a particular host is or is not
   permitted to send mail with a given identity.  Mail receivers that
   perform this check MUST correctly evaluate the check_host() function
   as described here.

   Implementations MAY use a different algorithm than the canonical
   algorithm defined here, so long as the results are the same.

4.1  Arguments

   The function check_host() takes these arguments:

   <ip>     - the IP address of the SMTP client that is injecting the
            mail, either IPv4 or IPv6.
   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
   <sender> - the "MAIL FROM" or "HELO" identity.

   The domain portion of <sender> will usually be the same as the
   <domain> argument when check_host() is initially evaluated.  However,
   it will generally not be true for recursive evaluations (see Section
   5.2 below).

   Actual implementations of the check_host() function will likely need
   additional arguments.

4.2  Results

   The function check_host() can result in one of seven results
   described in Section 2.5.  Based on the result, the action to be
   taken is determined by the local policies of the receiver.

4.3  Initial Processing

   If the <domain> is malformed or is not a fully qualified domain name,
   check_host() immediately returns the result "None".

   If the <sender> has no localpart, substitute the string "postmaster"
   for the localpart.

4.4  Record Lookup

   In accordance with how the records are published, see Section 3.1
   above, a DNS query needs to be made for the <domain> name, querying
   for either RR type TXT, SPF or both.

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   If the DNS lookup returns a server failure (RCODE 2), or other error
   (RCODE other than 0 or 3), or the query times out, check_host() exits
   immediately with the result "TempError"

4.5  Selecting Records

   Records begin with a version section:

   record           = version terms *SP
   version          = "v=spf1"

   Starting with the set of records that were returned by the lookup,
   record selection proceeds in two 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 a version section of exactly
       "v=spf1" are discarded.  Note that the version section is
       terminated either by a SP character or the end of the record.  A
       record with a version section of "v=spf10" does not match and
       must be discarded.

   After the above steps, there should be exactly one record remaining
   and evaluation can proceed.  If there are two or more records
   remaining, then check_host() exits immediately with the result of

   If no matching records are returned for the <domain;>, the SPF client
   MUST find the Zone Cut as defined in [RFC2181] section 6 and repeat
   the above steps.  The <domain>'s zone origin is then searched for SPF
   records.  If an SPF record is found at the zone origin, the <domain>
   is set to the zone origin as if a "redirect" modifier was executed.

   If no matching records are returned for either search, an SPF client
   MUST assume that the domain makes no SPF declarations.  SPF
   processing MUST abort and return "None".

4.6  Record Evaluation

   After one SPF record has been selected, the check_host() function
   parses and interprets it to find a result for the current test.  If
   there are any syntax errors, check_host() returns immediately with
   the result "PermError".

   Implementations MAY choose to parse the entire record first and
   return "PermError" if the record is not syntactically well formed.
   However, in all cases, any syntax errors anywhere in the record MUST
   be detected.

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4.6.1  Term Evaluation

   There are two types of terms: mechanisms and modifiers.  A record
   contains an ordered list of these as specified in the following ABNF.

   terms            = *( 1*SP ( directive / modifier ) )

   directive        = [ prefix ] mechanism
   prefix           = "+" / "-" / "?" / "~"
   mechanism        = ( all / include
                      / A / MX / PTR / IP4 / IP6 / exists )

   modifier         = redirect / explanation / unknown-modifier
   unknown-modifier = name "=" macro-string

   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )

   Most mechanisms allow a ":" or "/" character after the name.

   Modifiers always contain an equals ('=') character immediately after
   the name, and before any ":" or "/" characters that may be part of
   the macro-string.

   Terms that do not contain any of "=", ":" or "/" are mechanisms.

   As per the definition of the ABNF notation in [RFC2234], mechanism
   and modifier names are case-insensitive.

4.6.2  Mechanisms

   Each mechanism is considered in turn from left to right.  If there
   are no more mechanisms, the result is specified in Section 4.7.

   When a mechanism is evaluated, one of three things can happen: it can
   match, it can not match, or it can throw an exception.

   If it matches, processing ends and the prefix value is returned as
   the result of that record.  If it does not match, processing
   continues with the next mechanism.  If it throws an exception,
   mechanism processing ends and the exception value is returned.

   The possible prefixes, and the results they return are:
   "+" Pass
   "-" Fail
   "~" SoftFail

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   "?" Neutral

   The prefix is optional and defaults to "+".

   When a mechanism matches, and the prefix is "-" so that a "Fail"
   result is returned and the explanation string is computed as
   described in Section 6.2.

   Specific mechanisms are described in Section 5.

4.6.3  Modifiers

   Modifiers are not mechanisms: they do not return match or not-match.
   Instead they provide additional information.  While modifiers do not
   directly effect the evaluation of the record, the "redirect" modifier
   has an effect after all the mechanisms have been evaluated.

4.7  Default Result

   If none of the mechanisms match and there is no "redirect" modifier,
   then the check_host() returns a result of "Neutral".  If there is a
   "redirect" modifier, check_host() proceeds as defined in Section 6.1.

   Note that records SHOULD always either use a "redirect" modifier or
   an "all" mechanism to explicitly terminate processing.

   For example:

      v=spf1 +mx -all
      v=spf1 +mx redirect=_spf.example.com

4.8  Domain Specification

   Several of these mechanisms and modifiers have a <domain-spec>
   section.  The <domain-spec> string is macro expanded (see Section 8).
   The resulting string is the common presentation form of a fully
   qualified DNS name: A series of labels separated by periods.  This
   domain is called the <target-name> in the rest of this document.

   Note: The result of the macro expansion is not subject to any further
   escaping.  Hence, this facility cannot produce all characters that
   are legal in a DNS label (e.g.  the control characters).  However,
   this facility is powerful enough to express legal host names, and
   common utility labels (such as "_spf") that are used in DNS.

   For several mechanisms, the <domain-spec> is optional.  If it is not
   provided, the <domain> is used as the <target-name>.

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5.  Mechanism Definitions

   This section defines two types of mechanisms.

   Basic mechanisms contribute to the language framework.  They do not
   specify a particular type of authorization scheme.


   Designated sender mechanisms are used to designate a set of <ip>
   addresses as being permitted or not to use the <domain> for sending


   The following conventions apply to all mechanisms that perform a
   comparison between <ip> and an IP address at any point:

   If no CIDR-length is given in the directive, then <ip> and the IP
   address are compared for equality.

   If a CIDR-length is specified, then only the specified number of
   high-order bits of <ip> and the IP address are compared for equality.

   When any mechanism fetches host addresses to compare with <ip>, when
   <ip> is an IPv4 address, A records are fetched, when <ip> is an IPv6
   address, AAAA records are fetched.  Even if the SMTP connection is
   via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513] section
   2.5.5) MUST still be considered an IPv4 address.

   Several mechanisms rely on information fetched from DNS.  For these
   DNS queries, except where noted, if the DNS server returns an error
   (RCODE other than 0 or 3) or the query times out, the mechanism
   throws the exception "TempError".  If the server returns "domain does
   not exist" (RCODE 3), then evaluation of the mechanism continues as
   if the server returned no error (RCODE 0) and zero answer records.

5.1  "all"

   all              = "all"

   The "all" mechanism is a test that always matches.  It is used as the

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   rightmost mechanism in a record to provide an explicit default.

   For example:
      v=spf1 a mx -all

   Mechanisms after "all" will never be tested.  Any "redirect" modifier
   (Section 6.1) has no effect when there is an "all" mechanism.

5.2  "include"

   include          = "include"  ":" domain-spec

   The "include" mechanism triggers a recursive evaluation of
   check_host().  The domain-spec is expanded as per Section 8.  Then
   check_host() is evaluated with the resulting string as the <domain>.
   The <ip> and <sender> arguments remain the same as in the current
   evaluation of check_host().

   In hind sight, the name "include" was poorly chosen.  Only the
   evaluated results of the referenced SPF record is used, rather than
   acting as if the referenced SPF record was literally included in the
   first.  Better names for this mechanism would have been something
   like "on-pass" or "if-pass".

   The "include" mechanism makes it possible for one domain to designate
   multiple administratively independent domains.  For example, a vanity
   domain "example.net" might send mail using the servers of
   administratively independent domains example.com and example.org.

   Example.net could say

      "v=spf1 include:example.com include:example.org -all".

   That would direct check_host() to, in effect, check the records of
   example.com and example.org for a "pass" result.  Only if the host
   were not permitted for either of those domains would the result be

   Whether this mechanism matches or not, or throws an error depends on
   the result of the recursive evaluation of check_host():

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   | A recursive check_host() result | Causes the "include" mechanism  |
   | of:                             | to:                             |
   | Pass                            | match                           |
   |                                 |                                 |
   | Fail                            | not match                       |
   |                                 |                                 |
   | SoftFail                        | not match                       |
   |                                 |                                 |
   | Neutral                         | not match                       |
   |                                 |                                 |
   | TempError                       | throw TempError                 |
   |                                 |                                 |
   | PermError                       | throw PermError                 |
   |                                 |                                 |
   | None                            | throw PermError                 |

   The "include" mechanism is intended for crossing administrative
   boundaries.  While it is possible to use includes to consolidate
   multiple domains that share the same set of designated hosts, domains
   are encouraged to use redirects where possible, and to minimize the
   number of includes within a single administrative domain.  For
   example, if example.com and example.org were managed by the same
   entity, and if the permitted set of hosts for both domains were
   "mx:example.com", it would be possible for example.org to specify
   "include:example.com", but it would be preferable to specify
   "redirect=example.com" or even "mx:example.com".

5.3  "a"

   This mechanism matches if <ip> is one of the <target-name>'s IP

   A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]

   An address lookup is done on the <target-name>.  The <ip> is compared
   to the returned address(es).  If any address matches, the mechanism

5.4  "mx"

   This mechanism matches if <ip> is one of the MX hosts for a domain

   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]

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   check_host() first performs an MX lookup on the <target-name>.  Then
   it performs an address lookup on each MX name returned.  The <ip> is
   compared to each returned IP address.  To prevent DoS attacks, a
   limit of 10 MX names MUST be enforced (see Section 10).  If any
   address matches, the mechanism matches.

   Note regarding implicit MXes: If the <target-name> has no MX records,
   check_host() MUST NOT pretend the target is its single MX, and MUST
   NOT default to an A lookup on the <target-name> directly.  This
   behavior breaks with the legacy "implicit MX" rule.  See [RFC2821]
   Section 5.  If such behavior is desired, the publisher should specify
   an "a" directive.

5.5  "ptr"

   This mechanism tests if the DNS reverse mapping for <ip> exists and
   correctly points to a domain name within a particular domain.

   PTR              = "ptr"    [ ":" domain-spec ]

   First the <ip>'s name is looked up using this procedure: perform a
   DNS reverse-mapping for <ip>, looking up the corresponding PTR record
   in "in-addr.arpa." if the address is an IPv4 one and in "ip6.arpa."
   if it is an IPv6 address.  For each record returned, validate the
   domain name by looking up its IP address.  To prevent DoS attacks, a
   limit of 10 PTR names MUST be enforced (see Section 10).  If <ip> is
   among the returned IP addresses, then that domain name is validated.
   In pseudocode:

   sending-domain_names := ptr_lookup(sending-host_IP);
   if more than 10 sending-domain_names are found, use at most 10.
   for each name in (sending-domain_names) {
     IP_addresses := a_lookup(name);
     if the sending-domain_IP is one of the IP_addresses {
       validated-sending-domain_names += name;

   Check all validated domain names to see if they end in the
   <target-name> domain.  If any do, this mechanism matches.  If no
   validated domain name can be found, or if none of the validated
   domain names end in the <target-name>, this mechanism fails to match.
   If a DNS error occurs while doing the PTR RR lookup, then this
   mechanism fails to match.  If a DNS error occurs while doing an A RR
   lookup, then that domain name is skipped and the search continues.

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   for each name in (validated-sending-domain_names) {
     if name ends in <domain-spec>, return match.
     if name is <domain-spec>, return match.
   return no-match.

   This mechanism matches if the <target-name> is either an ancestor of
   a validated domain name, or if the <target-name> and a validated
   domain name are the same.  For example: "mail.example.com" is within
   the domain "example.com", but "mail.bad-example.com" is not.

   Note: Use of this mechanism is discouraged because it is slow, is not
   as reliable as other mechanisms in cases of DNS errors and it places
   a large burden on the arpa name servers.  If used, proper PTR records
   must be in place for the domain's hosts and the "ptr" mechanism
   should be one of the last mechanisms checked.

5.6  "ip4" and "ip6"

   These mechanisms test if <ip> is contained within a given IP network.

   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]

   ip4-cidr-length  = "/" 1*DIGIT
   ip6-cidr-length  = "/" 1*DIGIT
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]

   ip4-network      = ; as per conventional dotted quad notation,
             ; e.g.
   ip6-network      = ; as per [RFC 3513], section 2.2,
             ; e.g. 2001:DB8::CD30

   The <ip> is compared to the given network.  If CIDR-length high-order
   bits match, the mechanism matches.

   If ip4-cidr-length is omitted it is taken to be "/32".  If
   ip6-cidr-length is omitted it is taken to be "/128".  It is not
   permitted to omit parts of the IP address instead of using CIDR
   notations.  That is, use instead of 10.23.45.

5.7  "exists"

   This mechanism is used to construct an arbitrary domain name that is
   used for a DNS A record query.  It allows for complicated schemes
   involving arbitrary parts of the mail envelope to determine what is

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   exists           = "exists"   ":" domain-spec

   The domain-spec is expanded as per Section 8.  The resulting domain
   name is used for a DNS A RR lookup.  If any A record is returned,
   this mechanism matches.  The lookup type is 'A' even when the
   connection type is IPv6.

   Domains can use this mechanism to specify arbitrarily complex
   queries.  For example, suppose example.com publishes the record:

      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all

   The <target-name> might expand to
   "".  This makes fine-grained
   decisions possible at the level of the user and client IP address.

   This mechanism enables queries that mimic the style of tests that
   existing DNSBL lists use.

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6.  Modifier Definitions

   Modifiers are name/value pairs that provide additional information.
   Modifiers always have an "=" separating the name and the value.

   The modifiers defined in this document ("redirect" and "exp") MAY
   appear anywhere in the record, but SHOULD appear at the end, after
   all mechanisms.  Ordering of these two modifiers does not matter.
   These modifiers MUST NOT appear in a record more than once each.  If
   they do, then check_host() exits with a result of "PermError".

   Unrecognized modifiers SHOULD be ignored no matter where in a record,
   nor how often.  This allows implementations of this document to
   handle records with modifiers that are defined in other

6.1  redirect: Redirected Query

   If all mechanisms fail to match, and a "redirect" modifier is
   present, then processing proceeds as follows.

   redirect         = "redirect" "=" domain-spec

   The domain-spec portion of the redirect section is expanded as per
   the macro rules in Section 8.  Then check_host() is evaluated with
   the resulting string as the <domain>.  The <ip> and <sender>
   arguments remain the same as current evaluation of check_host().

   The result of this new evaluation of check_host() is then considered
   the result of the current evaluation.

   Note that the newly queried domain may itself specify redirect

   This facility is intended for use by organizations that wish to apply
   the same record to multiple domains.  For example:

     la.example.com. TXT "v=spf1 redirect=_spf.example.com"
     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
   _spf.example.com. TXT "v=spf1 mx:example.com -all"

   In this example, mail from any of the three domains is described by
   the same record.  This can be an administrative advantage.

   Note: In general, the domain "A" cannot reliably use a redirect to
   another domain "B" not under the same administrative control.  Since
   the <sender> stays the same, there is no guarantee that the record at

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   domain "B" will correctly work for addresses in domain "A",
   especially if domain "B" uses mechanisms involving localparts.  An
   "include" directive may be more appropriate.

   For clarity it is RECOMMENDED that any "redirect" modifier appear as
   the very last term in a record.

6.2  exp: Explanation

   explanation      = "exp" "=" domain-spec

   If check_host() results in a "Fail" due to a mechanism match (such as
   "-all"), and the "exp" modifier is present, then the explanation
   string returned is computed as described below.  If no "exp" modifier
   is present, then either a default explanation string or an empty
   explanation string may be returned.

   The <domain-spec> is macro expanded (see Section 8) and becomes the
   <target-name>.  The DNS TXT record for the <target-name> is fetched.

   If <domain-spec> is empty, or there are any processing errors (any
   RCODE other than 0), or if no records are returned, or if more than
   one record is returned, then proceed as if no exp modifier was given.

   The fetched TXT record's strings are concatenated with no spaces, and
   then treated as an <explain-string> which is macro-expanded.  This
   final result is the explanation string.

   Software evaluating check_host() can use this string to communicate
   information from the publishing domain in the form of a short message
   or URL.  Software should make it clear that the explanation string
   comes from a third party.  For example, it can prepend the macro
   string "%{o} explains: " to the explanation.

   Implementations MAY limit the length of the resulting explanation
   string to allow for other protocol constraints and/or reasonable
   processing limits.  The SPF client SHOULD make it clear when an
   explanation string is coming from a third party, such as shown in
   Section 2.5.4.

   Suppose example.com has this record

      v=spf1 mx -all exp=explain._spf.%{d}

   Here are some examples of possible explanation TXT records at

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      Example.com mail should only be sent by its own servers.

         -- a simple, constant message

      %{i} is not one of %{d}'s designated mail servers.

         -- a message with a little more info, including the IP address
            that failed the check

      See http://%{d}/why.html?s=%{S}&i=%{I}

         -- a complicated example that constructs a URL with the
            arguments to check_host() so that a web page can be
            generated with detailed, custom instructions

   Note: During recursion into an "include" mechanism, exp= modifiers do
   not propagate out.  In contrast, during execution of a "redirect"
   modifier, the explanation string from the target of the redirect is

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7.  Miscellaneous

7.1  Processing Limits

   During processing, an evaluation of check_host() may require
   additional evaluations of check_host() due to the "include" mechanism
   and/or the "redirect" modifier.

   In order to prevent Denial-of-Service (DoS) attacks, the total number
   of DNS lookups must be limited.  The subject of a DoS attack can be
   either the SPF client directly, the domain owner of the claimed
   sender, or some third party domain that is referenced in the SPF

   Of these, the case of a third party referenced in the SPF record is
   the easiest for a DoS attack to effectively exploit.  For example, a
   malicious person could create an SPF record with many references to a
   victim domain, send many e-mails to different SPF clients and the SPF
   clients would create a DoS attack.  In effect, the SPF clients are
   being used to amplify the attacker's bandwidth by using fewer bytes
   in the SMTP session than is generated by the DNS queries.  Using SPF
   clients also allows the attacker to hide the true source of the

   As a result, limits that may seem reasonable for an individual mail
   server can still allow an unreasonable amount of bandwidth
   amplification.  Therefore the processing limits need to be quite

   SPF implementations MUST limit the number of mechanism that do DNS
   lookups to at most 10, if this number is exceeded, a PermError MUST
   be returned.  The mechanisms that count against this limit are
   "include", "a", "mx", "ptr", "exists" and the "redirect" modifier.
   The "all", "ip4" and "ip6" mechanisms do not require DNS lookups and
   therefore do not count against this limit.  The "exp" modifier
   requires a DNS lookup, but it is not counted as it is used only in
   the case of errors.

   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
   there MUST be a limit of no more than 10 MX or PTR RRs looked up and

   SPF implementation SHOULD limit the total amount of data obtained
   from the DNS queries.  For example, when DNS over TCP or EDNS0 are
   available, there may need to be an explicit limit to how much data
   will be accepted to prevent excessive bandwidth usage or memory
   usage, and DoS attacks.

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   Implementations must be prepared to handle records that are set up
   incorrectly or maliciously.

   MTAs or other processors MAY also impose a limit on the maximum
   amount of elapsed time to evaluate check_host().  Such a limit SHOULD
   allow at least 20 seconds.  If such a limit is exceeded, the result
   of authentication SHOULD be "TempError".

   Domains publishing records SHOULD try to keep the number of "include"
   mechanisms and chained "redirect" modifiers to a minimum.  Domains
   SHOULD also try to minimize the amount of other DNS information
   needed to evaluate a record.  This can be done by choosing directives
   that require less DNS information and placing lower cost mechanisms
   earlier in the SPF record.

   For example, consider a domain set up as:

   example.com.      IN MX   10 mx.example.com.
   mx.example.com.   IN A
   a.example.com.    IN TXT  "v=spf1 mx:example.com -all"
   b.example.com.    IN TXT  "v=spf1 a:mx.example.com -all"
   c.example.com.    IN TXT  "v=spf1 ip4: -all"

   Evaluating check_host() for the domain "a.example.com" requires the
   MX records for "example.com", and then the A records for the listed
   hosts.  Evaluating for "b.example.com" only requires the A records.
   Evaluating for "c.example.com" requires none.

   However, there may be administrative considerations: Using "a" over
   "ip4" allows hosts to be renumbered easily.  Using "mx" over "a"
   allows the set of mail hosts to be changed easily.

7.2  The Received-SPF header

   It is RECOMMENDED that SMTP receivers record the result of SPF
   processing in the message headers.  If an SMTP receiver chooses to do
   so, it SHOULD use the "Received-SPF" header defined here.  This
   information is intended for the recipient.  (Information intended for
   the sender described in Section 6.2, Explanation.)

   The Received-SPF header is a trace field (See [RFC2822] section
   3.6.7) and SHOULD be prepended to existing headers, above the
   Received: header that is generated by the SMTP receiver.  It MUST
   appear above any other Received-SPF headers in the message.  The
   header has the format:

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   header           = "Received-SPF:" [CFWS] result [CFWS]
                      [ key-value-list ]

   result           = "Pass" / "Fail" / "TempError" / "SoftFail" /
                      "Neutral" / "None" / "PermError"

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )

   key-value-pair   = name [CFWS] "=" ( dot-atom / quoted-string )

   dot-atom         = ; unquoted word as per [RFC2822]

   quoted-string    = ; quoted string as per [RFC2822]

   CFWS             = ; comment or folding white space as per [RFC2822]

   The <comment-string> should convey supporting information for the
   result, such as <ip>, <sender> and <domain>.

   SPF clients may append zero or more of the following key-value-pairs
   at their discretion:

   receiver       the host name of the SPF client
   client-ip      the IP address of the SMTP client
   envelope-from  the envelope sender address
   helo           the host name given in the HELO or EHLO command
   mechanism      the mechanism that matched (if no mechanisms matched,
                  substitute the word "default".)
   problem        if an error was returned, details about the error

   Other key-value pairs may be defined by SPF clients.  Until a new key
   name becomes widely accepted, new key names should start with "x-".

   SPF clients MUST make sure that the Received-SPF header does not
   contain invalid characters, is excessively long, or contain malicious
   data that has been provided by the sender.

   Examples of various header styles that could be generated:

   Received-SPF: Pass (mybox.example.org: domain of
      myname@example.com designates as permitted sender)
         receiver=mybox.example.org; client-ip=;
         envelope-from=<myname@example.com>; helo=foo.example.com;

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   Received-SPF: Fail (mybox.example.org: domain of
                       myname@example.com does not designate
              as permitted sender)

   Received-SPF: SoftFail (mybox.example.org: domain of
                           transitioning myname@example.com discourages
                           use of as permitted sender)

   Received-SPF: Neutral (mybox.example.org: is neither
                          permitted nor denied by domain of

   Received-SPF: None (mybox.example.org: myname@example.com does
                       not designate permitted sender hosts)

   Received-SPF: PermError (mybox.example.org: domain
                            of myname@example.com used an invalid
                            SPF mechanism)

   Received-SPF: TempError (mybox.example.org: error in processing
                        during lookup of myname@example.com: DNS

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8.  Macros

8.1  Macro definitions

   Many mechanisms and modifiers perform macro interpolation on part of
   the term.

   domain-spec      = macro-string domain-end
   domain-end       = ( "." toplabel ) / macro-expand
   toplabel         = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
                      ; LDH rule (See [RFC3696])
   alphanum         = ALPHA / DIGIT
   macro-string     = *( macro-expand / macro-literal )
   explain-string   = *( macro-string / SP )
   macro-expand     = ( "%{" macro-letter transformer *delimiter "}" )
                      / "%%" / "%_" / "%-"
   macro-literal    = %x21-24 / %x26-7E
                      ; visible characters except "%"
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                      "c" / "r" / "t"
   transformer      = *DIGIT [ "r" ]
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

   A literal "%" is expressed by "%%".
      "%_" expands to a single " " space.
      "%-" expands to a URL-encoded space, viz.  "%20".

   The following macro letters are expanded in term arguments:

      s  = <sender>
      l  = local-part of <sender>
      o  = domain of <sender>
      d  = <domain>
      i  = <ip>
      p  = the validated domain name of <ip>
      v  = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is
      h  = HELO/EHLO domain

   The following macro letters are only allowed in "exp" text:

      c  = SMTP client IP (easily readable format)
      r  = domain name of host performing the check

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      t  = current timestamp

   A '%' character not followed by a '{', '%', '-', or '_' character is
   a syntax error.  So,
   is incorrect and will cause check_host() to return a "PermError".
   Instead, say

   Optional transformers are:

      *DIGIT : zero or more digits
      'r'    : reverse value, splitting on dots by default

   If transformers or delimiters are provided, the replacement value for
   a macro letter is split into parts.  After performing any reversal
   operation and/or removal of left-hand parts, the parts are rejoined
   using "." and not the original splitting characters.

   By default, strings are split on "." (dots).  Note that no special
   treatment is given to leading, trailing or consecutive delimiters,
   and so the list of parts may contain empty strings.  Macros may
   specify delimiter characters which are used instead of ".".

   The 'r' transformer indicates a reversal operation: if the client IP
   address were, the macro %{i} would expand to ""
   and the macro %{ir} would expand to "".

   The DIGIT transformer indicates the number of right-hand parts to
   use, after optional reversal.  If a DIGIT is specified, the value
   MUST be nonzero.  If no DIGITs are specified, or if the value
   specifies more parts than are available, all the available parts are
   used.  If the DIGIT was 5, and only 3 parts were available, the macro
   interpreter would pretend the DIGIT was 3.  Implementations MUST
   support at least a value of 128, as that is the maximum number of
   labels in a domain name.

   The "s" macro expands to the <sender> argument.  It is an e-mail
   address with a localpart, an "@" character, and a domain.  The "l"
   macro expands to just the localpart.  The "o" macro expands to just
   the domain part.  Note that these values remain the same during
   recursive and chained evaluations due to "include" and/or "redirect".
   Note also that if the original <sender> had no localpart, the
   localpart was set to "postmaster" in initial processing (see Section

   For IPv4 addresses, both the "i" and "c" macros expand to the
   standard dotted-quad format.

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   For IPv6 addresses, the "i" macro expands to a dot-format address; it
   is intended for use in %{ir}.  The "c" macro may expand to any of the
   hexadecimal colon-format addresses specified in [RFC3513] section
   2.2.  It is intended for humans to read.

   The "p" macro expands to the validated domain name of <ip>.  The
   procedure for finding the validated domain name is defined in Section
   5.5.  If the <domain> is present in the list of validated domains, it
   SHOULD be used.  Otherwise, if a subdomain of the <domain> is
   present, it SHOULD be used.  Otherwise, any name from the list may be
   used.  If there are no validated domain name or if a DNS error
   occurs, the string "unknown" is used.

   The "r" macro expands to the name of the receiving MTA.  This SHOULD
   be a fully qualified domain name, but if one does not exist (as when
   the checking is done by a script) or if policy restrictions dictate
   otherwise, the word "unknown" should be substituted.  The domain name
   may be different than the name found in the MX record that the client
   MTA used to locate the receiving MTA.

   The "t" macro expands to the decimal representation of the
   approximate number of seconds since the Epoch (Midnight, January 1st,
   1970, UTC).  This is the same value as returned by the POSIX time()
   function in most standards compliant libraries.

   When the result of macro expansion is used in a domain name query, if
   the expanded domain name exceeds 253 characters (the maximum length
   of a domain name), the left side is truncated to fit, by removing
   successive subdomains until the total length does not exceed 253

   Uppercased macros expand exactly as their lower case equivalents, and
   are then URL escaped.  URL escaping for the non-uric characters is
   described in [RFC2396].

   Note: Domains should avoid using the "s", "l", "o" or "h" macros in
   conjunction with any mechanism directive.  While these macros are
   powerful and allow per-user records to be published, they severely
   limit the ability of implementations to cache results of check_host()
   and they reduce the effectiveness of DNS caches.

   Implementations should be aware that if no directive processed during
   the evaluation of check_host() contains an "s", "l", "o" or "h"
   macro, then the results of the evaluation can be cached on the basis
   of <domain> and <ip> alone for as long as the shortest TTL of all the
   DNS records involved.

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8.2  Expansion Examples

      The <sender> is strong-bad@email.example.com.
      The IPv4 SMTP client IP is
      The IPv6 SMTP client IP is 5f05:2000:80ad:5800::1.
      The PTR domain name of the client IP is mx.example.org.

   macro                       expansion
   -------  ----------------------------
   %{s}     strong-bad@email.example.com
   %{o}                email.example.com
   %{d}                email.example.com
   %{d4}               email.example.com
   %{d3}               email.example.com
   %{d2}                     example.com
   %{d1}                             com
   %{dr}               com.example.email
   %{d2r}                  example.email
   %{l}                       strong-bad
   %{l-}                      strong.bad
   %{lr}                      strong-bad
   %{lr-}                     bad.strong
   %{l1r-}                        strong

   macro-string                                               expansion
   %{lr-}.lp._spf.%{d2}                  bad.strong.lp._spf.example.com





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9.  Implications

   This section outlines the major implications that adoption of this
   document will have on various entities involved in Internet e-mail.
   It is intended to make clear to the reader where this document
   knowingly affects the operation of such entities.  This section is
   not a "how-to" manual, nor a "best practices" document, and is not a
   comprehensive list of what such entities should do in light of this

   This section is non-normative.

9.1  Sending Domains

   Domains that wish to be compliant with this specification will need
   to determine the list of hosts that they allow to use their domain
   name in the "HELO" and "MAIL FROM" identities.  It is recognized that
   forming such a list is not just a simple technical exercise, but
   involves policy decisions with both technical and administrative

   It can be helpful to publish records that include a "tracking
   exists:" mechanism.  By looking at the name server logs, an
   incompletely list may be generated.  For example:
      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all

9.2  Mailing Lists

   Mailing lists must be aware of how they re-inject mail that is sent
   to the list.  Mailing lists MUST comply with the requirement in
   [RFC2821] Section 3.10 and [RFC1123] Section 5.3.6 that say that the
   reverse-path MUST be changed to be the address of a person or other
   entity who administers the list.  While the reasons for changing the
   reverse-path are many and long standing, SPF adds enforcement to this

   In practice, almost all mailing list software in use already complies
   with this requirement.  Mailing lists that do not comply, may or may
   not encounter problems depending on how access to the list is
   restricted.  Such lists that are entirely internal to a domain (only
   people in the domain can send to or receive from the list) are not

9.3  Forwarding Services and Aliases

   Forwarding services take mail that is received at a mailbox and
   direct it to some external mailbox.  At the time of this writing, the
   near-universal practice of such services is to use the original MAIL

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   FROM of a message when re-injecting it for delivery to the external
   mailbox.  [RFC1123] and [RFC2821] describe this action as an "alias"
   rather than a "mail list".  This means the external mailbox's MTA
   sees all such mail in a connection from a host of the forwarding
   service, and so the "MAIL FROM" identity will not in general pass

   There are several possible ways that this authorization failure can
   be ameliorated.  If the owner of the external mailbox wishes to trust
   the forwarding service, they can direct the external mailbox's MTA to
   skip such tests when the client host belongs to the forwarding
   service.  Tests against some other identity may also be used to
   override the test against the "MAIL FROM" identity.

   For larger domains, it may not be possible to have a complete or
   accurate list of forwarding services used by the owners of the
   domain's mailboxes.  In such cases, white lists of generally
   recognized forwarding services could be employed.

   Forwarding services can also solve the problem by using MAIL FROM
   that contain their own domain.  This means that mail bounced from the
   external mailbox will have to be re-bounced by the forwarding
   service.  Various schemes to do this exist though they vary widely in
   complexity and resource requirements on the part of the forwarding
   service.  Several popular MTAs can change "alias" semantics to
   "mailing list" semantics by including an adding another alias with
   "owner-" added to the beginning of the alias name.  (e.g.  an alias
   of "friends: george@example.com, fred@example.org" would need another
   alias of the form "owner-friends: localowner")

9.4  Mail Services

   Entities that offer mail services to other domains such as sending of
   bulk mail will may have to alter their mail in light of the
   authorization check in this document.  If the MAIL FROM used for such
   e-mail uses the domain of the mail service provider, then the
   provider needs only to ensure that their sending host is authorized
   by their own SPF record, if any.

   If the MAIL FROM does not use the mail service provider's domain,
   then extra care must be taken.  The SPF record format has several
   options for authorizing the sending MTAs of another domain (the
   service provider's)

9.5  MTA Relays

   The authorization check generally precludes the use of arbitrary MTA
   relays between sender and receiver of an e-mail message.

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   Within an organization, MTA relays can be effectively deployed.
   However, for purposes of this document, such relays are effectively
   invisible.  The "MAIL FROM" identity authorization check is a check
   between border MTAs.

   For mail senders, this means that published SPF records must
   authorize any MTAs that actually send across the Internet.  Usually,
   these are just the border MTAs as internal MTAs simply forward mail
   to these MTAs for delivery.

   Mail receivers will generally want to perform the authorization check
   at the border MTAs.  This allows mail that fails to be rejected
   during the SMTP session rather than bounced.  Internal MTAs then do
   not perform the authorization test.  To perform the authorization
   test other than at the border, the host that first transferred the
   message to the organization must be determined, which can be
   difficult to extract from headers.  Testing other than at the border
   is not recommended.

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10.  Security Considerations

   The "MAIL FROM" and "HELO" identity authorizations must not be
   construed to provide more assurance than it does.  It is entirely
   possible for a malicious sender to inject a message using their own
   domain in the identities used by SPF, to have that domain's SPF
   record authorize the sending host, and yet the message content can
   easily claim other identities in the headers.  Unless the user, or
   the MUA takes care to note that the authorized identity does not
   match the other, more commonly presented identities (such as the
   From: header), the user may be lulled into a false sense of security.

   There are two aspects of this protocol that malicious parties could
   exploit to undermine the validity of the check_host() function:

      The evaluation of check_host() relies heavily on DNS.  A malicious
      attacker could attack the DNS infrastructure and cause
      check_host() to see spoofed DNS data, and then return incorrect
      results.  This could include returning "Pass" for an <ip> value
      where the actual domain's record would evaluate to "Fail".  See
      [RFC3833] for a description of the DNS weaknesses.
      The client IP address, <ip>, is assumed to be correct.  A
      malicious attacker could spoof TCP sequence numbers to make mail
      appear to come from a permitted host for a domain that the
      attacker is impersonating.

   As with most aspects of mail, there are a number of ways that
   malicious parties could use the protocol as an avenue of a
   Denial-of-Service (DoS) attack.  The processing limits outlined in
   Section 7.1 are designed to prevent attacks such as:

      Malicious parties could create SPF records that make many
      references to the target's domain and then send large volume mail
      to other SPF clients that use this SPF record.  These legitimate
      machines would then present a DNS load on the target as they
      fetched the relevant DNS references.
      While implementations of check_host() need to limit the number of
      DNS lookups, malicious domains could publish records exceed these
      limits in an attempt to waste computation effort at their targets
      when they send them mail.  Malicious domains could also design SPF
      records that cause excessive memory or CPU usage.
      Malicious parties could send large volume mail purporting to come
      from the intended target to a wide variety of legitimate mail
      hosts.  These legitimate machines would then present a DNS load on
      the target as they fetched the relevant records.

   When the authorization check fails, an explanation string may be
   included in the reject response.  Both the sender and the rejecting

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   receiver need to be aware that the explanation was determined by the
   publisher of the SPF record checked and, in general, not the
   receiver.  The explanation may contain URLs that may be malicious,
   offensive and/or have misleading text.  This is probably less of a
   concern than it may initially seem since such messages are returned
   to the sender, and the source is the SPF record published by the
   domain in the identity claimed by that very sender.  To put it
   another way, the only people who see malicious explanation strings
   are people whose messages claim to be from domains that publish such
   strings in their SPF records.

   SPF uses information supplied by third parties, such as the HELO
   domain name, the MAIL FROM and SPF records.  This information is then
   sent to the receiver in the Received-SPF: mail headers and possibly
   returned to the client MTA in the form of an SMTP rejection message.
   This information must be checked for invalid characters and
   excessively long lines.

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11.  IANA Considerations

   The IANA needs to assign a new Resource Record Type and Qtype from
   the DNS Parameters Registry for the SPF RR type.

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12.  Contributors and Acknowledgements

   This document is largely based on the work of Meng Weng Wong and Mark
   Lentczner.  Mark is not listed as an author by his request.  While,
   as this section acknowledges, many people have contributed to this
   document, a very large portion of the writing and editing are due to
   Meng and Mark.

   This design owes a debt of parentage to [RMX] by Hadmut Danisch and
   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
   the legitimacy of an e-mail address traces its ancestry farther back
   through messages on the namedroppers mailing list by Paul Vixie
   [Vixie] (based on suggestion by Jim Miller) and by David Green

   Philip Gladstone contributed macros to the specification, multiplying
   the expressiveness of the language and making per-user and per-IP
   lookups possible.

   The authors would also like to thank the literally hundreds of
   individuals who have participated in the development of this design.
   There are far too numerous to name, but they include:

      The folks on the spf-discuss mailing list.
      The folks on the SPAM-L mailing list.
      The folks on the IRTF ASRG mailing list.
      The folks on the IETF MARID mailing list.
      The folks on #perl.

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13.  References

13.1  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2034]  Freed, N., "SMTP Service Extension for Returning Enhanced
              Error Codes", RFC 2034, October 1996.

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

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

13.2  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983, August

   [RFC2162]  Allocchio, C., "MaXIM-11 - Mapping between X.400 /
              Internet mail and Mail-11 mail", RFC 2162, January 1998.

   [RFC3696]  Klensin, J., "Application Techniques for Checking and
              Transformation of Names", RFC 3696, February 2004.

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   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, August 2004.

   [RMX]      Danish, H., "The RMX DNS RR Type for light weight sender
              authentication", October 2003.

              Work In Progress

   [DMP]      Fecyk, G., "Designated Mailers Protocol", December 2003.

              Work In Progress

   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.

   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.

Authors' Addresses

   Meng Weng Wong

   EMail: mengwong+spf@pobox.com
   URI:   http://spf.pobox.com/

   Wayne Schlitt
   4615 Meredeth #9
   Lincoln Nebraska, NE  68506
   United States of America

   EMail: wayne@schlitt.net
   URI:   http://www.schlitt.net/spf/

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Appendix A.  Collected ABNF

   This section is normative and any discrepancies with the ABNF
   fragments in the preceding text are to be resolved in favor of this

   See [RFC2234] for ABNF notation.  Please note that as per this ABNF
   definition, literal text strings (those in quotes) are
   case-insensitive.  Hence, "mx" matches "mx", "MX", "mX" and "Mx".

   record           = version terms *SP
   version          = "v=spf1"

   terms            = *( 1*SP ( directive / modifier ) )

   directive        = [ prefix ] mechanism
   prefix           = "+" / "-" / "?" / "~"
   mechanism        = ( all / include
                      / A / MX / PTR / IP4 / IP6 / exists )

   all              = "all"
   include          = "include"  ":" domain-spec
   A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
   PTR              = "ptr"    [ ":" domain-spec ]
   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
   exists           = "exists"   ":" domain-spec

   modifier         = redirect / explanation / unknown-modifier
   redirect         = "redirect" "=" domain-spec
   explanation      = "exp" "=" domain-spec
   unknown-modifier = name "=" macro-string

   ip4-cidr-length  = "/" 1*DIGIT
   ip6-cidr-length  = "/" 1*DIGIT
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]

   ip4-network      = ; as per conventional dotted quad notation,
             ; e.g.
   ip6-network      = ; as per [RFC 3513], section 2.2,
             ; e.g. 2001:DB8::CD30

   domain-spec      = macro-string domain-end
   domain-end       = ( "." toplabel ) / macro-expand
   toplabel         = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
                      ; LDH rule (See [RFC3696])
   alphanum         = ALPHA / DIGIT

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   macro-string     = *( macro-expand / macro-literal )
   explain-string   = *( macro-string / SP )
   macro-expand     = ( "%{" macro-letter transformer *delimiter "}" )
                      / "%%" / "%_" / "%-"
   macro-literal    = %x21-24 / %x26-7E
                      ; visible characters except "%"
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                      "c" / "r" / "t"
   transformer      = *DIGIT [ "r" ]
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )

   header           = "Received-SPF:" [CFWS] result [CFWS]
                      [ key-value-list ]

   result           = "Pass" / "Fail" / "TempError" / "SoftFail" /
                      "Neutral" / "None" / "PermError"

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )

   key-value-pair   = name [CFWS] "=" ( dot-atom / quoted-string )

   dot-atom         = ; unquoted word as per [RFC2822]

   quoted-string    = ; quoted string as per [RFC2822]

   CFWS             = ; comment or folding white space as per [RFC2822]

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Appendix B.  Extended Examples

   These examples are based on the following DNS setup:

   ; A domain with two mail servers, two hosts
   ; and two servers at the domain name

   $ORIGIN example.com.
   @           MX  10 mail-a
   MX  20 mail-b
   amy         A
   bob         A
   mail-a      A
   mail-b      A
   www         CNAME example.com.

   ; A related domain

   $ORIGIN example.org
   @           MX  10 mail-c
   mail-c      A

   ; The reverse IP for those addresses

   $ORIGIN 2.0.192.in-addr.arpa.
   10          PTR example.com.
   11          PTR example.com.
   65          PTR amy.example.com.
   66          PTR bob.example.com.
   129         PTR mail-a.example.com.
   130         PTR mail-b.example.com.
   140         PTR mail-c.example.org.

   ; A rogue reverse IP domain that claims to be
   ; something it's not

   $ORIGIN 0.0.10.in-addr.arpa.
   4           PTR bob.example.com.

B.1  Simple Examples

   These examples show various possible published records for
   example.com and which values if <ip> would cause check_host() to
   return "Pass".  Note that <domain> is "example.com".

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   v=spf1 +all
      -- any <ip> passes

   v=spf1 a -all
      -- hosts and pass

   v=spf1 a:example.org -all
      -- no sending hosts pass since example.org has no A records

   v=spf1 mx -all
      -- sending hosts and pass

   v=spf1 mx:example.org -all
      -- sending host passes

   v=spf1 mx mx:example.org -all
      -- sending hosts,, and pass

   v=spf1 mx/30 mx:example.org/30 -all
      -- any sending host in or passes

   v=spf1 ptr -all
      -- sending host passes (reverse IP is valid and in
      -- sending host fails (reverse IP is valid, but not in
      -- sending host fails (reverse IP is not valid)

   v=spf1 ip4: -all
      -- sending host fails
      -- sending host passes

B.2  Multiple Domain Example

   These examples show the effect of related records:

      example.org: "v=spf1 include:example.com include:example.net -all"

   This record would be used if mail from example.org actually came
   through servers at example.com and example.net.  Example.org's
   designated servers are the union of example.com and example.net's
   designated servers.

      la.example.org: "v=spf1 redirect=example.org"
      ny.example.org: "v=spf1 redirect=example.org"
      sf.example.org: "v=spf1 redirect=example.org"

   These records allow a set of domains that all use the same mail

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   system to make use of that mail system's record.  In this way, only
   the mail system's record needs to updated when the mail setup
   changes.  These domains' records never have to change.

B.3  DNSBL Style Example

   Imagine that, in addition to the domain records listed above, there
   are these:

   $Origin _spf.example.com.
   mary.mobile-users                   A
   fred.mobile-users                   A     A     A

   The following records describe users at example.com who mail from
   arbitrary servers, or who mail from personal servers.


   v=spf1 mx


   v=spf1 exists:%{l1r+}.%{d}


   v=spf1 exists:%{ir}.%{l1r+}.%{d}

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