<pre>Internet Engineering Task Force (IETF)                          W. Mills
Request for Comments: 7293                                   Yahoo! Inc.
Category: Standards Track                                   M. Kucherawy
ISSN: 2070-1721                                           Facebook, Inc.
                                                               July 2014


             <span class="h1">The Require-Recipient-Valid-Since Header Field</span>
                       <span class="h1">and SMTP Service Extension</span>

Abstract

   This document defines an extension for the Simple Mail Transfer
   Protocol (SMTP) called "RRVS" to provide a method for senders to
   indicate to receivers a point in time when the ownership of the
   target mailbox was known to the sender.  This can be used to detect
   changes of mailbox ownership and thus prevent mail from being
   delivered to the wrong party.  This document also defines a header
   field called "Require-Recipient-Valid-Since" that can be used to
   tunnel the request through servers that do not support the extension.

   The intended use of these facilities is on automatically generated
   messages, such as account statements or password change instructions,
   that might contain sensitive information, though it may also be
   useful in other applications.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7293">http://www.rfc-editor.org/info/rfc7293</a>.












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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
   <a href="#section-3">3</a>.  Description . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
     <a href="#section-3.1">3.1</a>.  The "RRVS" SMTP Extension . . . . . . . . . . . . . . . .   <a href="#page-5">5</a>
     <a href="#section-3.2">3.2</a>.  The "Require-Recipient-Valid-Since" Header Field  . . . .   <a href="#page-5">5</a>
     <a href="#section-3.3">3.3</a>.  Timestamps  . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
   <a href="#section-4">4</a>.  Use By Generators . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
   <a href="#section-5">5</a>.  Handling By Receivers . . . . . . . . . . . . . . . . . . . .   <a href="#page-7">7</a>
     <a href="#section-5.1">5.1</a>.  SMTP Extension Used . . . . . . . . . . . . . . . . . . .   <a href="#page-7">7</a>
       <a href="#section-5.1.1">5.1.1</a>.  Relays  . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-8">8</a>
     <a href="#section-5.2">5.2</a>.  Header Field Used . . . . . . . . . . . . . . . . . . . .   <a href="#page-9">9</a>
       <a href="#section-5.2.1">5.2.1</a>.  Design Choices  . . . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
     <a href="#section-5.3">5.3</a>.  Clock Synchronization . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-6">6</a>.  Relaying without RRVS Support . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-6.1">6.1</a>.  Header Field Conversion . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-7">7</a>.  Header Field with Multiple Recipients . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-8">8</a>.  Special Use Addresses . . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
     <a href="#section-8.1">8.1</a>.  Mailing Lists . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
     <a href="#section-8.2">8.2</a>.  Single-Recipient Aliases  . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
     <a href="#section-8.3">8.3</a>.  Multiple-Recipient Aliases  . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
     <a href="#section-8.4">8.4</a>.  Confidential Forwarding Addresses . . . . . . . . . . . .  <a href="#page-14">14</a>
     <a href="#section-8.5">8.5</a>.  Suggested Mailing List Enhancements . . . . . . . . . . .  <a href="#page-14">14</a>
   <a href="#section-9">9</a>.  Continuous Ownership  . . . . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-10">10</a>. Digital Signatures  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-11">11</a>. Authentication-Results Definitions  . . . . . . . . . . . . .  <a href="#page-16">16</a>
   <a href="#section-12">12</a>. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-16">16</a>
     <a href="#section-12.1">12.1</a>.  SMTP Extension Example . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>
     <a href="#section-12.2">12.2</a>.  Header Field Example . . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>
     <a href="#section-12.3">12.3</a>.  Authentication-Results Example . . . . . . . . . . . . .  <a href="#page-17">17</a>





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   <a href="#section-13">13</a>. Security Considerations . . . . . . . . . . . . . . . . . . .  <a href="#page-18">18</a>
     <a href="#section-13.1">13.1</a>.  Abuse Countermeasures  . . . . . . . . . . . . . . . . .  <a href="#page-18">18</a>
     <a href="#section-13.2">13.2</a>.  Suggested Use Restrictions . . . . . . . . . . . . . . .  <a href="#page-18">18</a>
     <a href="#section-13.3">13.3</a>.  False Sense of Security  . . . . . . . . . . . . . . . .  <a href="#page-18">18</a>
     <a href="#section-13.4">13.4</a>.  Reassignment of Mailboxes  . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
   <a href="#section-14">14</a>. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
     <a href="#section-14.1">14.1</a>.  The Tradeoff . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
     <a href="#section-14.2">14.2</a>.  Probing Attacks  . . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
     <a href="#section-14.3">14.3</a>.  Envelope Recipients  . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
     <a href="#section-14.4">14.4</a>.  Risks with Use . . . . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-15">15</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
     <a href="#section-15.1">15.1</a>.  SMTP Extension Registration  . . . . . . . . . . . . . .  <a href="#page-20">20</a>
     <a href="#section-15.2">15.2</a>.  Header Field Registration  . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
     <a href="#section-15.3">15.3</a>.  Enhanced Status Code Registration  . . . . . . . . . . .  <a href="#page-21">21</a>
     <a href="#section-15.4">15.4</a>.  Authentication Results Registration  . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-16">16</a>. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-17">17</a>. References  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
     <a href="#section-17.1">17.1</a>.  Normative References . . . . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
     <a href="#section-17.2">17.2</a>.  Informative References . . . . . . . . . . . . . . . . .  <a href="#page-23">23</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Email addresses sometimes get reassigned to a different person.  For
   example, employment changes at a company can cause an address used
   for an ex-employee to be assigned to a new employee, or a mail
   service provider (MSP) might expire an account and then let someone
   else register for the local-part that was previously used.  Those who
   sent mail to the previous owner of an address might not know that it
   has been reassigned.  This can lead to the sending of email to the
   correct address but the wrong recipient.  This situation is of
   particular concern with transactional mail related to purchases,
   online accounts, and the like.

   What is needed is a way to indicate an attribute of the recipient
   that will distinguish between the previous owner of an address and
   its current owner, if they are different.  Further, this needs to be
   done in a way that respects privacy.

   The mechanisms specified here allow the sender of the mail to
   indicate how "old" the address assignment is expected to be.  In
   effect, the sender is saying, "I know that the intended recipient was
   using this address at this point in time.  I don't want this message
   delivered to anyone else".  A receiving system can then compare this
   information against the point in time at which the address was
   assigned to its current user.  If the assignment was made later than
   the point in time indicated in the message, there is a good chance





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   the current user of the address is not the correct recipient.  The
   receiving system can then prevent delivery and, preferably, notify
   the original sender of the problem.

   The primary application is transactional mail (such as account
   information, password change requests, and other automatically
   generated messages) rather than user-authored content.  However, it
   may be useful in other contexts; for example, a personal address book
   could record the time an email address was added to it, and thus use
   that time with this extension.

   Because the use cases for this extension are strongly tied to privacy
   issues, attention to the Security Considerations (<a href="#section-13">Section 13</a>) and the
   Privacy Considerations (<a href="#section-14">Section 14</a>) is particularly important.  Note,
   especially, the limitation described in <a href="#section-13.3">Section 13.3</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Definitions</span>

   For a description of the email architecture, consult [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [<a href="#ref-KEYWORDS" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">KEYWORDS</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Description</span>

   To address the problem described in <a href="#section-1">Section 1</a>, a mail-sending client
   (usually an automated agent) needs to indicate to the server to which
   it is connecting that it expects the destination address of the
   message to have been under continuous ownership (see <a href="#section-9">Section 9</a>) since
   a specified point time.  That specified time would be the time when
   the intended recipient gave the address to the message author, or
   perhaps a more recent time when the intended recipient reconfirmed
   ownership of the address with the sender.

   Two mechanisms are defined here: an extension to the Simple Mail
   Transfer Protocol [<a href="#ref-SMTP" title="&quot;Simple Mail Transfer Protocol&quot;">SMTP</a>] and a new message header field.  The SMTP
   extension permits strong assurance of enforcement by confirming
   support at each handling step for a message and the option to demand
   support at all nodes in the handling path of the message (and
   returning of the message to the originator otherwise).  The header
   field can be used when the Message Delivery Agent (MDA) supports this
   function, but an intermediary system between the sending system and
   the MDA does not.  However, the header field does not provide the
   same strong assurance described above and is more prone to exposure
   of private information (see <a href="#section-14.1">Section 14.1</a>).





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   The SMTP extension is called "RRVS" and adds a parameter to the SMTP
   "RCPT" command that indicates the most recent point in time when the
   message author believed the destination mailbox to be under the
   continuous ownership of a specific party.  Similarly, the "Require-
   Recipient-Valid-Since" header field includes an intended recipient
   coupled with a timestamp indicating the same thing.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  The "RRVS" SMTP Extension</span>

   Extensions to SMTP are described in Section 2.2 of [<a href="#ref-SMTP" title="&quot;Simple Mail Transfer Protocol&quot;">SMTP</a>].

   The name of the extension is "RRVS", an abbreviation of "Require
   Recipient Valid Since".  Servers implementing the SMTP extension
   advertise an additional EHLO keyword of "RRVS", which has no
   associated parameters, introduces no new SMTP commands, and does not
   alter the MAIL command.

   A Message Transfer Agent (MTA) implementing RRVS can transmit or
   accept one new parameter to the RCPT command.  An MDA can also accept
   this new parameter.  The parameter is "RRVS", and the value is a
   timestamp expressed as "date-time" as defined in [<a href="#ref-DATETIME" title="&quot;Date and Time on the Internet: Timestamps&quot;">DATETIME</a>], with the
   added restriction that a "time-secfrac" MUST NOT be used.  The
   timestamp MAY optionally be followed by a semicolon character and a
   letter (known as the "no-support action"), indicating the action to
   be taken when a downstream MTA is discovered that does not support
   the extension.  Valid actions are "R" (reject; the default) and "C"
   (continue).

   Formally, the new parameter and its value are defined as follows:

       rrvs-param = "RRVS=" date-time [ ";" ( "C" / "R" ) ]

   Accordingly, this extension increases the maximum command length for
   the RCPT command by 33 characters.

   The meaning of this extension, when used, is described in
   <a href="#section-5.1">Section 5.1</a>.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  The "Require-Recipient-Valid-Since" Header Field</span>

   The general constraints on syntax and placement of header fields in a
   message are defined in "Internet Message Format" [<a href="#ref-MAIL" title="&quot;Internet Message Format&quot;">MAIL</a>].

   Using Augmented Backus-Naur Form [<a href="#ref-ABNF" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">ABNF</a>], the syntax for the field is:

     rrvs = "Require-Recipient-Valid-Since:" addr-spec ";" date-time
            CRLF




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   "date-time" is defined in <a href="#section-3.3">Section 3.3</a>, and "addr-spec" is defined in
   Section 3.4.1 of [<a href="#ref-MAIL" title="&quot;Internet Message Format&quot;">MAIL</a>].

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Timestamps</span>

   The header field version of this protocol has a different format for
   the date and time expression than the SMTP extension does.  This is
   because message header fields use a format to express date and time
   that is specific to message header fields, and this is consistent
   with that usage.

   Use of both date and time is done to be consistent with how current
   implementations typically store the timestamp and to make it easy to
   include the time zone.  In practice, granularity beyond the date may
   or may not be useful.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Use By Generators</span>

   When a message is generated whose content is sufficiently sensitive
   that an author or author's ADministrative Management Domain (ADMD),
   see [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>], wishes to protect against misdelivery using this
   protocol, it determines for each recipient mailbox on the message a
   timestamp at which it last confirmed ownership of that mailbox.  It
   then applies the SMTP extension when sending the message to its
   destination.

   In cases where the outgoing MTA does not support the extension, the
   header field defined above can be used to pass the request through
   that system.  However, use of the header field is only a "best-
   effort" approach to solving the stated goals, and it has some
   shortcomings:

   1.  The positive confirmation of support at each handling node, with
       the option to return the message to the originator when
       end-to-end support cannot be confirmed, will be unavailable;

   2.  The protocol is focused on affecting delivery (that is, the
       transaction) rather than content, and therefore use of a header
       field in the content is generally inappropriate;

   3.  The mechanism cannot be used with multiple recipients without
       unintentionally exposing information about one recipient to the
       others (see <a href="#section-7">Section 7</a>); and

   4.  There is a risk of the timestamp parameter being inadvertently
       forwarded, automatically or intentionally by the user (since user
       agents might not reveal the presence of the header field), and
       therefore exposed to unintended recipients.  (See <a href="#section-14.4">Section 14.4</a>.)



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   Thus, the header field format MUST NOT be used unless the originator
   or relay has specific knowledge that the receiving MDA or an
   intermediary MTA will apply it properly.  In any case, it SHOULD NOT
   be used for the multi-recipient case.

   Use of the header field mechanism is further restricted by the
   practices described in Section 7.2 of [<a href="#ref-SMTP" title="&quot;Simple Mail Transfer Protocol&quot;">SMTP</a>], Section 3.6.3 of
   [<a href="#ref-MAIL" title="&quot;Internet Message Format&quot;">MAIL</a>], and <a href="#section-7">Section 7</a> of this document.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Handling By Receivers</span>

   If a receiver implements this specification, then there are two
   possible evaluation paths:

   1.  The sending client uses the extension, and so there is an RRVS
       parameter on a RCPT TO command in the SMTP session, and the
       parameters of interest are taken only from there (and the header
       field, if present, is disregarded); or

   2.  The sending client does not use the extension, so the RRVS
       parameter is not present on the RCPT TO commands in the SMTP
       session, but the corresponding header field might be present in
       the message.

   When the continuous ownership test fails for transient reasons (such
   as an unavailable database or other condition that is likely
   temporary), normal transient failure handling for the message is
   applied.

   If the continuous ownership test cannot be completed because the
   necessary datum (the mailbox creation or reassignment date and time)
   was not recorded, the MDA doing the evaluation selects a date and
   time to use that is the latest possible point in time at which the
   mailbox could have been created or reassigned.  For example, this
   might be the earliest of all recorded mailbox creation/reassignment
   timestamps, or the time when the host was first installed.  If no
   reasonable substitute for the timestamp can be selected, the MDA
   rejects the message using an SMTP reply code, preferably with an
   enhanced mail system status code (see <a href="#section-15.3">Section 15.3</a>), that indicates
   the test cannot be completed.  A message originator can then decide
   whether to reissue the message without RRVS protection or find
   another way to reach the mailbox owner.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  SMTP Extension Used</span>

   For an MTA supporting the SMTP extension, the requirement is to
   continue enforcement of RRVS during the relaying process to the next
   MTA or the MDA.



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   A receiving MTA or MDA that implements the SMTP extension declared
   above and observes an RRVS parameter on a RCPT TO command checks
   whether the current owner of the destination mailbox has held it
   continuously, far enough back to include the given point in time, and
   delivers it unless that check returns in the negative.  Specifically,
   an MDA will do the following before continuing with delivery:

   1.  Ignore the parameter if the named mailbox is known to be a role
       account as listed in "Mailbox Names for Common Services, Roles
       and Functions" [<a href="#ref-ROLES" title="&quot;Mailbox Names for Common Services, Roles and Functions&quot;">ROLES</a>].

   2.  If the address is not known to be a role account, and if that
       address has not been under continuous ownership since the
       timestamp specified in the extension, return a 550 error to the
       RCPT command.  (See also <a href="#section-15.3">Section 15.3</a>.)

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Relays</span>

   An MTA that does not make mailbox ownership checks, such as an MTA
   positioned to do SMTP ingress at an organizational boundary, SHOULD
   relay the RRVS extension parameter to the next MTA or MDA so that it
   can be processed there.

   For the SMTP extension, the optional RRVS parameter defined in
   <a href="#section-5.1">Section 5.1</a> indicates the action to be taken when relaying a message
   to another MTA that does not advertise support for this extension.
   When this is the case and the no-support action was not specified or
   is "R" (reject), the MTA handling the message MUST reject the message
   by:

   1.  returning a 550 error to the DATA command, if synchronous service
       is being provided to the SMTP client that introduced the message,
       or

   2.  generating a Delivery Status Notification [<a href="#ref-DSN" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">DSN</a>] to indicate to
       the originator of the message that the non-delivery occurred and
       terminating further relay attempts.

   An enhanced mail system status code is defined for such rejections in
   <a href="#section-15.3">Section 15.3</a>.

   See <a href="#section-8.2">Section 8.2</a> for additional discussion.

   When relaying, an MTA MUST preserve the no-support action if it was
   used by the SMTP client.






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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Header Field Used</span>

   A receiving system that implements this specification, upon receiving
   a message bearing a "Require-Recipient-Valid-Since" header field when
   no corresponding RRVS SMTP extension was used, checks whether the
   destination mailbox owner has held it continuously, far enough back
   to include the given date-time, and delivers it unless that check
   returns in the negative.  Expressed as a sequence of steps:

   1.  Extract those Require-Recipient-Valid-Since fields from the
       message that contain a recipient for which no corresponding RRVS
       SMTP extension was used.

   2.  Discard any such fields that match any of these criteria:

       *  are syntactically invalid;

       *  name a role account as listed in [<a href="#ref-ROLES" title="&quot;Mailbox Names for Common Services, Roles and Functions&quot;">ROLES</a>];

       *  the "addr-spec" portion does not match a current recipient, as
          listed in the RCPT TO commands in the SMTP session; or

       *  the "addr-spec" portion does not refer to a mailbox handled
          for local delivery by this ADMD.

   3.  For each field remaining, determine if the named address has been
       under continuous ownership since the corresponding timestamp.  If
       it has not, reject the message.

   4.  RECOMMENDED: If local delivery is being performed, remove all
       instances of this field prior to delivery to a mailbox; if the
       message is being forwarded, remove those instances of this header
       field that were not discarded by step 2 above.

   Handling proceeds normally upon completion of the above steps if
   rejection has not been performed.

   The final step is not mandatory as not all mail handling agents are
   capable of stripping away header fields, and there are sometimes
   reasons to keep the field intact such as debugging or presence of
   digital signatures that might be invalidated by such a change.  See
   <a href="#section-10">Section 10</a> for additional discussion.

   If a message is to be rejected within the SMTP protocol itself
   (versus generating a rejection message separately), servers
   implementing this protocol SHOULD also implement the SMTP extension
   described in "Enhanced Mail System Status Codes" [<a href="#ref-ESC" title="&quot;Enhanced Mail System Status Codes&quot;">ESC</a>] and use the
   enhanced status codes described in <a href="#section-15.3">Section 15.3</a> as appropriate.



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   Implementation by this method is expected to be transparent to non-
   participants, since they would typically ignore this header field.

   This header field is not normally added to a message that is
   addressed to multiple recipients.  The intended use of this field
   involves an author seeking to protect transactional or otherwise
   sensitive data intended for a single recipient, and thus generating
   independent messages for each individual recipient is normal
   practice.  See <a href="#section-7">Section 7</a> for further discussion and restrictions.

<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>.  Design Choices</span>

   The presence of the address in the field content supports the case
   where a message bearing this header field is forwarded.  The specific
   use case is as follows:

   1.  A user subscribes to a service "S" at date-time "D" and confirms
       an email address at the user's current location, "A";

   2.  At some later date, the user intends to leave the current
       location and thus creates a new mailbox elsewhere, at "B";

   3.  The user configures address "A" to forward to "B";

   4.  "S" constructs a message to "A" claiming that the address was
       valid at date-time "D" and sends it to "A";

   5.  The receiving MTA for "A" determines that the forwarding in
       effect was created by the same party that owned the mailbox there
       and thus concludes that the continuous ownership test has been
       satisfied;

   6.  If possible, the MTA for "A" removes this header field from the
       message, and in either case, forwards it to "B"; and

   7.  On receipt at "B", either the header field has been removed or
       the header field does not refer to a current envelope recipient,
       and in either case the MTA delivers the message.

   <a href="#section-8">Section 8</a> discusses some interesting use cases, such as the case
   where "B" above results in further forwarding of the message.

   SMTP has never required any correspondence between addresses in the
   <a href="./rfc5321">RFC5321</a>.MailFrom and <a href="./rfc5321">RFC5321</a>.RcptTo parameters and header fields of a
   message, which is why the header field defined here contains the
   recipient address to which the timestamp applies.





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<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Clock Synchronization</span>

   The timestamp portion of this specification supports a precision at
   the seconds level.  Although uncommon, it is not impossible for a
   clock at either a generator or a receiver to be incorrect, leading to
   an incorrect result in the RRVS evaluation.

   To minimize the risk of such incorrect results, both generators and
   receivers implementing this specification MUST use a standard clock
   synchronization protocol such as [<a href="#ref-NTP" title="&quot;Network Time Protocol Version 4: Protocol and Algorithms Specification&quot;">NTP</a>] to synchronize to a common
   clock.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Relaying without RRVS Support</span>

   When a message is received using the SMTP extension defined here but
   will not be delivered locally (that is, it needs to be relayed
   further), the MTA to which the relay will take place might not be
   compliant with this specification.  Where the MTA in possession of
   the message observes it is going to relay the message to an MTA that
   does not advertise this extension, it needs to choose one of the
   following actions:

   1.  Decline to relay the message further, preferably generating a
       Delivery Status Notification [<a href="#ref-DSN" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">DSN</a>] to indicate failure
       (RECOMMENDED);

   2.  Downgrade the data thus provided in the SMTP extension to a
       header field, as described in <a href="#section-6.1">Section 6.1</a> below (SHOULD NOT
       unless the conditions in that section are satisfied, and only
       when the previous option is not available); or

   3.  Silently continue with delivery, dropping the protection offered
       by this protocol.

   Using options other than the first option needs to be avoided unless
   there is specific knowledge that further relaying with the degraded
   protections thus provided does not introduce undue risk.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Header Field Conversion</span>

   If an SMTP server ("B") receives a message bearing one or more
   "Require-Recipient-Valid-Since" header fields from a client ("A"),
   presumably because "A" does not support the SMTP extension, and needs
   to relay the corresponding message on to another server ("C")
   (thereby becoming a client), and "C" advertises support for the SMTP
   extension, "B" SHOULD delete the header field(s) and instead relay
   this information by making use of the SMTP extension.  Note that such
   modification of the header might affect later validation of the



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   header upon delivery; for example, a hash of the modified header
   would produce a different result.  This might be a valid cause for
   some operators to skip this delete operation.

   Conversely, if "B" has received a mailbox timestamp from "A" using
   the SMTP extension for which it must now relay the message on to "C",
   but "C" does not advertise the SMTP extension, and "B" does not
   reject the message because rejection was specifically declined by the
   client (see <a href="#section-5.1.1">Section 5.1.1</a>), "B" SHOULD add a Require-Recipient-Valid-
   Since header field matching the mailbox to which relaying is being
   done, and the corresponding valid-since timestamp for it, if it has
   prior information that the eventual MDA or another intermediate MTA
   supports this mechanism and will be able to process the header field
   as described in this specification.

   The admonitions about very cautious use of the header field described
   in <a href="#section-4">Section 4</a> apply to this relaying mechanism as well.  If multiple
   mailbox timestamps are received from "A", the admonitions in
   <a href="#section-7">Section 7</a> also apply.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Header Field with Multiple Recipients</span>

   Numerous issues arise when using the header field form of this
   extension, particularly when multiple recipients are specified for a
   single message resulting in multiple fields each with a distinct
   address and timestamp.

   Because of the nature of SMTP, a message bearing a multiplicity of
   Require-Recipient-Valid-Since header fields could result in a single
   delivery attempt for multiple recipients (in particular, if two of
   the recipients are handled by the same server), and if any one of
   them fails the test, the delivery fails to all of them; it then
   becomes necessary to do one of the following:

   o  reject the message on completion of the DATA phase of the SMTP
      session, which is a rejection of delivery to all recipients, or

   o  accept the message on completion of DATA, and then generate a
      Delivery Status Notification [<a href="#ref-DSN" title="&quot;An Extensible Message Format for Delivery Status Notifications&quot;">DSN</a>] message for each of the failed
      recipients.

   Additional complexity arises when a message is sent to two
   recipients, "A" and "B", presumably with different timestamps, both
   of which are then redirected to a common address "C".  The author is
   not necessarily aware of the current or past ownership of mailbox
   "C", or indeed that "A" and/or "B" have been redirected.  This might





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   result in either or both of the two deliveries failing at "C", which
   is likely to confuse the message author, who (as far as the author is
   aware) never sent a message to "C" in the first place.

   Finally, there is an obvious concern with the fan-out of a message
   bearing the timestamps of multiple users; tight control over the
   handling of the timestamp information is very difficult to assure as
   the number of handling agents increases.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Special Use Addresses</span>

   In [<a href="#ref-DSN-SMTP" title="&quot;Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)&quot;">DSN-SMTP</a>], an SMTP extension was defined to allow SMTP clients to
   request generation of DSNs and related information to allow such
   reports to be maximally useful.  <a href="#section-5.2.7">Section 5.2.7</a> of that document
   explored the issue of the use of that extension where the recipient
   is a mailing list.  This extension has similar concerns, which are
   covered here following that document as a model.

   For all cases described below, a receiving MTA SHOULD NOT introduce
   RRVS in either form (SMTP extension or header field) if the message
   did not arrive with RRVS in use.  This would amount to second
   guessing the message originator's intention and might lead to an
   undesirable outcome.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Mailing Lists</span>

   Delivery to a mailing list service is considered a final delivery.
   Where this protocol is in use, it is evaluated as per any normal
   delivery: if the same mailing list has been operating in place of the
   specified recipient mailbox since at least the timestamp given as the
   RRVS parameter, the message is delivered to the list service
   normally, and is otherwise not delivered.

   It is important, however, that the participating MDA passing the
   message to the list service needs to omit the RRVS parameter in
   either form (SMTP extension or header field) when doing so.  The
   emission of a message from the list service to its subscribers
   constitutes a new message not covered by the previous transaction.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Single-Recipient Aliases</span>

   Upon delivery of an RRVS-protected message to an alias (acting in
   place of a mailbox) that results in relaying of the message to a
   single other destination, the usual RRVS check is performed.  The
   continuous ownership test here might succeed if, for example, a
   conventional user inbox was replaced with an alias on behalf of that
   same user, and the time when this was done is recorded in a way that
   can be queried by the relaying MTA.



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   If the relaying system also performs some kind of step where
   ownership of the new destination address is confirmed, it SHOULD
   apply RRVS using the later of that timestamp and the one that was
   used inbound.  This also allows for changes to the alias without
   disrupting the protection offered by RRVS.

   If the relaying system has no such time records related to the new
   destination address, the RRVS SMTP extension is not used on the
   relaying SMTP session, and the header field relative to the local
   alias is removed, in accordance with <a href="#section-5">Section 5</a>.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Multiple-Recipient Aliases</span>

   Upon delivery of an RRVS-protected message to an alias (acting in
   place of a mailbox) that results in relaying of the message to
   multiple other destinations, the usual RRVS check is performed as in
   <a href="#section-8.2">Section 8.2</a>.  The MTA expanding such an alias then decides which of
   the options enumerated in that section is to be applied for each new
   recipient.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Confidential Forwarding Addresses</span>

   In the above cases, the original author could receive message
   rejections, such as DSNs, from the ultimate destination, where the
   RRVS check (or indeed, any other) fails and rejection is warranted.
   This can reveal the existence of a forwarding relationship between
   the original intended recipient and the actual final recipient.

   Where this is a concern, the initial delivery attempt is to be
   treated like a mailing list delivery, with RRVS evaluation done and
   then all RRVS information removed from the message prior to relaying
   it to its true destination.

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>.  Suggested Mailing List Enhancements</span>

   Mailing list services could store the timestamp at which a subscriber
   was added to a mailing list.  This specification could then be used
   in conjunction with that information in order to restrict list
   traffic to the original subscriber, rather than a different person
   now in possession of an address under which the original subscriber
   was added to the list.  Upon receiving a rejection caused by this
   specification, the list service can remove that address from further
   distribution.

   A mailing list service that receives a message containing the header
   field defined here needs to remove it from the message prior to
   redistributing it, limiting exposure of information regarding the
   relationship between the message's author and the mailing list.



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<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Continuous Ownership</span>

   For the purposes of this specification, an address is defined as
   having been under continuous ownership since a given date-time if a
   message sent to the address at any point since the given date-time
   would not go to anyone except the owner at that given date-time.
   That is, while an address may have been suspended or otherwise
   disabled for some period, any mail actually delivered would have been
   delivered exclusively to the same owner.  It is presumed that some
   sort of relationship exists between the message sender and the
   intended recipient.  Presumably, there has been some confirmation
   process applied to establish this ownership of the receiver's
   mailbox; however, the method of making such determinations is a local
   matter and outside the scope of this document.

   Evaluating the notion of continuous ownership is accomplished by
   doing any query that establishes whether the above condition holds
   for a given mailbox.

   Determining continuous ownership of a mailbox is a local matter at
   the receiving site.  The only possible answers to the continuous-
   ownership-since question are "yes", "no", and "unknown"; the action
   to be taken in the "unknown" case is a matter of local policy.

   For example, when control of a domain name is transferred, the new
   domain owner might be unable to determine whether the owner of the
   subject address has been under continuous ownership since the stated
   date-time if the mailbox history is not also transferred (or was not
   previously maintained).  It will also be "unknown" if whatever
   database contains mailbox ownership data is temporarily unavailable
   at the time a message arrives for delivery.  In this latter case,
   typical SMTP temporary failure handling is appropriate.

   To avoid exposing account details unnecessarily, if the address
   specified has had one continuous owner since it was created, any
   confirmation date-time SHOULD be considered to pass the test, even if
   that date-time is earlier than the account creation date and time.
   This is further discussed in <a href="#section-13">Section 13</a>.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Digital Signatures</span>

   This protocol mandates removal of the header field (when used) upon
   delivery in all but exceptional circumstances.  If a message with the
   header field were digitally signed in a way that included the header
   field, altering a message in this way would invalidate the signature.
   However, the header field is strictly for tunneling purposes and
   should be regarded by the rest of the transport system as purely
   trace information.



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   Accordingly, the header field MUST NOT be included in the content
   covered by digital signatures.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Authentication-Results Definitions</span>

   [<a id="ref-AUTHRES">AUTHRES</a>] defines a mechanism for indicating, via a header field, the
   results of message authentication checks.  <a href="#section-15">Section 15</a> registers RRVS
   as a new method that can be reported in this way, as well as
   corresponding result names.  The possible result names and their
   meanings are as follows:

   none:  The message had no recipient mailbox timestamp associated with
      it, either via the SMTP extension or header field method; this
      protocol was not in use.

   unknown:  At least one form of this protocol was in use, but
      continuous ownership of the recipient mailbox could not be
      determined.

   temperror:  At least one form of this protocol was in use, but some
      kind of error occurred during evaluation that was transient in
      nature; a later retry will likely produce a final result.

   permerror:  At least one form of this protocol was in use, but some
      kind of error occurred during evaluation that was not recoverable;
      a later retry will not likely produce a final result.

   pass:  At least one form of this protocol was in use, and the
      destination mailbox was confirmed to have been under continuous
      ownership since the timestamp thus provided.

   fail:  At least one form of this protocol was in use, and the
      destination mailbox was confirmed not to have been under
      continuous ownership since the timestamp thus provided.

   Where multiple recipients are present on a message, multiple results
   can be reported using the mechanism described in [<a href="#ref-AUTHRES" title="&quot;Message Header Field for Indicating Message Authentication Status&quot;">AUTHRES</a>].

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Examples</span>

   In the following examples, "C:" indicates data sent by an SMTP
   client, and "S:" indicates responses by the SMTP server.  Message
   content is CRLF terminated, though these are omitted here for ease of
   reading.







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<span class="h3"><a class="selflink" id="section-12.1" href="#section-12.1">12.1</a>.  SMTP Extension Example</span>

     C: [connection established]
     S: 220 server.example.com ESMTP ready
     C: EHLO client.example.net
     S: 250-server.example.com
     S: 250 RRVS
     C: MAIL FROM:&lt;sender@example.net&gt;
     S: 250 OK
     C: RCPT TO:&lt;receiver@example.com&gt; RRVS=2014-04-03T23:01:00Z
     S: 550 5.7.17 receiver@example.com is no longer valid
     C: QUIT
     S: 221 So long!

<span class="h3"><a class="selflink" id="section-12.2" href="#section-12.2">12.2</a>.  Header Field Example</span>

     C: [connection established]
     S: 220 server.example.com ESMTP ready
     C: HELO client.example.net
     S: 250 server.example.com
     C: MAIL FROM:&lt;sender@example.net&gt;
     S: 250 OK
     C: RCPT TO:&lt;receiver@example.com&gt;
     S: 250 OK
     C: DATA
     S: 354 Ready for message content
     C: From: Mister Sender &lt;sender@example.net&gt;
        To: Miss Receiver &lt;receiver@example.com&gt;
        Subject: Are you still there?
        Date: Fri, 28 Jun 2013 18:01:01 +0200
        Require-Recipient-Valid-Since: receiver@example.com;
          Sat, 1 Jun 2013 09:23:01 -0700

        Are you still there?
        .
     S: 550 5.7.17 receiver@example.com is no longer valid
     C: QUIT
     S: 221 So long!

<span class="h3"><a class="selflink" id="section-12.3" href="#section-12.3">12.3</a>.  Authentication-Results Example</span>

   Here is an example use of the Authentication-Results header field
   used to yield the results of an RRVS evaluation:

     Authentication-Results: mx.example.com; rrvs=pass
             smtp.rcptto=user@example.com





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   This indicates that the message arrived addressed to the mailbox
   user@example.com, the continuous ownership test was applied with the
   provided timestamp, and the check revealed that the test was
   satisfied.  The timestamp is not revealed.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Security Considerations</span>

<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>.  Abuse Countermeasures</span>

   The response of a server implementing this protocol can disclose
   information about the age of an existing email mailbox.
   Implementation of countermeasures against probing attacks is
   RECOMMENDED.  For example, an operator could track appearance of this
   field with respect to a particular mailbox and observe the timestamps
   being submitted for testing; if it appears that a variety of
   timestamps are being tried against a single mailbox in short order,
   the field could be ignored and the message silently discarded.  This
   concern is discussed further in <a href="#section-14">Section 14</a>.

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>.  Suggested Use Restrictions</span>

   If the mailbox named in the field is known to have had only a single
   continuous owner since creation, or not to have existed at all (under
   any owner) prior to the date-time specified in the field, then the
   field SHOULD be silently ignored and normal message handling applied
   so that this information is not disclosed.  Such fields are likely
   the product of either gross error or an attack.

   A message author using this specification might restrict inclusion of
   the header field such that it is only done for recipients known also
   to implement this specification, in order to reduce the possibility
   of revealing information about the relationship between the author
   and the mailbox.

   If ownership of an entire domain is transferred, the new owner may
   not know what addresses were assigned in the past by the prior owner.
   Hence, no address can be known not to have had a single owner, or to
   have existed (or not) at all.  In this case, the "unknown" result is
   likely appropriate.

<span class="h3"><a class="selflink" id="section-13.3" href="#section-13.3">13.3</a>.  False Sense of Security</span>

   Senders implementing this protocol likely believe their content is
   being protected by doing so.  It has to be considered, however, that
   receiving systems might not implement this protocol correctly, or at
   all.  Furthermore, use of RRVS by a sending system constitutes
   nothing more than a request to the receiving system; that system
   could choose not to prevent delivery for some local policy, for legal



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   or operational reasons, which compromises the security the sending
   system believed was a benefit to using RRVS.  This could mean the
   timestamp information involved in the protocol becomes inadvertently
   revealed.

   This concern lends further support to the notion that senders would
   do well to avoid using this protocol other than when sending to
   known, trusted receivers.

<span class="h3"><a class="selflink" id="section-13.4" href="#section-13.4">13.4</a>.  Reassignment of Mailboxes</span>

   This specification is a direct response to the risks involved with
   reassignment or recycling of email addresses, an inherently dangerous
   practice.  It is typically expected that email addresses will not
   have a high rate of turnover or ownership change.

   It is RECOMMENDED to have a substantial period of time between
   mailbox owners during which the mailbox accepts no mail, giving
   message generators an opportunity to detect that the previous owner
   is no longer at that address.

<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  Privacy Considerations</span>

<span class="h3"><a class="selflink" id="section-14.1" href="#section-14.1">14.1</a>.  The Tradeoff</span>

   That some MSPs allow for expiration of account names when they have
   been unused for a protracted period forces a choice between two
   potential types of privacy vulnerabilities, one of which presents
   significantly greater threats to users than the other.  Automatically
   generated mail is often used to convey authentication credentials
   that can potentially provide access to extremely sensitive
   information.  Supplying such credentials to the wrong party after a
   mailbox ownership change could allow the previous owner's data to be
   exposed without his or her authorization or knowledge.  In contrast,
   the information that may be exposed to a third party via the proposal
   in this document is limited to information about the mailbox history.
   Given that MSPs have chosen to allow transfers of mailbox ownership
   without the prior owner's involvement, the information leakage from
   the extensions specified here creates far lower overall risk than the
   potential for delivering mail to the wrong party.

<span class="h3"><a class="selflink" id="section-14.2" href="#section-14.2">14.2</a>.  Probing Attacks</span>

   As described above, use of this extension or header field in probing
   attacks can disclose information about the history of the mailbox.
   The harm that can be done by leaking any kind of private information
   is difficult to predict, so it is prudent to be sensitive to this
   sort of disclosure, either inadvertently or in response to probing by



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   an attacker.  It bears restating, then, that implementing
   countermeasures against abuse of this capability needs strong
   consideration.

<span class="h3"><a class="selflink" id="section-14.3" href="#section-14.3">14.3</a>.  Envelope Recipients</span>

   The email To and Cc header fields are not required to be populated
   with addresses that match the envelope recipient set, and Cc may even
   be absent.  However, the algorithm in <a href="#section-3">Section 3</a> requires that this
   header field contain a match for an envelope recipient in order to be
   actionable.  As such, use of this specification can reveal some or
   all of the original intended recipient set to any party that can see
   the message in transit or upon delivery.

   For a message destined to a single recipient, this is unlikely to be
   a concern, which is one of the reasons use of this specification on
   multi-recipient messages is discouraged.

<span class="h3"><a class="selflink" id="section-14.4" href="#section-14.4">14.4</a>.  Risks with Use</span>

   MDAs might not implement the recommendation to remove the header
   field defined here when messages are delivered, either out of
   ignorance or due to error.  Since user agents often do not render all
   of the header fields present, the message could be forwarded to
   another party that would then inadvertently have the content of this
   header field.

   A bad actor may detect use of either form of the RRVS protocol and
   interpret it as an indication of high-value content.

<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-15.1" href="#section-15.1">15.1</a>.  SMTP Extension Registration</span>

   Section 2.2.2 of [<a href="#ref-SMTP" title="&quot;Simple Mail Transfer Protocol&quot;">SMTP</a>] sets out the procedure for registering a new
   SMTP extension.  IANA has registered the SMTP extension using the
   details provided in <a href="#section-3.1">Section 3.1</a> of this document.

<span class="h3"><a class="selflink" id="section-15.2" href="#section-15.2">15.2</a>.  Header Field Registration</span>

   IANA has added the following entry to the "Permanent Message Header
   Field Names" registry, as per the procedure found in [<a href="#ref-IANA-HEADERS">IANA-HEADERS</a>]:

     Header field name: Require-Recipient-Valid-Since
     Applicable protocol: mail ([<a href="#ref-MAIL" title="&quot;Internet Message Format&quot;">MAIL</a>])
     Status: standard
     Author/Change controller: IETF
     Specification document(s): <a href="./rfc7293">RFC 7293</a>



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     Related information:
       Requesting review of any proposed changes and additions to
       this field is recommended.

<span class="h3"><a class="selflink" id="section-15.3" href="#section-15.3">15.3</a>.  Enhanced Status Code Registration</span>

   IANA has registered the following in the Enumerated Status Codes
   table of the "Simple Mail Transfer Protocol (SMTP) Enhanced Status
   Codes Registry":

     Code:               X.7.17
     Sample Text:        Mailbox owner has changed
     Associated basic status code:  5XX
     Description:        This status code is returned when a message is
                         received with a Require-Recipient-Valid-Since
                         field or RRVS extension and the receiving
                         system is able to determine that the intended
                         recipient mailbox has not been under continuous
                         ownership since the specified date-time.
     Reference:          <a href="./rfc7293">RFC 7293</a>
     Submitter:          M. Kucherawy
     Change controller:  IESG

      Code:               X.7.18
      Sample Text:        Domain owner has changed
      Associated basic status code:  5XX
      Description:        This status code is returned when a message is
                          received with a Require-Recipient-Valid-Since
                          field or RRVS extension and the receiving
                          system wishes to disclose that the owner of
                          the domain name of the recipient has changed
                          since the specified date-time.
      Reference:          <a href="./rfc7293">RFC 7293</a>
      Submitter:          M. Kucherawy
      Change controller:  IESG

      Code:               X.7.19
      Sample Text:        RRVS test cannot be completed
      Associated basic status code:  5XX
      Description:        This status code is returned when a message is
                          received with a Require-Recipient-Valid-Since
                          field or RRVS extension and the receiving
                          system cannot complete the requested
                          evaluation because the required timestamp was
                          not recorded.  The message originator needs to
                          decide whether to reissue the message without
                          RRVS protection.
      Reference:          <a href="./rfc7293">RFC 7293</a>



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      Submitter:          M. Kucherawy
      Change controller:  IESG

<span class="h3"><a class="selflink" id="section-15.4" href="#section-15.4">15.4</a>.  Authentication Results Registration</span>

   IANA has registered the following in the "Email Authentication
   Methods" registry:

   Method:  rrvs

   Specifying Document:  <a href="./rfc7293">RFC 7293</a>

   ptype:  smtp

   Property:  rcptto

   Value:  envelope recipient

   Status:  active

   Version:  1

   IANA has also registered the following in the "Email Authentication
   Result Names" registry:

   Codes:  none, unknown, temperror, permerror, pass, fail

   Defined:  <a href="./rfc7293">RFC 7293</a>

   Auth Method(s):  rrvs

   Meaning:  <a href="./rfc7293#section-11">Section&nbsp;11 of RFC 7293</a>

   Status:  active

<span class="h2"><a class="selflink" id="section-16" href="#section-16">16</a>.  Acknowledgments</span>

   Erling Ellingsen proposed the idea.

   Reviews and comments were provided by Michael Adkins, Kurt Andersen,
   Eric Burger, Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed,
   John Levine, Alexey Melnikov, Jay Nancarrow, Hector Santos, Gregg
   Stefancik, and Ed Zayas.








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<span class="h2"><a class="selflink" id="section-17" href="#section-17">17</a>.  References</span>

<span class="h3"><a class="selflink" id="section-17.1" href="#section-17.1">17.1</a>.  Normative References</span>

   [<a id="ref-ABNF">ABNF</a>]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>, January 2008.

   [<a id="ref-DATETIME">DATETIME</a>] Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", <a href="./rfc3339">RFC 3339</a>, July 2002.

   [<a id="ref-IANA-HEADERS">IANA-HEADERS</a>]
              Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", <a href="https://www.rfc-editor.org/bcp/bcp90">BCP 90</a>, <a href="./rfc3864">RFC 3864</a>,
              September 2004.

   [<a id="ref-KEYWORDS">KEYWORDS</a>] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-MAIL">MAIL</a>]     Resnick, P., Ed., "Internet Message Format", <a href="./rfc5322">RFC 5322</a>,
              October 2008.

   [<a id="ref-NTP">NTP</a>]      Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", <a href="./rfc5905">RFC 5905</a>, June 2010.

   [<a id="ref-ROLES">ROLES</a>]    Crocker, D., "Mailbox Names for Common Services, Roles and
              Functions", <a href="./rfc2142">RFC 2142</a>, May 1997.

   [<a id="ref-SMTP">SMTP</a>]     Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc5321">RFC 5321</a>,
              October 2008.

<span class="h3"><a class="selflink" id="section-17.2" href="#section-17.2">17.2</a>.  Informative References</span>

   [<a id="ref-AUTHRES">AUTHRES</a>]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", <a href="./rfc7001">RFC 7001</a>, September 2013.

   [<a id="ref-DSN">DSN</a>]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", <a href="./rfc3464">RFC 3464</a>, January
              2003.

   [<a id="ref-DSN-SMTP">DSN-SMTP</a>] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)", <a href="./rfc3461">RFC</a>
              <a href="./rfc3461">3461</a>, January 2003.

   [<a id="ref-EMAIL-ARCH">EMAIL-ARCH</a>]
              Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>, July
              2009.




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   [<a id="ref-ESC">ESC</a>]      Vaudreuil, G., "Enhanced Mail System Status Codes", <a href="./rfc3463">RFC</a>
              <a href="./rfc3463">3463</a>, January 2003.

Authors' Addresses

   William J. Mills
   Yahoo! Inc.

   EMail: wmills_92105@yahoo.com


   Murray S. Kucherawy
   Facebook, Inc.
   1 Hacker Way
   Menlo Park, CA  94025
   USA

   EMail: msk@fb.com

































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