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rfc:rfc7293

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
           The Require-Recipient-Valid-Since Header Field
                     and SMTP Service Extension

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 Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7293.

Mills & Kucherawy Standards Track [Page 1] RFC 7293 Require-Recipient-Valid-Since July 2014

Copyright Notice

 Copyright (c) 2014 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
 (http://trustee.ietf.org/license-info) 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

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

Mills & Kucherawy Standards Track [Page 2] RFC 7293 Require-Recipient-Valid-Since July 2014

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

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.  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

Mills & Kucherawy Standards Track [Page 3] RFC 7293 Require-Recipient-Valid-Since July 2014

 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 (Section 13) and the
 Privacy Considerations (Section 14) is particularly important.  Note,
 especially, the limitation described in Section 13.3.

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 in Section 1, 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 Section 9) 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 [SMTP] 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 Section 14.1).

Mills & Kucherawy Standards Track [Page 4] RFC 7293 Require-Recipient-Valid-Since July 2014

 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.

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 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 [DATETIME], 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
 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

Mills & Kucherawy Standards Track [Page 5] RFC 7293 Require-Recipient-Valid-Since July 2014

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

3.3. Timestamps

 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.

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 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 Section 7); 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 Section 14.4.)

Mills & Kucherawy Standards Track [Page 6] RFC 7293 Require-Recipient-Valid-Since July 2014

 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 [SMTP], Section 3.6.3 of
 [MAIL], and Section 7 of this document.

5. Handling By Receivers

 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 Section 15.3), 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.

5.1. SMTP Extension Used

 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.

Mills & Kucherawy Standards Track [Page 7] RFC 7293 Require-Recipient-Valid-Since July 2014

 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" [ROLES].
 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 Section 15.3.)

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 or MDA so that it
 can be processed there.
 For the SMTP extension, the optional RRVS parameter defined in
 Section 5.1 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 [DSN] 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
 Section 15.3.
 See Section 8.2 for additional discussion.
 When relaying, an MTA MUST preserve the no-support action if it was
 used by the SMTP client.

Mills & Kucherawy Standards Track [Page 8] RFC 7293 Require-Recipient-Valid-Since July 2014

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 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 [ROLES];
  • 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
 Section 10 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" [ESC] and use the
 enhanced status codes described in Section 15.3 as appropriate.

Mills & Kucherawy Standards Track [Page 9] RFC 7293 Require-Recipient-Valid-Since July 2014

 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 7 for further discussion and restrictions.

5.2.1. Design Choices

 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.
 Section 8 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
 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.

Mills & Kucherawy Standards Track [Page 10] RFC 7293 Require-Recipient-Valid-Since July 2014

5.3. Clock Synchronization

 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 [NTP] to synchronize to a common
 clock.

6. 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
 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 6.1 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.

6.1. Header Field Conversion

 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

Mills & Kucherawy Standards Track [Page 11] RFC 7293 Require-Recipient-Valid-Since July 2014

 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 Section 5.1.1), "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 Section 4 apply to this relaying mechanism as well.  If multiple
 mailbox timestamps are received from "A", the admonitions in
 Section 7 also apply.

7. 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 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 [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

Mills & Kucherawy Standards Track [Page 12] RFC 7293 Require-Recipient-Valid-Since July 2014

 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.

8. 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.
 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.

8.1. 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.
 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.

8.2. Single-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 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.

Mills & Kucherawy Standards Track [Page 13] RFC 7293 Require-Recipient-Valid-Since July 2014

 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 Section 5.

8.3. 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 8.2.  The MTA expanding such an alias then decides which of
 the options enumerated in that section is to be applied for each new
 recipient.

8.4. 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.

8.5. 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
 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.

Mills & Kucherawy Standards Track [Page 14] RFC 7293 Require-Recipient-Valid-Since July 2014

9. 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-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 Section 13.

10. Digital Signatures

 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.

Mills & Kucherawy Standards Track [Page 15] RFC 7293 Require-Recipient-Valid-Since July 2014

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

11. Authentication-Results Definitions

 [AUTHRES] defines a mechanism for indicating, via a header field, the
 results of message authentication checks.  Section 15 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 [AUTHRES].

12. 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.

Mills & Kucherawy Standards Track [Page 16] RFC 7293 Require-Recipient-Valid-Since July 2014

12.1. 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=2014-04-03T23:01:00Z
   S: 550 5.7.17 receiver@example.com is no longer valid
   C: QUIT
   S: 221 So long!

12.2. 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 5.7.17 receiver@example.com is no longer valid
   C: QUIT
   S: 221 So long!

12.3. Authentication-Results Example

 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

Mills & Kucherawy Standards Track [Page 17] RFC 7293 Require-Recipient-Valid-Since July 2014

 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.

13. Security Considerations

13.1. Abuse Countermeasures

 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 Section 14.

13.2. 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-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.

13.3. False Sense of Security

 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

Mills & Kucherawy Standards Track [Page 18] RFC 7293 Require-Recipient-Valid-Since July 2014

 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.

13.4. Reassignment of Mailboxes

 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.

14. Privacy Considerations

14.1. The Tradeoff

 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.

14.2. 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

Mills & Kucherawy Standards Track [Page 19] RFC 7293 Require-Recipient-Valid-Since July 2014

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

14.3. 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.

14.4. Risks with Use

 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.

15. IANA Considerations

15.1. SMTP Extension Registration

 Section 2.2.2 of [SMTP] sets out the procedure for registering a new
 SMTP extension.  IANA has registered the SMTP extension using the
 details provided in Section 3.1 of this document.

15.2. Header Field Registration

 IANA has added the following entry to the "Permanent Message Header
 Field Names" 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): RFC 7293

Mills & Kucherawy Standards Track [Page 20] RFC 7293 Require-Recipient-Valid-Since July 2014

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

15.3. Enhanced Status Code Registration

 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:          RFC 7293
   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:          RFC 7293
    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:          RFC 7293

Mills & Kucherawy Standards Track [Page 21] RFC 7293 Require-Recipient-Valid-Since July 2014

    Submitter:          M. Kucherawy
    Change controller:  IESG

15.4. Authentication Results Registration

 IANA has registered the following in the "Email Authentication
 Methods" registry:
 Method:  rrvs
 Specifying Document:  RFC 7293
 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:  RFC 7293
 Auth Method(s):  rrvs
 Meaning:  Section 11 of RFC 7293
 Status:  active

16. Acknowledgments

 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.

Mills & Kucherawy Standards Track [Page 22] RFC 7293 Require-Recipient-Valid-Since July 2014

17. References

17.1. Normative References

 [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [DATETIME] Klyne, G., Ed. 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., Ed., "Internet Message Format", RFC 5322,
            October 2008.
 [NTP]      Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
            Time Protocol Version 4: Protocol and Algorithms
            Specification", RFC 5905, June 2010.
 [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.

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.

Mills & Kucherawy Standards Track [Page 23] RFC 7293 Require-Recipient-Valid-Since July 2014

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

Mills & Kucherawy Standards Track [Page 24]

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