Using TLS in Applications D. Margolis
Internet-Draft M. Risher
Intended status: Standards Track Google, Inc
Expires: May 12, 2018 B. Ramakrishnan
Yahoo!, Inc
A. Brotman
Comcast, Inc
J. Jones
Microsoft, Inc
November 8, 2017

SMTP MTA Strict Transport Security (MTA-STS)


SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections, and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on May 12, 2018.

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Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and hosts to negotiate the use of a TLS channel for encrypted mail transmission.

While this opportunistic encryption protocol by itself provides a high barrier against passive man-in-the-middle traffic interception, any attacker who can delete parts of the SMTP session (such as the "250 STARTTLS" response) or who can redirect the entire SMTP session (perhaps by overwriting the resolved MX record of the delivery domain) can perform downgrade or interception attacks.

This document defines a mechanism for recipient domains to publish policies specifying:

1.1. Terminology

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 [RFC2119].

We also define the following terms for further use in this document:

2. Related Technologies

The DANE TLSA record [RFC7672] is similar, in that DANE is also designed to upgrade unauthenticated encryption or plaintext transmission into authenticated, downgrade-resistant encrypted transmission. DANE requires DNSSEC [RFC4033] for authentication; the mechanism described here instead relies on certificate authorities (CAs) and does not require DNSSEC, at a cost of risking malicious downgrades. For a thorough discussion of this trade-off, see Section 9, "Security Considerations".

In addition, MTA-STS provides an optional report-only mode, enabling soft deployments to detect policy failures; partial deployments can be achieved in DANE by deploying TLSA records only for some of a domain's MXs, but such a mechanism is not possible for the per-domain policies used by MTA-STS.

The primary motivation of MTA-STS is to provide a mechanism for domains to upgrade their transport security even when deploying DNSSEC is undesirable or impractical. However, MTA-STS is designed not to interfere with DANE deployments when the two overlap; in particular, senders who implement MTA-STS validation MUST NOT allow a "valid" or "report-only" MTA-STS validation to override a failing DANE validation.

3. Policy Discovery

MTA-STS policies are distributed via HTTPS from a "well-known" [RFC5785] path served within the Policy Domain, and their presence and current version are indicated by a TXT record at the Policy Domain. These TXT records additionally contain a policy id field, allowing sending MTAs to check the currency of a cached policy without performing an HTTPS request.

To discover if a recipient domain implements MTA-STS, a sender need only resolve a single TXT record. To see if an updated policy is available for a domain for which the sender has a previously cached policy, the sender need only check the TXT record's version id against the cached value.

3.1. MTA-STS TXT Records

The MTA-STS TXT record is a TXT record with the name _mta-sts at the Policy Domain. For the domain, this record would be MTA-STS TXT records MUST be US-ASCII, semicolon-separated key/value pairs containing the following fields:

An example TXT record is as below: IN TXT "v=STSv1; id=20160831085700Z;"

The formal definition of the _mta-sts TXT record, defined using [RFC7405], is as follows:

sts-text-record = sts-version 1*(field-delim sts-field) [field-delim]

sts-field       = sts-id /                        ; Note that sts-id record
                  sts-extension                   ; is required.

field-delim     = *WSP ";" *WSP

sts-version     = %s"v=STSv1"

sts-id          = %s"id=" 1*32(ALPHA / DIGIT)     ; id=...

sts-extension   = sts-ext-name "=" sts-ext-value  ; name=value

sts-ext-name    = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")

sts-ext-value   = 1*(%x21-3A / %x3C / %x3E-7E)    ; chars excluding "=",
                                                  ; ";", SP, and control
                                                  ; chars

If multiple TXT records for _mta-sts are returned by the resolver, records which do not begin with v=STSv1; are discarded. If the number of resulting records is not one, senders MUST assume the recipient domain does not implement MTA-STS and skip the remaining steps of policy discovery. If the resulting TXT record contains multiple strings, then the record MUST be treated as if those strings are concatenated together without adding spaces.

3.2. MTA-STS Policies

The policy itself is a set of key/value pairs (similar to [RFC5322] header fields) served via the HTTPS GET method from the fixed [RFC5785] "well-known" path of .well-known/mta-sts.txt served by the mta-sts host at the Policy Domain. Thus for the path is

The [RFC7231] "Content-Type" media type for this resource MUST be "text/plain". When fetching a policy, senders SHOULD validate that the media type is "text/plain" to guard against cases where webservers allow untrusted users to host non-text content (typically, HTML or images) at a user-defined path. Additional "Content-Type" parameters are ignored.

This resource contains the following line-separated key/value pairs:

An example policy is as below:

version: STSv1
mode: enforce
max_age: 123456

The formal definition of the policy resource, defined using [RFC7405], is as follows:

sts-policy-record        = *WSP sts-policy-field *WSP
                           *(CRLF *WSP sts-policy-field *WSP)

sts-policy-field         = sts-policy-version /           ; required once
                           sts-policy-mode    /           ; required once
                           sts-policy-max-age /           ; required once
                           0*(sts-policy-mx *WSP CRLF) /  ; required at
                                                          ; least once
                                                          ; except when mode
                                                          ; is "none"
                           sts-policy-extension           ; other fields

field-delim              = ":" *WSP

sts-policy-version       = sts-policy-version-field field-delim

sts-policy-version-field = %s"version"

sts-policy-version-value = %s"STSv1"

sts-policy-mode          = sts-policy-mode-field field-delim

sts-policy-mode-field    = %s"mode"

sts-policy-model-value   =  %s"report" / %s"enforce" / %s"none"

sts-policy-mx            = sts-policy-mx-field field-delim

sts-policy-mx-field      = %s"mx"

sts-policy-mx-value      = 1*(ALPHA / DIGIT / "_" / "-" / ".")

sts-policy-max-age       = sts-policy-max-age-field field-delim

sts-policy-max-age-field = %s"max_age"

sts-policy-max-age-value = 1*10(DIGIT)

sts-policy-extension     = sts-policy-ext-name field-delim ; additional
                           sts-policy-ext-value            ; extension
                                                           ; fields

sts-policy-ext-name      = (ALPHA / DIGIT)
                           *31(ALPHA / DIGIT / "_" / "-" / ".")

sts-policy-ext-value     = 1*(%x21-3A / %x3C / %x3E-7E)    ; chars excluding
                                                           ; "=", ";", SP,
                                                           ; and control
                                                           ; chars

Parsers MUST accept TXT records and policy files which are syntactically valid (i.e. valid key/value pairs separated by semi-colons for TXT records) and but containing additional key/value pairs not specified in this document, in which case unknown fields SHALL be ignored. If any non-repeated field--i.e. all fields excepting mx--is duplicated, all entries except for the first SHALL be ignored. If any field is not specified, the policy SHALL be treated as invalid.

3.3. HTTPS Policy Fetching

When fetching a new policy or updating a policy, the HTTPS endpoint MUST present a X.509 certificate which is valid for the mta-sts host (e.g. as described below, chain to a root CA that is trusted by the sending MTA, and be non-expired. It is expected that sending MTAs use a set of trusted CAs similar to those in widely deployed Web browsers and operating systems.

The certificate is valid for the mta-sts host with respect to the rules described in [RFC6125], with the following application-specific considerations:

The certificate MAY be checked for revocation via the Online Certificate Status Protocol (OCSP) [RFC6960], certificate revocation lists (CRLs), or some other mechanism.

Policies fetched via HTTPS are only valid if the HTTP response code is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP caching (as specified in [RFC7234]) MUST NOT be used.

Senders may wish to rate-limit the frequency of attempts to fetch the HTTPS endpoint even if a valid TXT record for the recipient domain exists. In the case that the HTTPS GET fails, we suggest implementions may limit further attempts to a period of five minutes or longer per version ID, to avoid overwhelming resource-constrained recipients with cascading failures.

Senders MAY impose a timeout on the HTTPS GET and/or a limit on the maximum size of the response body to avoid long delays or resource exhaustion during attempted policy updates. A suggested timeout is one minute, and a suggested maximum policy size 64 kilobytes; policy hosts SHOULD respond to requests with a complete policy body within that timeout and size limit.

If a valid TXT record is found but no policy can be fetched via HTTPS (for any reason), and there is no valid (non-expired) previously-cached policy, senders MUST continue with delivery as though the domain has not implemented MTA-STS.

Conversely, if no "live" policy can be discovered via DNS or fetched via HTTPS, but a valid (non-expired) policy exists in the sender's cache, the sender MUST apply that cached policy.

3.4. Policy Selection for Smart Hosts and Subdomains

When sending mail via a "smart host"--an intermediate SMTP relay rather than the message recipient's server--compliant senders MUST treat the smart host domain as the policy domain for the purposes of policy discovery and application.

When sending mail to a mailbox at a subdomain, compliant senders MUST NOT attempt to fetch a policy from the parent zone. Thus for mail sent to "", the policy can be fetched only from "", not "".

#Policy Validation

When sending to an MX at a domain for which the sender has a valid and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST validate:

  1. That the recipient MX supports STARTTLS and offers a valid PKIX-based TLS certificate.
  2. That at least one of the policy's "mx" patterns matches at least one of the identities presented in the MX's X.509 certificate, as described in "MX Certificate Validation".

This section does not dictate the behavior of sending MTAs when policies fail to validate; in particular, validation failures of policies which specify mode values of "report" or "none" MUST NOT be interpreted as delivery failures, as described in Section 4, "Policy Application".

3.5. MX Certificate Validation

The certificate presented by the receiving MX MUST chain to a root CA that is trusted by the sending MTA and be non-expired. The certificate MUST have a CN-ID ([RFC6125]) or subject alternative name (SAN, [RFC5280]) with a DNS-ID matching the mx pattern. The MX's certificate MAY also be checked for revocation via OCSP [RFC6960], CRLs [RFC6818], or some other mechanism.

Because the mx patterns are not hostnames, however, matching is not identical to other common cases of X.509 certificate authentication (as described, for example, in [RFC6125]). Consider the example policy given above, with an mx pattern containing In this case, if the MX server's X.509 certificate contains a SAN matching *, we are required to implement "wildcard-to-wildcard" matching.

To simplify this case, we impose the following constraints on wildcard certificates, identical to those in [RFC7672] section 3.2.3 and [@?RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN-IDs, but must be the entire first label of the identifier (that is, *, not mail* Senders who are comparing a "suffix" MX pattern with a wildcard identifier should thus strip the wildcard and ensure that the two sides match label-by-label, until all labels of the shorter side (if unequal length) are consumed.

Note that a wildcard must match a label; an mx pattern of thus does not match a SAN of, nor does a SAN of * match an mx of

A simple pseudocode implementation of this algorithm is presented in the Appendix.

4. Policy Application

When sending to an MX at a domain for which the sender has a valid, non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies the result of a policy validation failure one of two ways, depending on the value of the policy mode field:

  1. enforce: In this mode, sending MTAs MUST NOT deliver the message to hosts which fail MX matching or certificate validation.
  2. report: In this mode, sending MTAs which also implement the TLSRPT specification (TODO: add ref) merely send a report indicating policy application failures (so long as TLSRPT is also implemented by the recipient domain).
  3. none: In this mode, sending MTAs should treat the policy domain as though it does not have any active policy; see Section 7.3, "Removing MTA-STS", for use of this mode value.

When a message fails to deliver due to an enforce policy, a compliant MTA MUST NOT permanently fail to deliver messages before checking for the presence of an updated policy at the Policy Domain. (In all cases, MTAs SHOULD treat such failures as transient errors and retry delivery later.) This allows implementing domains to update long-lived policies on the fly.

4.1. Policy Application Control Flow

An example control flow for a compliant sender consists of the following steps:

  1. Check for a cached policy whose time-since-fetch has not exceeded its max_age. If none exists, attempt to fetch a new policy (perhaps asynchronously, so as not to block message delivery). Optionally, sending MTAs may unconditionally check for a new policy at this step.
  2. For each candidate MX, in order of MX priority, attempt to deliver the message, enforcing STARTTLS and, assuming a policy is present, PKIX certificate validation as described in Section 3.5, "MX Certificate Validation."
  3. A message delivery MUST NOT be permanently failed until the sender has first checked for the presence of a new policy (as indicated by the id field in the _mta-sts TXT record). If a new policy is not found, existing rules for the case of temporary message delivery failures apply (as discussed in [RFC5321] section

5. Reporting Failures

MTA-STS is intended to be used along with TLSRPT (TODO: add ref) in order to ensure implementing domains can detect cases of both benign and malicious failures, and to ensure that failures that indicate an active attack are discoverable. As such, senders who also implement TLSRPT SHOULD treat the following events as reportable failures:

6. Interoperability Considerations

6.1. SNI Support

To ensure that the server sends the right certificate chain, the SMTP client MUST have support for the TLS SNI extension [RFC6066]. When connecting to a HTTP server to retrieve the MTA-STS policy, the SNI extension MUST contain the name of the policy host (e.g. When connecting to an SMTP server, the SNI extension MUST contain the MX hostname.

HTTP servers used to deliver MTA-STS policies MUST have support for the TLS SNI extension and MAY rely on SNI to determine which certificate chain to present to the client. In either case, HTTP servers MUST respond with a certificate chain that matches the policy hostname or abort the TLS handshake if unable to do so.

SMTP servers MUST have support for the TLS SNI extension and MAY rely on SNI to determine which certificate chain to present to the client. If the client sends no SNI extension or sends an SNI extension for an unsupported server name, the server MUST simply send a fallback certificate chain of its choice. The reason for not enforcing strict matching of the requested SNI hostname is that MTA-STS TLS clients may be typically willing to accept multiple server names but can only send one name in the SNI extension. The server's fallback certificate may match a different name that is acceptable to the client, e.g., the original next-hop domain.

6.2. Minimum TLS Version Support

MTAs supporting MTA-STS MUST have support for TLS version 1.2 [RFC5246] or higher. The general TLS usage guidance in [RFC7525] SHOULD be followed.

7. Operational Considerations

7.1. Policy Updates

Updating the policy requires that the owner make changes in two places: the _mta-sts TXT record in the Policy Domain's DNS zone and at the corresponding HTTPS endpoint. As a result, recipients should expect a policy will continue to be used by senders until both the HTTPS and TXT endpoints are updated and the TXT record's TTL has passed.

In other words, a sender who is unable to successfully deliver a message while applying a cache of the recipient's now-outdated policy may be unable to discover that a new policy exists until the DNS TTL has passed. Recipients should therefore ensure that old policies continue to work for message delivery during this period of time, or risk message delays.

Recipients should also prefer to update the HTTPS policy body before updating the TXT record; this ordering avoids the risk that senders, seeing a new TXT record, mistakenly cache the old policy from HTTPS.

7.2. Policy Delegation

Domain owners commonly delegate SMTP hosting to a different organization, such as an ISP or a Web host. In such a case, they may wish to also delegate the MTA-STS policy to the same organization which can be accomplished with two changes.

First, the Policy Domain must point the _mta-sts record, via CNAME, to the _mta-sts record maintained by the hosting organization. This allows the hosting organization to control update signaling.

Second, the Policy Domain must point the "well-known" policy location to the hosting organization. This can be done either by setting the mta-sts record to a host or CNAME specified by the hosting organization and by giving the hosting organization a TLS certificate which is valid for that host, or by setting up a "reverse proxy" (also known as a "gateway") server that serves as the Policy Domain's policy the policy currently served by the hosting organization.

For example, given a user domain hosted by a mail provider, the following configuration would allow policy delegation:



> GET /.well-known/mta-sts.txt
> Host:
< HTTP/1.1 200 OK  # Response proxies content from

Note that while sending MTAs MUST NOT use HTTP caching when fetching policies via HTTPS, such caching may nonetheless be useful to a reverse proxy configured as described in this section. An HTTPS policy endpoint expecting to be proxied for multiple hosted domains--as with a large mail hosting provider or similar--may wish to indicate an HTTP Cache-Control max-age response directive (as specified in [RFC7234]) of 60 seconds as a reasonable value to save reverse proxies an unnecessarily high-rate of proxied policy fetching.

7.3. Removing MTA-STS

In order to facilitate clean opt-out of MTA-STS by implementing policy domains, and to distinguish clearly between failures which indicate attacks and those which indicate such opt-outs, MTA-STS implements the none mode, which allows validated policies to indicate authoritatively that the policy domain wishes to no longer implement MTA-STS and may, in the future, remove the MTA-STS TXT and policy endpoints entirely.

A suggested workflow to implement such an opt out is as follows:

  1. Publish a new policy with mode equal to none and a small max_age (e.g. one day).
  2. Publish a new TXT record to trigger fetching of the new policy.
  3. When all previously served policies have expired--normally this is the time the previously published policy was last served plus that policy's max_age, but note that older policies may have been served with a greater max_age, allowing overlapping policy caches--safely remove the TXT record and HTTPS endpoint.

8. IANA Considerations

8.1. Well-Known URIs Registry

A new .well-known URI will be registered in the Well-Known URIs registry as described below:

URI Suffix: mta-sts.txt Change Controller: IETF

8.2. MTA-STS TXT Record Fields

IANA is requested to create a new registry titled "MTA-STS TXT Record Fields". The initial entries in the registry are:

Field Name Description Reference
v Record version Section 3.1 of RFC XXX
id Policy instance ID Section 3.1 of RFC XXX

New fields are added to this registry using IANA's "Expert Review" policy.

8.3. MTA-STS Policy Fields

IANA is requested to create a new registry titled "MTA-STS Policy Fields". The initial entries in the registry are:

Field Name Description Reference
version Policy version Section 3.2 of RFC XXX
mode Enforcement behavior Section 3.2 of RFC XXX
max_age Policy lifetime Section 3.2 of RFC XXX
mx MX identities Section 3.2 of RFC XXX

New fields are added to this registry using IANA's "Expert Review" policy.

9. Security Considerations

SMTP MTA Strict Transport Security attempts to protect against an active attacker who wishes to intercept or tamper with mail between hosts who support STARTTLS. There are two classes of attacks considered:

MTA-STS can thwart such attacks only if the sender is able to previously obtain and cache a policy for the recipient domain, and only if the attacker is unable to obtain a valid certificate that complies with that policy. Below, we consider specific attacks on this model.

9.1. Obtaining a Signed Certificate

SMTP MTA-STS relies on certificate validation via PKIX based TLS identity checking [RFC6125]. Attackers who are able to obtain a valid certificate for the targeted recipient mail service (e.g. by compromising a certificate authority) are thus able to circumvent STS authentication.

9.2. Preventing Policy Discovery

Since MTA-STS uses DNS TXT records for policy discovery, an attacker who is able to block DNS responses can suppress the discovery of an MTA-STS Policy, making the Policy Domain appear not to have an MTA-STS Policy. The sender policy cache is designed to resist this attack by decreasing the frequency of policy discovery and thus reducing the window of vulnerability; it is nonetheless a risk that attackers who can predict or induce policy discovery--for example, by inducing a victim sending domain to send mail to a never-before-contacted recipient while carrying out a man-in-the-middle attack--may be able to foil policy discovery and effectively downgrade the security of the message delivery.

Since this attack depends upon intercepting initial policy discovery, we strongly recommend implementers to prefer policy max_age values to be as long as is practical.

Because this attack is also possible upon refresh of a cached policy, we suggest implementers do not wait until a cached policy has expired before checking for an update; if senders attempt to refresh the cache regularly (for instance, by checking their cached version string against the TXT record on each successful send, or in a background task that runs daily or weekly), an attacker would have to foil policy discovery consistently over the lifetime of a cached policy to prevent a successful refresh.

Additionally, MTAs should alert administrators to repeated policy refresh failures long before cached policies expire (through warning logs or similar applicable mechanisms), allowing administrators to detect such a persistent attack on policy refresh. (However, they should not implement such alerts if the cached policy has a none mode, to allow clean MTA-STS removal, as described in Section 7.3.)

Resistance to downgrade attacks of this nature--due to the ability to authoritatively determine "lack of a record" even for non-participating recipients--is a feature of DANE, due to its use of DNSSEC for policy discovery.

9.3. Denial of Service

We additionally consider the Denial of Service risk posed by an attacker who can modify the DNS records for a victim domain. Absent MTA-STS, such an attacker can cause a sending MTA to cache invalid MX records, but only for however long the sending resolver caches those records. With MTA-STS, the attacker can additionally advertise a new, long-max_age MTA-STS policy with mx constraints that validate the malicious MX record, causing senders to cache the policy and refuse to deliver messages once the victim has resecured the MX records.

This attack is mitigated in part by the ability of a victim domain to (at any time) publish a new policy updating the cached, malicious policy, though this does require the victim domain to both obtain a valid CA-signed certificate and to understand and properly configure MTA-STS.

Similarly, we consider the possibility of domains that deliberately allow untrusted users to serve untrusted content on user-specified subdomains. In some cases (e.g. the service this takes the form of providing HTTPS hosting of user-registered subdomains; in other cases (e.g. dynamic DNS providers) this takes the form of allowing untrusted users to register custom DNS records at the provider's domain.

In these cases, there is a risk that untrusted users would be able to serve custom content at the mta-sts host, including serving an illegitimate MTA-STS policy. We believe this attack is rendered more difficult by the need for the attacker to also serve the _mta-sts TXT record on the same domain--something not, to our knowledge, widely provided to untrusted users. This attack is additionally mitigated by the aforementioned ability for a victim domain to update an invalid policy at any future date.

9.4. Weak Policy Constraints

Even if an attacker cannot modify a served policy, the potential exists for configurations that allow attackers on the same domain to receive mail for that domain. For example, an easy configuration option when authoring an MTA-STS Policy for is to set the mx equal to; recipient domains must consider in this case the risk that any user possessing a valid hostname and CA-signed certificate (for example, will, from the perspective of MTA-STS Policy validation, be a valid MX host for that domain.

9.5. Compromise of the Web PKI System

A host of risks apply to the PKI system used for certificate authentication, both of the mta-sts HTTPS host's certificate and the SMTP servers' certificates. These risks are broadly applicable within the Web PKI ecosystem and are not specific to MTA-STS; nonetheless, they deserve some consideration in this context.

Broadly speaking, attackers may compromise the system by obtaining certificates under fraudulent circumstances (i.e. by impersonating the legitimate owner of the victim domain), by compromising a Certificate Authority or Delegate Authority's private keys, by obtaining a legitimate certificate issued to the victim domain, and similar.

One approach commonly employed by Web browsers to help mitigate against some of these attacks is to allow for revocation of compromised or fraudulent certificates via OCSP [RFC6960] or CRLs [RFC6818]. Such mechanisms themselves represent tradeoffs and are not universally implemented; we nonetheless recommend implementors of MTA-STS to implement revocation mechanisms which are most applicable to their implementations.

10. Contributors

Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)

Wei Chuang Google, Inc weihaw (at) google (dot com)

Brandon Long Google, Inc blong (at) google (dot com)

Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com)

Klaus Umbach 1&1 Mail & Media Development & Technology GmbH klaus.umbach (at) 1und1 (dot de)

Markus Laber 1&1 Mail & Media Development & Technology GmbH markus.laber (at) 1und1 (dot de)

11. Appendix 1: MTA-STS example record & policy

The owner of wishes to begin using MTA-STS with a policy that will solicit reports from senders without affecting how the messages are processed, in order to verify the identity of MXs that handle mail for, confirm that TLS is correctly used, and ensure that certificates presented by the recipient MX validate.

MTA-STS policy indicator TXT RR:  IN TXT "v=STSv1; id=20160831085700Z;"

MTA-STS Policy file served as the response body at <>

version: STSv1
mode: report
max_age: 12345678

12. Appendix 2: Message delivery pseudocode

Below is pseudocode demonstrating the logic of a compliant sending MTA.

While this pseudocode implementation suggests synchronous policy retrieval in the delivery path, in a working implementation that may be undesirable, and we expect some implementers to instead prefer a background fetch that does not block delivery if no cached policy is present.

func isEnforce(policy) {
  // Return true if the policy mode is "enforce".

func isNonExpired(policy) {
  // Return true if the policy is not expired.

func tryStartTls(connection) {
  // Attempt to open an SMTP connection with STARTTLS with the MX.

func isWildcardMatch(pat, host) {
  // Literal matches are true.
  if pat == host {
    return true
  // Leading '.' matches a wildcard against the first part, i.e.
  // matches but not
  if pat[0] == '.' {
    parts = SplitN(host, '.', 2)  // Split on the first '.'.
    if len(parts) > 1 && parts[1] == pat[1:] {
      return true
  return false

func certMatches(connection, policy) {
  // Assume a handy function to return CN and DNS-ID SANs.
  for san in getDnsIdSansAndCnFromCert(connection) {
    for mx in {
      // Return if the server certificate from "connection" matches the "mx"
      // host.
      if san[0] == '*' {
        // Invalid wildcard!
        if san[1] != '.' continue
        san = san[1:]
      if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
        return true
  return false

func tryDeliverMail(connection, message) {
  // Attempt to deliver "message" via "connection".

func tryGetNewPolicy(domain) {
  // Check for an MTA-STS TXT record for "domain" in DNS, and return the
  // indicated policy.

func cachePolicy(domain, policy) {
  // Store "policy" as the cached policy for "domain".

func tryGetCachedPolicy(domain) {
  // Return a cached policy for "domain".

func reportError(error) {
  // Report an error via TLSRPT.

func tryMxAccordingTo(message, mx, policy) {
  connection := connect(mx)
  if !connection {
    return false  // Can't connect to the MX so it's not an MTA-STS error.
  secure := true
  if !tryStartTls(connection) {
    secure = false
  } else if !certMatches(connection, policy) {
    secure = false
  if secure || !isEnforce(policy) {
    return tryDeliverMail(connection, message)
  return false

func tryWithPolicy(message, domain, policy) {
  mxes := getMxForDomain(domain)
  for mx in mxes {
    if tryMxAccordingTo(message, mx, policy) {
      return true
  return false

func handleMessage(message) {
  domain := ... // domain part after '@' from recipient
  policy := tryGetNewPolicy(domain)
  if policy {
    cachePolicy(domain, policy)
  } else {
    policy = tryGetCachedPolicy(domain)
  if policy {
    return tryWithPolicy(message, domain, policy)
  // Try to deliver the message normally (i.e. without MTA-STS).

13. References

13.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10.17487/RFC5785, April 2010.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.

13.2. Informative References

[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, February 2002.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 2013.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013.
[RFC7234] Fielding, R., Nottingham, M. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10.17487/RFC7672, October 2015.

Authors' Addresses

Daniel Margolis Google, Inc EMail: dmargolis (at)
Mark Risher Google, Inc EMail: risher (at) google (dot com)
Binu Ramakrishnan Yahoo!, Inc EMail: rbinu (at) yahoo-inc (dot com)
Alexander Brotman Comcast, Inc EMail: alex_brotman (at)
Janet Jones Microsoft, Inc EMail: janet.jones (at) microsoft (dot com)