Yahoo! Inc.

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Facebook, Inc.

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Applications 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 point in 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.

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 last time the intended recipient was known to be using this address was this point in time." 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 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. One important point is that the protocols presented here provide a way for a sending system to make a request to receiving systems with respect to handling of a message. In the end, there is no guarantee that the request will have the desired effect.

For a description of the email architecture, consult . 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 .

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 ) since some point in 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 and a new message header field. The SMTP extension permits strong assurance of enforcement by confirming support at each handling step for a message. The header field does not provide the strong assurance, but only requires adoption by the receiving Message Delivery Agent (MDA). The SMTP extension is called "RRVS" (Require Recipient Valid Since), 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.

Extensions to SMTP are described in Section 2.2 of . 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 a new parameter to the RCPT command. An MDA can also accept this new parameter. The new parameter is "RRVS", which takes a value that is a timestamp expressed as a "date-time" as defined in , with the added restriction that a "time-secfrac" MUST NOT be used. Accordingly, this extension increases the maximum command length for the RCPT command by 31 characters. The meaning of this extension, when used, is described in .

The general constraints on syntax and placement of header fields in a message are defined in Internet Message Format . Using Augmented Backus-Naur Form , 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 .

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 time and date 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.

When a message is generated whose content is sufficiently sensitive that an author or author's Administrative Management Domain (ADMD; see ) 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 because of: the positive confirmation of support at each handling node; the fact that the protocol is focused on affecting delivery (that is, the transaction) rather than on content; and the fact that there is less risk of the timestamp parameter being inadvertently forwarded (see ). The header field mechanism is defined only to enable passage of the request between and through systems that do not implement the SMTP extension.

If a receiver implements this specification, then there are two possible evaluation paths: The sending client implements the extension, and so there was 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 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, but the corresponding header field might be present in the message.

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. 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: 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 . (See .) 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 .) If any Require-Recipient-Valid-Since header fields are present and refer to the named address, they SHOULD be removed prior to delivery or relaying. (See and for discussion.)

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. See for additional discussion.

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: Extract those Require-Recipient-Valid-Since fields from the message that contain a recipient for which no corresponding RRVS SMTP extension was used. Discard any such fields that match any of these criteria: are syntactically invalid; name a role account as listed in (see ); 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. 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. 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. See 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 and use the enhanced status codes described in 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 for further discussion.

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: A user subscribes to a service "S" on date "D" and confirms an email address at the user's current location, "A"; At some later date, the user intends to leave the current location, and thus creates a new mailbox elsewhere, at "B"; The user replaces address "A" with forwarding to "B"; "S" constructs a message to "A" claiming that address was valid at date "D" and sends it to "A"; 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; If possible, "A" removes this header field from the message, and in either case, forwards it to "B"; 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.

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 .

It is necessary not to interfere with delivery of messages to role mailboxes (see ), but it could be useful to notify users sending to those mailboxes that a change of ownership might have taken place, if such notification is possible.

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: Decline to relay the message further, preferably generating a Delivery Status Notification to indicate failure (RECOMMENDED); Downgrade the data thus provided in the SMTP extension to a header field, as described in below (RECOMMENDED when the previous option is not available); or 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.

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.

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 them fails the test, the delivery fails to all of them; it then becomes necessary to do one of the following: reject the message on completion of the DATA phase of the SMTP session, which is a rejection of delivery to all recipients; or accept the message on completion of DATA, and then generate a Delivery Status Notification 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.

In , 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.

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.

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 this information is recorded someplace. If the message is thus accepted, the relaying MTA can choose to do one of the following: Do not include an RRVS parameter or header field when relaying to the new address. (RECOMMENDED) 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. 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.

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

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.

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.

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 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. To avoid exposing account details unnecessarily, if the address specified has had one continuous owner since it was created, any confirmation date SHOULD be considered to pass the test, even if that date is earlier than the account creation date. This is further discussed in .

This protocol mandates removal of the header field (when used) upon delivery in all but exceptional circumstances. Altering a message in this way will invalidate a digital signature intended to guard against message modification in transit, which can interfere with delivery. Section 5.4.1 of DomainKeys Identified Mail proposes a strategy for selecting header fields to sign. Specifically, it advises including in the signed portion of the message only those header fields that comprise part of the core content of the message. As the header field version of this protocol is ephemeral, it cannot be considered core content. Accordingly, applying digital signatures that attempt to protect the content of this header field is NOT RECOMMENDED.

defines a mechanism for indicating, via a header field, the results of message authentication checks. 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: 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. At least one form of this protocol was in use, but continuous ownership of the recipient mailbox could not be determined. 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. 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. 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. 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 .

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.

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!

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!

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

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 test was satisfied. The timestamp is not revealed.

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 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 .

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.

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, legal or operational reason, 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.

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 potential for delivering mail to the wrong party.

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 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.

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.

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

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

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.

IANA is requested to register 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: 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.7.18 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

IANA is requested to register the following in the "Email Authentication Methods" Registry: rrvs [this document] smtp rcptto envelope recipient active 1 IANA is also requested to register the following in the "Email Authentication Result Names" Registry: none, unknown, temperror, permerror, pass, fail [this document] rrvs of [this document] active

Brandenburg InternetWorking THUS plc. Clearswift Corporation Sun Microsystems Harvard University Qualcomm, Inc. U. Delaware ISC JHU/APL JHU/APL Brandenburg InternetWorking AT&T Laboratories Cloudmark University of Tennessee Lucent Technologies University of Tennessee Brandenburg InternetWorking Lucent Technologies

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, Ed Zayas, (others)