The Require-Recipient-Valid-Since Header Field and SMTP Service Extension
draft-ietf-appsawg-rrvs-header-field-05

Network Working Group                                           W. Mills
Internet-Draft                                               Yahoo! Inc.
Intended status: Standards Track                            M. Kucherawy
Expires: June 14, 2014                                    Facebook, Inc.
                                                       December 11, 2013

    The Require-Recipient-Valid-Since Header Field and SMTP Service
                               Extension
                draft-ietf-appsawg-rrvs-header-field-05

Abstract

   This document defines an extension for the Simple Mail Transfer
   Protocol called RRVS, and a header field called Require-Recipient-
   Valid-Since, to provide a method for senders to indicate to receivers
   the time when the sender last confirmed the ownership of the target
   mailbox.  This can be used to detect changes of mailbox ownership,
   and thus prevent mail from being delivered to the wrong party.

   The intended use of these facilities is on automatically generated
   messages that might contain sensitive information, though it may also
   be useful in other applications.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Description  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  The 'RRVS' SMTP Extension  . . . . . . . . . . . . . . . .  5
     3.2.  The 'Require-Recipient-Valid-Since' Header Field . . . . .  5
   4.  Use By Generators  . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Handling By Receivers  . . . . . . . . . . . . . . . . . . . .  6
     5.1.  SMTP Extension Used  . . . . . . . . . . . . . . . . . . .  6
       5.1.1.  Relays . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Header Field Used  . . . . . . . . . . . . . . . . . . . .  7
   6.  Role Accounts  . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Relaying Without RRVS Support  . . . . . . . . . . . . . . . .  8
     7.1.  Header Field Conversion  . . . . . . . . . . . . . . . . .  9
   8.  Header Field with Multiple Recipients  . . . . . . . . . . . .  9
   9.  Special Use Addresses  . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 10
     9.2.  Single-Recipient Alaises . . . . . . . . . . . . . . . . . 10
     9.3.  Multiple-Recipient Aliases . . . . . . . . . . . . . . . . 11
     9.4.  Confidential Forwarding Addresses  . . . . . . . . . . . . 11
     9.5.  Suggested Mailing List Enhancements  . . . . . . . . . . . 11
   10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   11. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 13
   12. Authentication-Results Definitions . . . . . . . . . . . . . . 13
   13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     13.1. SMTP Extension Example . . . . . . . . . . . . . . . . . . 14
     13.2. Header Field Example . . . . . . . . . . . . . . . . . . . 15
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     14.1. Abuse Countermeasures  . . . . . . . . . . . . . . . . . . 15
     14.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 16
   15. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 16
     15.1. Probing Attacks  . . . . . . . . . . . . . . . . . . . . . 16
     15.2. Envelope Recipients  . . . . . . . . . . . . . . . . . . . 17
     15.3. Risks with Use of Header Field . . . . . . . . . . . . . . 17
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
     16.1. SMTP Extension Registration  . . . . . . . . . . . . . . . 17
     16.2. Header Field Registration  . . . . . . . . . . . . . . . . 17
     16.3. Enhanced Status Code Registration  . . . . . . . . . . . . 18
     16.4. Authentication Results Registration  . . . . . . . . . . . 18
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     17.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   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.

   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, "The person to whom I am sending to had
   this address assigned to as far back as this date-time."  A receiving
   system can then compare this information against the date and time
   the address was assigned to its current user.  If the assignment was
   made later than the date-time indicated in the message, there is a
   good chance the current user of the address is not the correct
   recipient.  The receiving system can then choose to prevent delivery
   and, possibly, to notify the original sender of the problem.

   The primary application is automatically generated messages rather
   than user-authored content, though it may be useful in other
   contexts.

2.  Definitions

   For a description of the email architecture, consult [EMAIL-ARCH].

   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 [KEYWORDS].

3.  Description

   To address the problem described above, a mail sending client needs
   to indicate to the server to which it is connecting that there is an
   expectation that the destination of the message has been under
   continuous ownership (see Section 11) since some date-time,
   presumably the most recent time the message author had confirmed its
   understanding of who owned that mailbox.  Two mechanisms are defined
   here: an extension to the Simple Mail Transfer Protocol [SMTP], for

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   use between a client and server that both implement the extension,
   and a header field that can be used when passing a message to a
   server that appears not to implement this extension.

   The SMTP extenion is called "RRVS" (Require Recipient Valid Since),
   and adds a parameter to the SMTP "RCPT" command that indicates the
   most recent date and 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.

3.1.  The 'RRVS' SMTP Extension

   Extensions to SMTP are described in Section 2.2 of [SMTP].

   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 verbs, and does not
   alter the MAIL verb.

   An MTA implementing RRVS can transmit or accept a new parameter to
   the RCPT command.  The new parameter is "RRVS", which takes a value
   that is a timestamp expressed as a "date-time" as defiend in
   [DATETIME], with the added restriction that a "time-secfrac" MUST NOT
   be used.  Accordingly, this extension increases the maximum command
   length for the RCPT verb by 31 characters.

   The meaning of this extension, when used, is described in
   Section 5.1.

3.2.  The 'Require-Recipient-Valid-Since' Header Field

   The general constraints on syntax and placement of header fields in a
   message are defined in Internet Message Format [MAIL].

   Using Augmented Backus-Naur Form [ABNF], the syntax for the field is:

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

   "date-time" is defined in Section 3.3, and "addr-spec" is defined in
   Section 3.4.1, of [MAIL].

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4.  Use By Generators

   When a message is generated whose content is sufficiently sensitive
   that an author or author's Administrative Management Domain (ADMD;
   see [EMAIL-ARCH]) 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 either the SMTP extension or the header field defined
   above when sending the message to its destination.

   Use of the SMTP extension provided here is preferable over the header
   field method, since the additional detail about the relationship
   between the message author and its intended recipient is at best a
   property of the message transaction and not part of the message
   itself.  Further, SMTP parameters are not typically recorded in the
   message upon delivery, so detail about the relationship between the
   author or author's ADMD and the intended recipient are not recorded.

   The header field mechanism is defined only to enable passage of the
   request between and through systems that that do not implement the
   SMTP extension.

5.  Handling By Receivers

   If a receiver implements the RRVS SMTP extension, then there are two
   possible evaluation paths:

   1.  The sending client implements the extension, and so there was an
       RRVS parameter on a RCPT TO command in the SMTP session; or

   2.  The sending client does not (or elected not to) implement the
       extension, so the RRVS parameter was not present on the RCPT TO
       commands in the SMTP session.

5.1.  SMTP Extension Used

   A receiving system 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 inclue the given date-time, and
   delivers it unless that check returns in the negative.  Expressed as
   a sequence of steps:

   1.  Ignore the parameter if the named mailbox is a role account as
       listed in Mailbox Names For Common Services, Roles And Functions
       [ROLES].  (See Section 6.)

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   2.  Determine if the named address is serviced for local delivery.
       If so, and if that address has not been under continuous
       ownership since the specified timestamp, return a 550 error to
       the RCPT command.  (See also Section 16.3.)

   3.  RECOMMENDED: If any Require-Recipient-Valid-Since header fields
       are present and refer to the named address, remove them prior to
       delivery or relaying.  (See Section 5.2 for discussion.)

   Where a message arrives using the SMTP extension that also contains
   the Require-Recipient-Valid-Since header field in its header, the
   header field SHOULD be removed.  (See Section 7.1 for further
   discussion about removal of the header field.)

5.1.1.  Relays

   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 so that it can be
   processed there.

   See Section 9.2 for additional discussion.

5.2.  Header Field Used

   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 the set of Require-Recipient-Valid-Since fields from the
       message for which no corresponding RRVS SMTP extension was used.

   2.  Discard any such fields that are syntactically invalid.

   3.  Discard any such fields that name a role account as listed in
       [ROLES] (see Section 6).

   4.  Discard any such fields for which the "addr-spec" portion does
       not match a current recipient, as listed in the RCPT TO commands
       in the SMTP session.

   5.  Discard any such fields for which the "addr-spec" portion does
       not refer to a mailbox handled for local delivery by this MTA.

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   6.  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.

   7.  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 steps 1-4 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.

   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 [ESC] and use the
   enhanced status codes described in Section 16.3 as appropriate.

   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 Section 8 for further discussion.

6.  Role Accounts

   It is necessary not to interfere with delivery of messages to role
   mailboxes (see [ROLES]), but it could be useful to indicate to users
   handling those mailboxes that a change of ownership might have taken
   place where doing so is possible.

7.  Relaying Without RRVS Support

   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

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   following actions:

   1.  Decline to relay the message further, preferably generating a
       Delivery Status Notification [DSN] to indicate failure
       (RECOMMENDED);

   2.  Downgrade the data thus provided in the SMTP extension to a
       header field, as described in Section 7.1 below (RECOMMENDED when
       the previous option is not available); or

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

   Using 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.

7.1.  Header Field Conversion

   If an SMTP server ("B") that has received mailbox timestamps from a
   client ("A") using this extension but then needs to relay the
   corresponding message on to another server ("C") (thereby becoming a
   client), but "C" does not advertise the SMTP extension and "B" elects
   not to reject the message, "B" SHOULD add Require-Recipient-Valid-
   Since header fields matching each mailbox to which relaying is being
   done, and the corresponding valid-since timestamp for each.

   Similarly, if "B" receives a message bearing one or more Require-
   Recipient-Valid-Since header fields from "A" for which it must now
   relay the message, 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
   header upon delivery; for example, a hash of the header would produce
   a different result.  This might be a valid cause for some operators
   to skip this delete operation.

8.  Header Field with Multiple Recipients

   Numerous issues arise when using the header field form of this
   extension, particularly when multiple recipients are specified for a
   single message resulting in one 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

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   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 [DSN] 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
   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.

9.  Special Use Addresses

   In [DSN-SMTP], 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.  Section 5.2.7 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.

9.1.  Mailing Lists

   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.  However, the MTA passing
   the message to the list service does not convey the RRVS parameter in
   either form (SMTP extension or header field) to the list service.
   The emission of a message from the list service to its subscribers
   constitutes a new message not covered by the previous transaction.

9.2.  Single-Recipient Alaises

   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 a conventional user
   inbox was replaced with an alias on behalf of that same user, and

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   this information is recorded someplace.  If the message is thus
   accepted, the relaying MTA can choose to do one of the following:

   1.  Do not include an RRVS parameter or header field when relaying to
       the new address.  (RECOMMENDED)

   2.  If the relaying system records the time when the alias was
       established, independent of confirming the validity of the new
       destination address, it MAY add an RRVS parameter for the new
       target address that includes that time.

   3.  If an explicit confirmation of the new destination was done, it
       MAY add an RRVS parameter for the new target address that
       includes that time.

   There is risk and additional administrative burden associated with
   all but the first option in that list which are believed to make them
   not worth pursuing.

9.3.  Multiple-Recipient Aliases

   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
   Section 9.2.  The MTA expanding such an alias then decides which of
   the options enumerated in that section is to be applied for each new
   recipient.

9.4.  Confidential Forwarding Addresses

   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.

9.5.  Suggested Mailing List Enhancements

   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

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   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.

10.  Discussion

   To further obscure account details on the receiving system, the
   receiver SHOULD ignore the SMTP extension or the header field if the
   address specified has had one continuous owner since it was created,
   regardless of the purported confirmation date of the address.  This
   is further discussed in Section 14.

   The presence of the intended 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" on date "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 replaces address "A" with forwarding to "B";

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

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

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

   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 delivers the message.

   SMTP has never required any correspondence between addresses in the
   RFC5321.MailFrom and RFC5321.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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11.  Continuous Ownership

   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 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 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 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.

12.  Authentication-Results Definitions

   [AUTHRES] defines a mechanism for indicating, via a header field, the
   results of message authentication checks.  Section 16 registers RRVS
   as a new method that can be reported in this way, and 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.

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   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 constant
      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 constant
      ownership since the timestamp thus provided.

   Where multiple recipients are present on a message, multiple results
   can be reported using the mechanism described in [AUTHRES].

13.  Examples

   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.

13.1.  SMTP Extension Example

     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:<sender@example.net>
     S: 250 OK
     C: RCPT TO:<receiver@example.com> RRVS=1381993177
     S: 550 5.7.15 receiver@example.com is no longer valid
     C: QUIT
     S: 221 So long!

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13.2.  Header Field Example

     C: [connection established]
     S: 220 server.example.com ESMTP ready
     C: HELO client.example.net
     S: 250 server.example.com
     C: MAIL FROM:<sender@example.net>
     S: 250 OK
     C: RCPT TO:<receiver@example.com>
     S: 250 OK
     C: DATA
     S: 354 Ready for message content
     C: From: Mister Sender <sender@example.net>
        To: Miss Receiver <receiver@example.com>
        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 5.7.15 receiver@example.com is no longer valid
     C: QUIT
     S: 221 So long!

   If an authentication scheme is applied to claim the added header
   field is valid, but the scheme fails, a receiver might reject the
   message with an SMTP reply such as:

     S: 550-5.7.7 Use of Require-Recipient-Valid-Since header
     S: 550 field requires a valid signature

14.  Security Considerations

14.1.  Abuse Countermeasures

   The response of a server implementing this protocol can disclose
   information about the age of existing email mailbox.  Implementation
   of countermeasures against probing attacks is advised.  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 a variety of timestamps is 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
   Section 15.

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14.2.  Suggested Use Restrictions

   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 specified in the field, then the field
   can 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.

15.  Privacy Considerations

15.1.  Probing Attacks

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

   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 fewer risks than the

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   potential for delivering mail to the wrong party.

15.2.  Envelope Recipients

   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 Section 3 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.

15.3.  Risks with Use of Header Field

   MTAs might not implement the recommendation to remove the header
   field defined here, 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.

16.  IANA Considerations

16.1.  SMTP Extension Registration

   Section 2.2.2 of [MAIL] sets out the procedure for registering a new
   SMTP extension.  IANA is requested to register the SMTP extension
   using the details provided in Section 3.1 of this document.

16.2.  Header Field Registration

   IANA is requested to add the following entry to the Permanent Message
   Header Field registry, as per the procedure found in [IANA-HEADERS]:

     Header field name: Require-Recipient-Valid-Since
     Applicable protocol: mail ([MAIL])
     Status: Standard
     Author/Change controller: IETF
     Specification document(s): [this document]
     Related information:
       Requesting review of any proposed changes and additions to
       this field is recommended.

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16.3.  Enhanced Status Code Registration

   IANA is requested to register the following in the SMTP Enhanced
   Status Codes registry:

      Code:               X.7.15
      Sample Text:        Mailbox owner has changed
      Associated basic status code:  5
      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.
      Reference:          [this document]
      Submitter:          M. Kucherawy
      Change controller:  IESG

      Code:               X.7.16
      Sample Text:        Domain owner has changed
      Associated basic status code:  5
      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.
      Reference:          [this document]
      Submitter:          M. Kucherawy
      Change controller:  IESG

16.4.  Authentication Results Registration

   IANA is requested to register the following in the "Email
   Authentication Methods" Registry:

   Method:  rrvs

   Specifying Document:  [this document]

   ptype:  smtp

   Property:  rcptto

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   Value:  envelope recipient

   Status:  active

   Version:  1

   IANA is also requested to register the following in the "Email
   Authentication Result Names" Registry:

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

   Defined:  [this document]

   Auth Method(s):  rrvs

   Meaning:  Section 12 of [this document]

   Status:  active

17.  References

17.1.  Normative References

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

   [DATETIME]      Klyne, G. and C. Newman, "Date and Time on the
                   Internet: Timestamps", RFC 3339, July 2002.

   [IANA-HEADERS]  Klyne, G., Nottingham, M., and J. Mogul,
                   "Registration Procedures for Message Header Fields",
                   BCP 90, RFC 3864, September 2004.

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

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

   [ROLES]         Crocker, D., "Mailbox Names For Common Services,
                   Roles And Functions", RFC 2142, May 1997.

   [SMTP]          Klensin, J., "Simple Mail Transfer Protocol",
                   RFC 5321, October 2008.

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17.2.  Informative References

   [AUTHRES]       Kucherawy, M., "Message Header Field for Indicating
                   Message Authentication Status", RFC 7001,
                   September 2013.

   [DSN]           Moore, K. and G. Vaudreuil, "An Extensible Message
                   Format for Delivery Status Notifications", RFC 3464,
                   January 2003.

   [DSN-SMTP]      Moore, K., "Simple Mail Transfer Protocol (SMTP)
                   Service Extension for Delivery Status Notifications
                   (DSNs)", RFC 3461, January 2003.

   [EMAIL-ARCH]    Crocker, D., "Internet Mail Architecture", RFC 5598,
                   July 2009.

   [ESC]           Vaudreuil, G., "Enhanced Mail System Status Codes",
                   RFC 3463, January 2003.

Appendix A.  Acknowledgments

   Erling Ellingsen proposed the idea.

   Reviews and comments were provided by Michael Adkins, Kurt Andersen,
   Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine,
   Alexey Melnikov, Hector Santos, Gregg Stefancik, Ed Zayas, (others)

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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