    Internet Engineering Task Force (IETF)                       D. Margolis
    Request for Comments: 8461                                     M. Risher
    Category: Standards Track                                   Google, Inc.
    ISSN: 2070-1721                                          B. Ramakrishnan
                                                                  Oath, Inc.
                                                                  A. Brotman
                                                               Comcast, Inc.
                                                                    J. Jones
                                                             Microsoft, Inc.
                                                              September 2018

                  SMTP MTA Strict Transport Security (MTA-STS)

    Abstract

       SMTP MTA Strict Transport Security (MTA-STS) is a mechanism enabling
       mail service providers (SPs) 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 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 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8461.

    Margolis, et al.             Standards Track                    [Page 1]

    RFC 8461                         MTA-STS                  September 2018

    Copyright Notice

       Copyright (c) 2018 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
       (https://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.

    Margolis, et al.             Standards Track                    [Page 2]

    RFC 8461                         MTA-STS                  September 2018

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
         1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
       2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   5
       3.  Policy Discovery  . . . . . . . . . . . . . . . . . . . . . .   5
         3.1.  MTA-STS TXT Records . . . . . . . . . . . . . . . . . . .   6
         3.2.  MTA-STS Policies  . . . . . . . . . . . . . . . . . . . .   7
         3.3.  HTTPS Policy Fetching . . . . . . . . . . . . . . . . . .  10
         3.4.  Policy Selection for Smart Hosts and Subdomains . . . . .  11
       4.  Policy Validation . . . . . . . . . . . . . . . . . . . . . .  11
         4.1.  MX Host Validation  . . . . . . . . . . . . . . . . . . .  12
         4.2.  Recipient MTA Certificate Validation  . . . . . . . . . .  12
       5.  Policy Application  . . . . . . . . . . . . . . . . . . . . .  12
         5.1.  Policy Application Control Flow . . . . . . . . . . . . .  13
       6.  Reporting Failures  . . . . . . . . . . . . . . . . . . . . .  13
       7.  Interoperability Considerations . . . . . . . . . . . . . . .  14
         7.1.  SNI Support . . . . . . . . . . . . . . . . . . . . . . .  14
         7.2.  Minimum TLS Version Support . . . . . . . . . . . . . . .  14
       8.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
         8.1.  Policy Updates  . . . . . . . . . . . . . . . . . . . . .  15
         8.2.  Policy Delegation . . . . . . . . . . . . . . . . . . . .  15
         8.3.  Removing MTA-STS  . . . . . . . . . . . . . . . . . . . .  16
         8.4.  Preserving MX Candidate Traversal . . . . . . . . . . . .  17
       9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
         9.1.  Well-Known URIs Registry  . . . . . . . . . . . . . . . .  17
         9.2.  MTA-STS TXT Record Fields . . . . . . . . . . . . . . . .  17
         9.3.  MTA-STS Policy Fields . . . . . . . . . . . . . . . . . .  18
       10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
         10.1.  Obtaining a Signed Certificate . . . . . . . . . . . . .  18
         10.2.  Preventing Policy Discovery  . . . . . . . . . . . . . .  19
         10.3.  Denial of Service  . . . . . . . . . . . . . . . . . . .  19
         10.4.  Weak Policy Constraints  . . . . . . . . . . . . . . . .  20
         10.5.  Compromise of the Web PKI System . . . . . . . . . . . .  20
       11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
         11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
         11.2.  Informative References . . . . . . . . . . . . . . . . .  23
       Appendix A.  MTA-STS Example Record and Policy  . . . . . . . . .  25
       Appendix B.  Message Delivery Pseudocode  . . . . . . . . . . . .  25
       Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

    Margolis, et al.             Standards Track                    [Page 3]

    RFC 8461                         MTA-STS                  September 2018

    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, via a combination of DNS and HTTPS, specifying:

       o  whether MTAs sending mail to this domain can expect PKIX-
          authenticated TLS support

       o  what a conforming client should do with messages when TLS cannot
          be successfully negotiated

    1.1.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

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

       o  MTA-STS Policy: A commitment by the Policy Domain to support TLS
          authenticated with PKIX [RFC5280] for the specified MX hosts.

       o  Policy Domain: The domain for which an MTA-STS Policy is defined.
          This is the next-hop domain; when sending mail to
          "alice@example.com", this would ordinarily be "example.com", but
          this may be overridden by explicit routing rules (as described in
          Section 3.4, "Policy Selection for Smart Hosts and Subdomains").

       o  Policy Host: The HTTPS host that serves the MTA-STS Policy for a
          Policy Domain.  Rules for constructing the hostname are described
          in Section 3.2, "MTA-STS Policies".

       o  Sender or Sending MTA: The SMTP MTA sending an email message.

    Margolis, et al.             Standards Track                    [Page 4]

    RFC 8461                         MTA-STS                  September 2018

       o  ABNF: Augmented Backus-Naur Form, a syntax for formally specifying
          syntax, defined in [RFC5234] and [RFC7405].

    2.  Related Technologies

       The DNS-Based Authentication of a Named Entities (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 certification authorities (CAs) and does not
       require DNSSEC, at a cost of risking malicious downgrades.  For a
       thorough discussion of this trade-off, see Section 10, "Security
       Considerations".

       In addition, MTA-STS provides an optional testing-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 MXes, 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 ensure 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 MTA-STS
       Policy 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 that a cached policy is still current
       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.

    Margolis, et al.             Standards Track                    [Page 5]

    RFC 8461                         MTA-STS                  September 2018

    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 "example.com", this record would
       be "_mta-sts.example.com".  MTA-STS TXT records MUST be US-ASCII,
       semicolon-separated key/value pairs containing the following fields:

       o  "v" (plaintext, required): Currently, only "STSv1" is supported.

       o  "id" (plaintext, required): A short string used to track policy
          updates.  This string MUST uniquely identify a given instance of a
          policy, such that senders can determine when the policy has been
          updated by comparing to the "id" of a previously seen policy.
          There is no implied ordering of "id" fields between revisions.

       An example TXT record is as below:

       _mta-sts.example.com.  IN TXT "v=STSv1; id=20160831085700Z;"

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

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

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

       sts-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 CTLs

       The TXT record MUST begin with the sts-version field; the order of
       other fields is not significant.  If multiple TXT records for
       "_mta-sts" are returned by the resolver, records that do not begin
       with "v=STSv1;" are discarded.  If the number of resulting records is
       not one, or if the resulting record is syntactically invalid, senders
       MUST assume the recipient domain does not have an available MTA-STS

    Margolis, et al.             Standards Track                    [Page 6]

    RFC 8461                         MTA-STS                  September 2018

       Policy and skip the remaining steps of policy discovery.  (Note that
       the absence of a usable TXT record is not by itself sufficient to
       remove a sender's previously cached policy for the Policy Domain, as
       discussed in Section 5.1, "Policy Application Control Flow".)  If the
       resulting TXT record contains multiple strings, then the record MUST
       be treated as if those strings are concatenated without adding
       spaces.

       The "_mta-sts" record MAY return a CNAME that points (directly or via
       other CNAMEs) to a TXT record, in which case senders MUST follow the
       CNAME pointers.  This can be used for policy delegation, as described
       in Section 8.2.

    3.2.  MTA-STS Policies

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

       Thus, for a Policy Domain of "example.com", the full URL is
       "https://mta-sts.example.com/.well-known/mta-sts.txt".

       When fetching a policy, senders SHOULD validate that the media type
       is "text/plain" to guard against cases where web servers allow
       untrusted users to host non-text content (typically, HTML or images)
       at a user-defined path.  All parameters other than charset=utf-8 or
       charset=us-ascii are ignored.  Additional "Content-Type" parameters
       are also ignored.

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

       o  "version": Currently, only "STSv1" is supported.

       o  "mode": One of "enforce", "testing", or "none", indicating the
          expected behavior of a Sending MTA in the case of a policy
          validation failure.  See Section 5, "Policy Application", for more
          details about the three modes.

       o  "max_age": Max lifetime of the policy (plaintext non-negative
          integer seconds, maximum value of 31557600).  Well-behaved clients
          SHOULD cache a policy for up to this value from the last policy
          fetch time.  To mitigate the risks of attacks at policy refresh
          time, it is expected that this value typically be in the range of
          weeks or greater.

    Margolis, et al.             Standards Track                    [Page 7]

    RFC 8461                         MTA-STS                  September 2018

       o  "mx": Allowed MX patterns.  One or more patterns matching allowed
          MX hosts for the Policy Domain.  As an example,

                            mx: mail.example.com <CRLF>
                            mx: *.example.net

       indicates that mail for this domain might be handled by MX
       "mail.example.com" or any MX at "example.net".  Valid patterns can be
       either fully specified names ("example.com") or suffixes prefixed by
       a wildcard ("*.example.net").  If a policy specifies more than one
       MX, each MX MUST have its own "mx:" key, and each MX key/value pair
       MUST be on its own line in the policy file.  In the case of
       Internationalized Domain Names [RFC5891], the "mx" value MUST specify
       the Punycode-encoded A-label [RFC3492] to match against, and not the
       Unicode-encoded U-label.  The full semantics of certificate
       validation (including the use of wildcard patterns) are described in
       Section 4.1, "MX Host Validation".

       An example policy is as below:

                             version: STSv1
                             mode: enforce
                             mx: mail.example.com
                             mx: *.example.net
                             mx: backupmx.example.com
                             max_age: 604800

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

    sts-policy-record        = sts-policy-field *WSP
                               *(sts-policy-term sts-policy-field *WSP)
                               [sts-policy-term]

    sts-policy-field         = sts-policy-version /      ; required once
                               sts-policy-mode    /      ; required once
                               sts-policy-max-age /      ; required once
                               sts-policy-mx /
                               ; required at least once, except when
                               ; mode is "none"
                               sts-policy-extension      ; other fields

    sts-policy-field-delim   = ":" *WSP

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

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

    Margolis, et al.             Standards Track                    [Page 8]

    RFC 8461                         MTA-STS                  September 2018

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

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

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

    sts-policy-mode-value    =  %s"testing" / %s"enforce" / %s"none"

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

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

    sts-policy-mx-value      = ["*."] Domain

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

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

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

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

    sts-policy-ext-name      = (sts-policy-alphanum)
                               *31(sta-policy-alphanum / "_" / "-" / ".")

    sts-policy-term          = LF / CRLF

    sts-policy-ext-value     = sts-policy-vchar
                               [*(%x20 / sts-policy-vchar)
                               sts-policy-vchar]
                               ; chars, including UTF-8 [RFC3629],
                               ; excluding CTLs and no
                               ; leading/trailing spaces

    sts-policy-alphanum     = ALPHA / DIGIT

    sts-policy-vchar        = %x21-7E / UTF8-2 / UTF8-3 / UTF8-4

    UTF8-2          =   <Defined in Section 4 of [RFC3629]>

    UTF8-3          =   <Defined in Section 4 of [RFC3629]>

    UTF8-4          =   <Defined in Section 4 of [RFC3629]>

    Margolis, et al.             Standards Track                    [Page 9]

    RFC 8461                         MTA-STS                  September 2018

    Domain          =   <Defined in Section 4.1.2 of [RFC5321]>

       Parsers MUST accept TXT records and policy files that are
       syntactically valid (i.e., valid key/value pairs separated by
       semicolons for TXT records), possibly 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.

    3.3.  HTTPS Policy Fetching

       Policy bodies are, as described above, retrieved by Sending MTAs via
       HTTPS [RFC2818].  During the TLS handshake initiated to fetch a new
       or updated policy from the Policy Host, the Policy Host HTTPS server
       MUST present an X.509 certificate that is valid for the "mta-sts"
       DNS-ID [RFC6125] (e.g., "mta-sts.example.com") 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.  See [RFC5280] for more details about certificate
       verification.

       The certificate is valid for the Policy Host (i.e., "mta-sts"
       prepended to the Policy Domain) with respect to the rules described
       in [RFC6125], with the following application-specific considerations:

       o  Matching is performed only against the DNS-ID identifiers.

       o  DNS domain names in server certificates MAY contain the wildcard
          character '*' as the complete left-most label within the
          identifier.

       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 where the HTTPS GET fails, implementers SHOULD
       limit further attempts to a period of five minutes or longer per
       version ID, to avoid overwhelming resource-constrained recipients
       with cascading failures.

    Margolis, et al.             Standards Track                   [Page 10]

    RFC 8461                         MTA-STS                  September 2018

       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 is 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.

       Finally, to mitigate the risk of persistent interference with policy
       refresh, as discussed in-depth in Section 10, MTAs SHOULD proactively
       refresh cached policies before they expire; a suggested refresh
       frequency is once per day.  To enable administrators to discover
       problems with policy refresh, MTAs SHOULD alert administrators
       (through the use of logs or similar) when such attempts fail, unless
       the cached policy mode is "none".

    3.4.  Policy Selection for Smart Hosts and Subdomains

       When sending mail via a "smart host" -- an administratively
       configured intermediate SMTP relay, which is different from the
       message recipient's server as determined from DNS -- compliant
       senders MUST treat the smart host domain as the Policy Domain for the
       purposes of policy discovery and application.  This specification
       does not provide a means of associating policies with email addresses
       that employ Address Literals [RFC5321].

       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 "user@mail.example.com", the policy can be fetched only from
       "mail.example.com", not "example.com".

    4.  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
       check whether:

       1.  At least one of the policy's "mx" patterns matches the selected
           MX host, as described in Section 4.1, "MX Host Validation".

    Margolis, et al.             Standards Track                   [Page 11]

    RFC 8461                         MTA-STS                  September 2018

       2.  The recipient mail server supports STARTTLS and offers a PKIX-
           based TLS certificate, during TLS handshake, which is valid for
           that host, as described in Section 4.2, "Recipient MTA
           Certificate Validation".

       When these conditions are not met, a policy is said to fail to
       validate.  This section does not dictate the behavior of Sending MTAs
       when the above conditions are not met; see Section 5, "Policy
       Application", for a description of Sending MTA behavior when policy
       validation fails.

    4.1.  MX Host Validation

       A receiving candidate MX host is valid according to an applied MTA-
       STS Policy if the MX record name matches one or more of the "mx"
       fields in the applied policy.  Matching is identical to the rules
       given in [RFC6125], with the restriction that the wildcard character
       '*' may only be used to match the entire left-most label in the
       presented identifier.  Thus, the mx pattern "*.example.com" matches
       "mail.example.com" but not "example.com" or "foo.bar.example.com".

    4.2.  Recipient MTA Certificate Validation

       The certificate presented by the receiving MTA MUST not be expired
       and MUST chain to a root CA that is trusted by the Sending MTA.  The
       certificate MUST have a subject alternative name (SAN) [RFC5280] with
       a DNS-ID [RFC6125] matching the hostname, per the rules given in
       [RFC6125].  The MX's certificate MAY also be checked for revocation
       via OCSP [RFC6960], CRLs [RFC6818], or some other mechanism.

    5.  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 in 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 that fail MX matching or certificate validation
           or that do not support STARTTLS.

       2.  "testing": In this mode, Sending MTAs that also implement the
           TLSRPT (TLS Reporting) specification [RFC8460] send a report
           indicating policy application failures (as long as TLSRPT is also
           implemented by the recipient domain); in any case, messages may
           be delivered as though there were no MTA-STS validation failure.

    Margolis, et al.             Standards Track                   [Page 12]

    RFC 8461                         MTA-STS                  September 2018

       3.  "none": In this mode, Sending MTAs should treat the Policy Domain
           as though it does not have any active policy; see Section 8.3,
           "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, via DNS, 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.

    5.1.  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.  If a policy is present with an "enforce"
           mode, when attempting to deliver to each candidate MX, ensure
           STARTTLS support and host identity validity as described in
           Section 4, "Policy Validation".  If a candidate fails validation,
           continue to the next candidate (if there is one).

       3.  A message delivery attempt 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 4.5.4.1).

    6.  Reporting Failures

       MTA-STS is intended to be used along with TLSRPT [RFC8460] in order
       to ensure that 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 that also implement
       TLSRPT SHOULD treat the following events as reportable failures:

       o  HTTPS policy fetch failures when a valid TXT record is present.

       o  Policy fetch failures of any kind when a valid policy exists in
          the policy cache, except if that policy's mode is "none".

    Margolis, et al.             Standards Track                   [Page 13]

    RFC 8461                         MTA-STS                  September 2018

       o  Delivery attempts in which a contacted MX does not support
          STARTTLS or does not present a certificate that validates
          according to the applied policy, except if that policy's mode is
          "none".

    7.  Interoperability Considerations

    7.1.  SNI Support

       To ensure that the server sends the right certificate chain, the SMTP
       client MUST have support for the TLS Server Name Indication (SNI)
       extension [RFC6066].  When connecting to an HTTP server to retrieve
       the MTA-STS Policy, the SNI extension MUST contain the name of the
       Policy Host (e.g., "mta-sts.example.com").  When connecting to an
       SMTP server, the SNI extension MUST contain the MX hostname.

       HTTP servers used to deliver MTA-STS policies MAY rely on SNI to
       determine which certificate chain to present to the client.  HTTP
       servers MUST respond with a certificate chain that matches the policy
       hostname or abort the TLS handshake if unable to do so.  Clients that
       do not send SNI information may not see the expected certificate
       chain.

       SMTP servers MAY rely on SNI to determine which certificate chain to
       present to the client.  However, servers that have one identity and a
       single matching certificate do not require SNI support.  Servers MUST
       NOT enforce the use of SNI by clients, as the client may be using
       unauthenticated opportunistic TLS and may not expect any particular
       certificate from the server.  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.

    7.2.  Minimum TLS Version Support

       MTAs supporting MTA-STS MUST have support for TLS 1.2 [RFC5246] or
       TLS 1.3 [RFC8446] or higher.  The general TLS usage guidance in
       [RFC7525] SHOULD be followed.

    Margolis, et al.             Standards Track                   [Page 14]

    RFC 8461                         MTA-STS                  September 2018

    8.  Operational Considerations

    8.1.  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 that 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 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.

    8.2.  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 provider.  This allows the
       provider to control update signaling.

       Second, the Policy Domain must point the "well-known" policy location
       to the provider.  This can be done either by setting the "mta-sts"
       record to an IP address or CNAME specified by the provider and by
       giving the provider a TLS certificate that is valid for that host or
       by setting up a "reverse proxy" (also known as a "gateway") server
       for the Policy Domain's Policy Host, configured to serve proxied
       responses from the Policy Host of the provider.

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

       DNS:

            _mta-sts.user.example.  IN CNAME _mta-sts.provider.example.

    Margolis, et al.             Standards Track                   [Page 15]

    RFC 8461                         MTA-STS                  September 2018

       Policy:

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

       Note that in all such cases, the policy endpoint
       ("https://mta-sts.user.example/.well-known/mta-sts.txt" in this
       example) must still present a certificate valid for the Policy Host
       ("mta-sts.user.example"), and not for that host at the provider's
       domain ("mta-sts.provider.example").

       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.

    8.3.  Removing MTA-STS

       In order to facilitate clean opt-out of MTA-STS by implementing
       Policy Domains, and to distinguish clearly between failures that
       indicate attacks and those that 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 policies older than the
           previously published policy may have been served with a greater
           "max_age" than the previously published policy, allowing
           overlapping policy caches -- safely remove the TXT record and
           HTTPS endpoint.

    Margolis, et al.             Standards Track                   [Page 16]

    RFC 8461                         MTA-STS                  September 2018

    8.4.  Preserving MX Candidate Traversal

       Implementers of send-time MTA-STS validation in mail transfer agents
       should take note of the risks of modifying the logic of traversing MX
       candidate lists.  Because an MTA-STS Policy can be used to prefilter
       invalid MX candidates from the MX candidate list, it is tempting to
       implement a "two-pass" model, where MX candidates are first filtered
       for possible validity according to the MTA-STS Policy, and then the
       remaining candidates are attempted in order as without an MTA-STS
       Policy.  This may lead to incorrect implementations, such as message
       loops; instead, it is recommended that implementers traverse the MX
       candidate list as usual, and treat invalid candidates as though they
       were unreachable (i.e., as though there were some transient error
       when trying to deliver to that candidate).

       One consequence of validating MX hosts in order of ordinary candidate
       traversal is that in the event a higher-priority MX is MTA-STS valid
       and a lower-priority MX is not, senders may never encounter the
       lower-priority MX, leading to a risk that policy misconfigurations
       that apply only to "backup" MXes may only be discovered in the case
       of primary MX failure.

    9.  IANA Considerations

    9.1.  Well-Known URIs Registry

       A new "well-known" URI as described in Section 3 has been registered
       in the "Well-Known URIs" registry as described below:

       URI Suffix: mta-sts.txt

       Change Controller: IETF

    9.2.  MTA-STS TXT Record Fields

       IANA has created 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 8461 |
           | id         | Policy instance ID | Section 3.1 of RFC 8461 |
           +------------+--------------------+-------------------------+

       New fields are added to this registry using IANA's "Expert Review"
       policy [RFC8126].

    Margolis, et al.             Standards Track                   [Page 17]

    RFC 8461                         MTA-STS                  September 2018

    9.3.  MTA-STS Policy Fields

       IANA has created 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 8461 |
          | mode       | Enforcement behavior | Section 3.2 of RFC 8461 |
          | max_age    | Policy lifetime      | Section 3.2 of RFC 8461 |
          | mx         | MX identities        | Section 3.2 of RFC 8461 |
          +------------+----------------------+-------------------------+

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

    10.  Security Considerations

       SMTP MTA-STS attempts to protect against an active attacker trying to
       intercept or tamper with mail between hosts that support STARTTLS.
       There are two classes of attacks considered:

       o  Foiling TLS negotiation (for example, by deleting the "250
          STARTTLS" response from a server or altering TLS session
          negotiation).  This would result in the SMTP session occurring
          over plaintext, despite both parties supporting TLS.

       o  Impersonating the destination mail server, whereby the sender
          might deliver the message to an impostor, who could then monitor
          and/or modify messages despite opportunistic TLS.  This
          impersonation could be accomplished by spoofing the DNS MX record
          for the recipient domain or by redirecting client connections
          intended for the legitimate recipient server (for example, by
          altering BGP routing tables).

       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.

    10.1.  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 CA) are thus able to circumvent STS authentication.

    Margolis, et al.             Standards Track                   [Page 18]

    RFC 8461                         MTA-STS                  September 2018

    10.2.  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 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,
       implementers SHOULD prefer policy "max_age" values to be as long as
       is practical.

       Because this attack is also possible upon refresh of a cached policy,
       implementers SHOULD NOT wait until a cached policy has expired before
       checking for an update; if senders attempt to refresh the cache
       regularly (for example, by fetching the current live policy in a
       background task that runs daily or weekly, regardless of the state of
       the "_mta-sts" TXT record, and updating their cache's "max age"
       accordingly), 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 8.3.)

       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.

    10.3.  Denial of Service

       We additionally consider the Denial-of-Service risk posed by an
       attacker who can modify the DNS records for a recipient 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

    Margolis, et al.             Standards Track                   [Page 19]

    RFC 8461                         MTA-STS                  September 2018

       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 "tumblr.com"), 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.

    10.4.  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 "example.com" is to set
       the "mx" equal to "*.example.com"; in this case, recipient domains
       must consider the risk that any user possessing a valid hostname and
       CA-signed certificate (for example, "dhcp-123.example.com") will,
       from the perspective of MTA-STS Policy validation, be a valid MX host
       for that domain.

    10.5.  Compromise of the Web PKI System

       A number of risks apply to the PKI system that is 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.

    Margolis, et al.             Standards Track                   [Page 20]

    RFC 8461                         MTA-STS                  September 2018

       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 CA 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 trade-offs and are
       not universally implemented; we nonetheless recommend implementers of
       MTA-STS to implement revocation mechanisms that are most applicable
       to their implementations.

    11.  References

    11.1.  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,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
                  DOI 10.17487/RFC2818, May 2000,
                  <https://www.rfc-editor.org/info/rfc2818>.

       [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
                  Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
                  February 2002, <https://www.rfc-editor.org/info/rfc3207>.

       [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
                  for Internationalized Domain Names in Applications
                  (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
                  <https://www.rfc-editor.org/info/rfc3492>.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                  2003, <https://www.rfc-editor.org/info/rfc3629>.

       [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <https://www.rfc-editor.org/info/rfc5234>.

    Margolis, et al.             Standards Track                   [Page 21]

    RFC 8461                         MTA-STS                  September 2018

       [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
                  (TLS) Protocol Version 1.2", RFC 5246,
                  DOI 10.17487/RFC5246, August 2008,
                  <https://www.rfc-editor.org/info/rfc5246>.

       [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,
                  <https://www.rfc-editor.org/info/rfc5280>.

       [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                  DOI 10.17487/RFC5321, October 2008,
                  <https://www.rfc-editor.org/info/rfc5321>.

       [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
                  Uniform Resource Identifiers (URIs)", RFC 5785,
                  DOI 10.17487/RFC5785, April 2010,
                  <https://www.rfc-editor.org/info/rfc5785>.

       [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
                  Extensions: Extension Definitions", RFC 6066,
                  DOI 10.17487/RFC6066, January 2011,
                  <https://www.rfc-editor.org/info/rfc6066>.

       [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, <https://www.rfc-editor.org/info/rfc6125>.

       [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
                  RFC 7405, DOI 10.17487/RFC7405, December 2014,
                  <https://www.rfc-editor.org/info/rfc7405>.

       [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, <https://www.rfc-editor.org/info/rfc7525>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    Margolis, et al.             Standards Track                   [Page 22]

    RFC 8461                         MTA-STS                  September 2018

       [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
                  Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
                  <https://www.rfc-editor.org/info/rfc8446>.

       [RFC8460]  Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
                  and M. Risher, "SMTP TLS Reporting", RFC 8460,
                  DOI 10.17487/RFC8460, September 2018,
                  <https://www.rfc-editor.org/info/rfc8460>.

    11.2.  Informative References

       [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,
                  <https://www.rfc-editor.org/info/rfc4033>.

       [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
                  DOI 10.17487/RFC5322, October 2008,
                  <https://www.rfc-editor.org/info/rfc5322>.

       [RFC5891]  Klensin, J., "Internationalized Domain Names in
                  Applications (IDNA): Protocol", RFC 5891,
                  DOI 10.17487/RFC5891, August 2010,
                  <https://www.rfc-editor.org/info/rfc5891>.

       [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, <https://www.rfc-editor.org/info/rfc6818>.

       [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,
                  <https://www.rfc-editor.org/info/rfc6960>.

       [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                  Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                  RFC 7234, DOI 10.17487/RFC7234, June 2014,
                  <https://www.rfc-editor.org/info/rfc7234>.

       [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,
                  <https://www.rfc-editor.org/info/rfc7672>.

    Margolis, et al.             Standards Track                   [Page 23]

    RFC 8461                         MTA-STS                  September 2018

       [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

    Margolis, et al.             Standards Track                   [Page 24]

    RFC 8461                         MTA-STS                  September 2018

    Appendix A.  MTA-STS Example Record and Policy

       The owner of "example.com" 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 MXes
       that handle mail for "example.com", confirm that TLS is correctly
       used, and ensure that certificates presented by the recipient MX
       validate.

       MTA-STS Policy indicator TXT RR:

           _mta-sts.example.com.  IN TXT "v=STSv1; id=20160831085700Z;"

       MTA-STS Policy file served as the response body at
       "https://mta-sts.example.com/.well-known/mta-sts.txt":

                             version: STSv1
                             mode: testing
                             mx: mx1.example.com
                             mx: mx2.example.com
                             mx: mx.backup-example.com
                             max_age: 1296000

    Appendix B.  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, that may be undesirable in a working
       implementation, and we expect some implementers to instead prefer a
       background fetch that does not block delivery when 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 STARTTLS connection with the MX.
       }

       func certMatches(connection, host) {

    Margolis, et al.             Standards Track                   [Page 25]

    RFC 8461                         MTA-STS                  September 2018

         // Assume a handy function to return if the server
         // certificate presented in "connection" is valid for "host".
       }

       func policyMatches(candidate, policy) {
         for mx in policy.mx {
           // Literal match.
           if mx == candidate {
             return true
           }
           // Wildcard matches only the leftmost label.
           // Wildcards must always be followed by a '.'.
           if mx[0] == '*' {
             parts = SplitN(candidate, '.', 2)  // Split on the first '.'.
             if len(parts) > 1 && parts[1] == mx[2:] {
               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.

    Margolis, et al.             Standards Track                   [Page 26]

    RFC 8461                         MTA-STS                  September 2018

         }
         secure := true
         if !policyMatches(mx, policy) {
           secure = false
           reportError(E_HOST_MISMATCH)
         } else if !tryStartTls(connection) {
           secure = false
           reportError(E_NO_VALID_TLS)
         } else if !certMatches(connection, policy) {
           secure = false
           reportError(E_CERT_MISMATCH)
         }
         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).
       }

    Margolis, et al.             Standards Track                   [Page 27]

    RFC 8461                         MTA-STS                  September 2018

    Contributors

       Wei Chuang
       Google, Inc.
       weihaw@google.com

       Viktor Dukhovni
       ietf-dane@dukhovni.de

       Markus Laber
       1&1 Mail & Media Development & Technology GmbH
       markus.laber@1und1.de

       Nicolas Lidzborski
       Google, Inc.
       nlidz@google.com

       Brandon Long
       Google, Inc.
       blong@google.com

       Franck Martin
       LinkedIn, Inc.
       fmartin@linkedin.com

       Klaus Umbach
       1&1 Mail & Media Development & Technology GmbH
       klaus.umbach@1und1.de

    Margolis, et al.             Standards Track                   [Page 28]

    RFC 8461                         MTA-STS                  September 2018

    Authors' Addresses

       Daniel Margolis
       Google, Inc.

       Email: dmargolis@google.com

       Mark Risher
       Google, Inc.

       Email: risher@google.com

       Binu Ramakrishnan
       Oath, Inc.

       Email: prbinu@yahoo.com

       Alexander Brotman
       Comcast, Inc.

       Email: alex_brotman@comcast.com

       Janet Jones
       Microsoft, Inc.

       Email: janet.jones@microsoft.com

    Margolis, et al.             Standards Track                   [Page 29]