<pre>Internet Engineering Task Force (IETF)                          K. Moore
Request for Comments: 8314                                Windrock, Inc.
Updates: <a href="./rfc1939">1939</a>, <a href="./rfc2595">2595</a>, <a href="./rfc3501">3501</a>, <a href="./rfc5068">5068</a>, <a href="./rfc6186">6186</a>, <a href="./rfc6409">6409</a>                    C. Newman
Category: Standards Track                                         Oracle
ISSN: 2070-1721                                             January 2018


  <span class="h1">Cleartext Considered Obsolete: Use of Transport Layer Security (TLS)</span>
                    <span class="h1">for Email Submission and Access</span>

Abstract

   This specification outlines current recommendations for the use of
   Transport Layer Security (TLS) to provide confidentiality of email
   traffic between a Mail User Agent (MUA) and a Mail Submission Server
   or Mail Access Server.  This document updates RFCs 1939, 2595, 3501,
   5068, 6186, and 6409.

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 <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc8314">https://www.rfc-editor.org/info/rfc8314</a>.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) 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.





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Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. How This Document Updates Previous RFCs ....................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Conventions and Terminology Used in This Document ...............<a href="#page-4">4</a>
   <a href="#section-3">3</a>. Implicit TLS ....................................................<a href="#page-5">5</a>
      <a href="#section-3.1">3.1</a>. Implicit TLS for POP .......................................<a href="#page-5">5</a>
      <a href="#section-3.2">3.2</a>. Implicit TLS for IMAP ......................................<a href="#page-5">5</a>
      <a href="#section-3.3">3.3</a>. Implicit TLS for SMTP Submission ...........................<a href="#page-6">6</a>
      3.4. Implicit TLS Connection Closure for POP, IMAP, and
           SMTP Submission ............................................<a href="#page-7">7</a>
   4. Use of TLS by Mail Access Servers and Message Submission
      Servers .........................................................<a href="#page-7">7</a>
      4.1. Deprecation of Services Using Cleartext and TLS Versions
           Less Than 1.1 ..............................................<a href="#page-8">8</a>
      <a href="#section-4.2">4.2</a>. Mail Server Use of Client Certificate Authentication .......<a href="#page-9">9</a>
      <a href="#section-4.3">4.3</a>. Recording TLS Ciphersuite in "Received" Header Field .......<a href="#page-9">9</a>
      <a href="#section-4.4">4.4</a>. TLS Server Certificate Requirements .......................<a href="#page-10">10</a>
      <a href="#section-4.5">4.5</a>. Recommended DNS Records for Mail Protocol Servers .........<a href="#page-11">11</a>
           <a href="#section-4.5.1">4.5.1</a>. MX Records .........................................<a href="#page-11">11</a>
           <a href="#section-4.5.2">4.5.2</a>. SRV Records ........................................<a href="#page-11">11</a>
           <a href="#section-4.5.3">4.5.3</a>. DNSSEC .............................................<a href="#page-11">11</a>
           <a href="#section-4.5.4">4.5.4</a>. TLSA Records .......................................<a href="#page-11">11</a>
      <a href="#section-4.6">4.6</a>. Changes to Internet-Facing Servers ........................<a href="#page-11">11</a>
   <a href="#section-5">5</a>. Use of TLS by Mail User Agents .................................<a href="#page-12">12</a>
      <a href="#section-5.1">5.1</a>. Use of SRV Records in Establishing Configuration ..........<a href="#page-13">13</a>
      <a href="#section-5.2">5.2</a>. Minimum Confidentiality Level .............................<a href="#page-14">14</a>
      <a href="#section-5.3">5.3</a>. Certificate Validation ....................................<a href="#page-15">15</a>
      <a href="#section-5.4">5.4</a>. Certificate Pinning .......................................<a href="#page-15">15</a>
      <a href="#section-5.5">5.5</a>. Client Certificate Authentication .........................<a href="#page-16">16</a>
   6. Considerations Related to Antivirus/Antispam Software
      and Services ...................................................<a href="#page-17">17</a>
   <a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-17">17</a>
      <a href="#section-7.1">7.1</a>. POP3S Port Registration Update ............................<a href="#page-17">17</a>
      <a href="#section-7.2">7.2</a>. IMAPS Port Registration Update ............................<a href="#page-18">18</a>
      <a href="#section-7.3">7.3</a>. Submissions Port Registration .............................<a href="#page-18">18</a>
      <a href="#section-7.4">7.4</a>. Additional Registered Clauses for "Received" Fields .......<a href="#page-19">19</a>
   <a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-19">19</a>
   <a href="#section-9">9</a>. References .....................................................<a href="#page-20">20</a>
      <a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-20">20</a>
      <a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-22">22</a>
   <a href="#appendix-A">Appendix A</a>. Design Considerations .................................<a href="#page-24">24</a>
   Acknowledgements ..................................................<a href="#page-26">26</a>
   Authors' Addresses ................................................<a href="#page-26">26</a>







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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Software that provides email service via the Internet Message Access
   Protocol (IMAP) [<a href="./rfc3501" title="&quot;INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1&quot;">RFC3501</a>], the Post Office Protocol (POP) [<a href="./rfc1939" title="&quot;Post Office Protocol - Version 3&quot;">RFC1939</a>],
   and/or Simple Mail Transfer Protocol (SMTP) Submission [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>]
   usually has Transport Layer Security (TLS) [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>] support but
   often does not use it in a way that maximizes end-user
   confidentiality.  This specification describes current
   recommendations for the use of TLS in interactions between Mail User
   Agents (MUAs) and Mail Access Servers, and also between MUAs and Mail
   Submission Servers.

   In brief, this memo now recommends that:

   o  TLS version 1.2 or greater be used for all traffic between MUAs
      and Mail Submission Servers, and also between MUAs and Mail Access
      Servers.

   o  MUAs and Mail Service Providers (MSPs) (a) discourage the use of
      cleartext protocols for mail access and mail submission and
      (b) deprecate the use of cleartext protocols for these purposes as
      soon as practicable.

   o  Connections to Mail Submission Servers and Mail Access Servers be
      made using "Implicit TLS" (as defined below), in preference to
      connecting to the "cleartext" port and negotiating TLS using the
      STARTTLS command or a similar command.

   This memo does not address the use of TLS with SMTP for message relay
   (where Message Submission [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>] does not apply).  Improving the
   use of TLS with SMTP for message relay requires a different approach.
   One approach to address that topic is described in [<a href="./rfc7672" title="&quot;SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)&quot;">RFC7672</a>]; another
   is provided in [<a href="#ref-MTA-STS" title="&quot;SMTP MTA Strict Transport Security (MTA-STS)&quot;">MTA-STS</a>].

   The recommendations in this memo do not replace the functionality of,
   and are not intended as a substitute for, end-to-end encryption of
   electronic mail.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  How This Document Updates Previous RFCs</span>

   This document updates POP (<a href="./rfc1939">RFC 1939</a>), IMAP (<a href="./rfc3501">RFC 3501</a>), and Submission
   (<a href="./rfc6409">RFC 6409</a>, <a href="./rfc5068">RFC 5068</a>) in two ways:

   1.  By adding Implicit TLS ports as Standards Track ports for these
       protocols as described in <a href="#section-3">Section 3</a>.

   2.  By updating TLS best practices that apply to these protocols as
       described in Sections <a href="#section-4">4</a> and <a href="#section-5">5</a>.



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   This document updates <a href="./rfc2595">RFC 2595</a> by replacing <a href="./rfc2595#section-7">Section&nbsp;7 of RFC 2595</a>
   with the preference for Implicit TLS as described in Sections <a href="#section-1">1</a> and <a href="#section-3">3</a>
   of this document, as well as by updating TLS best practices that
   apply to the protocols in <a href="./rfc2595">RFC 2595</a> as described in Sections <a href="#section-4">4</a> and <a href="#section-5">5</a>
   of this document.

   This document updates <a href="./rfc6186">RFC 6186</a> as described herein, in <a href="#section-5.1">Section 5.1</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Conventions and Terminology Used in This Document</span>

   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
   <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] [<a href="./rfc8174" title="&quot;Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words&quot;">RFC8174</a>] when, and only when, they appear in all
   capitals, as shown here.

   The term "Implicit TLS" refers to the automatic negotiation of TLS
   whenever a TCP connection is made on a particular TCP port that is
   used exclusively by that server for TLS connections.  The term
   "Implicit TLS" is intended to contrast with the use of STARTTLS and
   similar commands in POP, IMAP, SMTP Message Submission, and other
   protocols, that are used by the client and the server to explicitly
   negotiate TLS on an established cleartext TCP connection.

   The term "Mail Access Server" refers to a server for POP, IMAP, and
   any other protocol used to access or modify received messages, or to
   access or modify a mail user's account configuration.

   The term "Mail Submission Server" refers to a server for the protocol
   specified in [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>] (or one of its predecessors or successors) for
   submission of outgoing messages for delivery to recipients.

   The term "Mail Service Provider" (or "MSP") refers to an operator of
   Mail Access Servers and/or Mail Submission Servers.

   The term "Mail Account" refers to a user's identity with an MSP, that
   user's authentication credentials, any user email that is stored by
   the MSP, and any other per-user configuration information maintained
   by the MSP (for example, instructions for filtering spam).  Most MUAs
   support the ability to access multiple Mail Accounts.

   For each account that an MUA accesses on its user's behalf, it must
   have the server names, ports, authentication credentials, and other
   configuration information specified by the user.  This information,
   which is used by the MUA, is referred to as "Mail Account
   Configuration".





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   This specification expresses syntax using the Augmented Backus-Naur
   Form (ABNF) as described in [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>], including the core rules
   provided in <a href="./rfc5234#appendix-B">Appendix&nbsp;B of [RFC5234]</a> and the rules provided in
   [<a href="./rfc5322" title="&quot;Internet Message Format&quot;">RFC5322</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Implicit TLS</span>

   Previous standards for the use of email protocols with TLS used the
   STARTTLS mechanism: [<a href="./rfc2595" title="&quot;Using TLS with IMAP, POP3 and ACAP&quot;">RFC2595</a>], [<a href="./rfc3207" title="&quot;SMTP Service Extension for Secure SMTP over Transport Layer Security&quot;">RFC3207</a>], and [<a href="./rfc3501" title="&quot;INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1&quot;">RFC3501</a>].  With
   STARTTLS, the client establishes a cleartext application session and
   determines whether to issue a STARTTLS command based on server
   capabilities and client configuration.  If the client issues a
   STARTTLS command, a TLS handshake follows that can upgrade the
   connection.  Although this mechanism has been deployed, an alternate
   mechanism where TLS is negotiated immediately at connection start on
   a separate port (referred to in this document as "Implicit TLS") has
   been deployed more successfully.  To encourage more widespread use of
   TLS and to also encourage greater consistency regarding how TLS is
   used, this specification now recommends the use of Implicit TLS for
   POP, IMAP, SMTP Submission, and all other protocols used between an
   MUA and an MSP.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Implicit TLS for POP</span>

   When a TCP connection is established for the "pop3s" service (default
   port 995), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in
   [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>].  Once the TLS session is established, POP3 [<a href="./rfc1939" title="&quot;Post Office Protocol - Version 3&quot;">RFC1939</a>]
   protocol messages are exchanged as TLS application data for the
   remainder of the TCP connection.  After the server sends an +OK
   greeting, the server and client MUST enter the AUTHORIZATION state,
   even if a client certificate was supplied during the TLS handshake.

   See Sections <a href="#section-5.5">5.5</a> and <a href="#section-4.2">4.2</a> for additional information on client
   certificate authentication.  See <a href="#section-7.1">Section 7.1</a> for port registration
   information.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Implicit TLS for IMAP</span>

   When a TCP connection is established for the "imaps" service (default
   port 993), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in
   [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>].  Once the TLS session is established, IMAP [<a href="./rfc3501" title="&quot;INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1&quot;">RFC3501</a>]
   protocol messages are exchanged as TLS application data for the
   remainder of the TCP connection.  If a client certificate was
   provided during the TLS handshake that the server finds acceptable,
   the server MAY issue a PREAUTH greeting, in which case both the




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   server and the client enter the AUTHENTICATED state.  If the server
   issues an OK greeting, then both the server and the client enter the
   NOT AUTHENTICATED state.

   See Sections <a href="#section-5.5">5.5</a> and <a href="#section-4.2">4.2</a> for additional information on client
   certificate authentication.  See <a href="#section-7.2">Section 7.2</a> for port registration
   information.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Implicit TLS for SMTP Submission</span>

   When a TCP connection is established for the "submissions" service
   (default port 465), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in
   [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>].  Once the TLS session is established, Message Submission
   protocol data [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>] is exchanged as TLS application data for the
   remainder of the TCP connection.  (Note: The "submissions" service
   name is defined in <a href="#section-7.3">Section 7.3</a> of this document and follows the usual
   convention that the name of a service layered on top of Implicit TLS
   consists of the name of the service as used without TLS, with an "s"
   appended.)

   The STARTTLS mechanism on port 587 is relatively widely deployed due
   to the situation with port 465 (discussed in <a href="#section-7.3">Section 7.3</a>).  This
   differs from IMAP and POP services where Implicit TLS is more widely
   deployed on servers than STARTTLS.  It is desirable to migrate core
   protocols used by MUA software to Implicit TLS over time, for
   consistency as well as for the additional reasons discussed in
   <a href="#appendix-A">Appendix A</a>.  However, to maximize the use of encryption for
   submission, it is desirable to support both mechanisms for Message
   Submission over TLS for a transition period of several years.  As a
   result, clients and servers SHOULD implement both STARTTLS on
   port 587 and Implicit TLS on port 465 for this transition period.
   Note that there is no significant difference between the security
   properties of STARTTLS on port 587 and Implicit TLS on port 465 if
   the implementations are correct and if both the client and the server
   are configured to require successful negotiation of TLS prior to
   Message Submission.

   Note that the "submissions" port provides access to a Message
   Submission Agent (MSA) as defined in [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>], so requirements and
   recommendations for MSAs in that document, including the requirement
   to implement SMTP AUTH [<a href="./rfc4954" title="&quot;SMTP Service Extension for Authentication&quot;">RFC4954</a>] and the requirements of Email
   Submission Operations [<a href="./rfc5068" title="&quot;Email Submission Operations: Access and Accountability Requirements&quot;">RFC5068</a>], also apply to the submissions port.

   See Sections <a href="#section-5.5">5.5</a> and <a href="#section-4.2">4.2</a> for additional information on client
   certificate authentication.  See <a href="#section-7.3">Section 7.3</a> for port registration
   information.




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<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Implicit TLS Connection Closure for POP, IMAP, and SMTP Submission</span>

   When a client or server wishes to close the connection, it SHOULD
   initiate the exchange of TLS close alerts before TCP connection
   termination.  The client MAY, after sending a TLS close alert,
   gracefully close the TCP connection (e.g., call the close() function
   on the TCP socket or otherwise issue a TCP CLOSE (<a href="./rfc793#section-3.5">[RFC793],
   Section&nbsp;3.5</a>)) without waiting for a TLS response from the server.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Use of TLS by Mail Access Servers and Message Submission Servers</span>

   The following requirements and recommendations apply to Mail Access
   Servers and Mail Submission Servers, or, if indicated, to MSPs:

   o  MSPs that support POP, IMAP, and/or Message Submission MUST
      support TLS access for those protocol servers.

   o  Servers provided by MSPs other than POP, IMAP, and/or Message
      Submission SHOULD support TLS access and MUST support TLS access
      for those servers that support authentication via username and
      password.

   o  MSPs that support POP, IMAP, and/or Message Submission SHOULD
      provide and support instances of those services that use Implicit
      TLS.  (See <a href="#section-3">Section 3</a>.)

   o  For compatibility with existing MUAs and existing MUA
      configurations, MSPs SHOULD also, in the near term, provide
      instances of these services that support STARTTLS.  This will
      permit legacy MUAs to discover new availability of TLS capability
      on servers and may increase the use of TLS by such MUAs.  However,
      servers SHOULD NOT advertise STARTTLS if the use of the STARTTLS
      command by a client is likely to fail (for example, if the server
      has no server certificate configured).

   o  MSPs SHOULD advertise their Mail Access Servers and Mail
      Submission Servers, using DNS SRV records according to [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>].
      (In addition to making correct configuration easier for MUAs, this
      provides a way by which MUAs can discover when an MSP begins to
      offer TLS-based services.)  Servers supporting TLS SHOULD be
      advertised in preference to cleartext servers (if offered).  In
      addition, servers using Implicit TLS SHOULD be advertised in
      preference to servers supporting STARTTLS (if offered).  (See also
      <a href="#section-4.5">Section 4.5</a>.)

   o  MSPs SHOULD deprecate the use of cleartext Mail Access Servers and
      Mail Submission Servers as soon as practicable.  (See
      <a href="#section-4.1">Section 4.1</a>.)



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   o  MSPs currently supporting such use of cleartext SMTP (on port 25)
      as a means of Message Submission by their users (whether or not
      requiring authentication) SHOULD transition their users to using
      TLS (either Implicit TLS or STARTTLS) as soon as practicable.

   o  Mail Access Servers and Mail Submission Servers MUST support
      TLS 1.2 or later.

   o  All Mail Access Servers and Mail Submission Servers SHOULD
      implement the recommended TLS ciphersuites described in [<a href="./rfc7525" title="&quot;Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)&quot;">RFC7525</a>]
      or a future BCP or Standards Track revision of that document.

   o  As soon as practicable, MSPs currently supporting Secure Sockets
      Layer (SSL) 2.x, SSL 3.0, or TLS 1.0 SHOULD transition their users
      to TLS 1.1 or later and discontinue support for those earlier
      versions of SSL and TLS.

   o  Mail Submission Servers accepting mail using TLS SHOULD include in
      the Received field of the outgoing message the TLS ciphersuite of
      the session in which the mail was received.  (See <a href="#section-4.3">Section 4.3</a>.)

   o  All Mail Access Servers and Mail Submission Servers implementing
      TLS SHOULD log TLS cipher information along with any connection or
      authentication logs that they maintain.

   Additional considerations and details appear below.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Deprecation of Services Using Cleartext and TLS Versions</span>
<span class="h3">      Less Than 1.1</span>

   The specific means employed for deprecation of cleartext Mail Access
   Servers and Mail Submission Servers MAY vary from one MSP to the next
   in light of their user communities' needs and constraints.  For
   example, an MSP MAY implement a gradual transition in which, over
   time, more and more users are forbidden to authenticate to cleartext
   instances of these servers, thus encouraging those users to migrate
   to Implicit TLS.  Access to cleartext servers should eventually be
   either (a) disabled or (b) limited strictly for use by legacy systems
   that cannot be upgraded.

   After a user's ability to authenticate to a server using cleartext is
   revoked, the server denying such access MUST NOT provide any
   indication over a cleartext channel of whether the user's
   authentication credentials were valid.  An attempt to authenticate as
   such a user using either invalid credentials or valid credentials
   MUST both result in the same indication of access being denied.





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   Also, users previously authenticating with passwords sent as
   cleartext SHOULD be required to change those passwords when migrating
   to TLS, if the old passwords were likely to have been compromised.
   (For any large community of users using the public Internet to access
   mail without encryption, the compromise of at least some of those
   passwords should be assumed.)

   Transition of users from SSL or TLS 1.0 to later versions of TLS MAY
   be accomplished by a means similar to that described above.  There
   are multiple ways to accomplish this.  One way is for the server to
   refuse a ClientHello message from any client sending a
   ClientHello.version field corresponding to any version of SSL or
   TLS 1.0.  Another way is for the server to accept ClientHello
   messages from some client versions that it does not wish to support
   but later refuse to allow the user to authenticate.  The latter
   method may provide a better indication to the user of the reason for
   the failure but (depending on the protocol and method of
   authentication used) may also risk exposure of the user's password
   over a channel that is known to not provide adequate confidentiality.

   It is RECOMMENDED that new users be required to use TLS version 1.1
   or greater from the start.  However, an MSP may find it necessary to
   make exceptions to accommodate some legacy systems that support only
   earlier versions of TLS or only cleartext.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Mail Server Use of Client Certificate Authentication</span>

   Mail Submission Servers and Mail Access Servers MAY implement client
   certificate authentication on the Implicit TLS port.  Such servers
   MUST NOT request a client certificate during the TLS handshake unless
   the server is configured to accept some client certificates as
   sufficient for authentication and the server has the ability to
   determine a mail server authorization identity matching such
   certificates.  How to make this determination is presently
   implementation specific.

   If the server accepts the client's certificate as sufficient for
   authorization, it MUST enable the Simple Authentication and Security
   Layer (SASL) EXTERNAL mechanism [<a href="./rfc4422" title="&quot;Simple Authentication and Security Layer (SASL)&quot;">RFC4422</a>].  An IMAPS server MAY issue
   a PREAUTH greeting instead of enabling SASL EXTERNAL.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Recording TLS Ciphersuite in "Received" Header Field</span>

   The ESMTPS transmission type [<a href="./rfc3848" title="&quot;ESMTP and LMTP Transmission Types Registration&quot;">RFC3848</a>] provides trace information
   that can indicate that TLS was used when transferring mail.  However,
   TLS usage by itself is not a guarantee of confidentiality or
   security.  The TLS ciphersuite provides additional information about
   the level of security made available for a connection.  This section



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   defines a new SMTP "tls" Received header additional-registered-clause
   that is used to record the TLS ciphersuite that was negotiated for
   the connection.  This clause SHOULD be included whenever a Submission
   server generates a Received header field for a message received via
   TLS.  The value included in this additional clause SHOULD be the
   registered ciphersuite name (e.g.,
   TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in the "TLS Cipher
   Suite Registry".  In the event that the implementation does not know
   the name of the ciphersuite (a situation that should be remedied
   promptly), a four-digit hexadecimal ciphersuite identifier MAY be
   used.  In addition, the Diffie-Hellman group name associated with the
   ciphersuite MAY be included (when applicable and known) following the
   ciphersuite name.  The ABNF for the field follows:

   tls-cipher-clause  =  CFWS "tls" FWS tls-cipher
                         [ CFWS tls-dh-group-clause ]

   tls-cipher         =  tls-cipher-name / tls-cipher-hex

   tls-cipher-name    =  ALPHA *(ALPHA / DIGIT / "_")
   ; as registered in the IANA "TLS Cipher Suite Registry"
   ; &lt;<a href="https://www.iana.org/assignments/tls-parameters">https://www.iana.org/assignments/tls-parameters</a>&gt;

   tls-cipher-hex     =  "0x" 4HEXDIG

   tls-dh-group-clause = "group" FWS dh-group
   ; not to be used except immediately after tls-cipher

   dh-group           = ALPHA *(ALPHA / DIGIT / "_" / "-")
   ; as registered in the IANA "TLS Supported Groups Registry"
   ; &lt;<a href="https://www.iana.org/assignments/tls-parameters">https://www.iana.org/assignments/tls-parameters</a>&gt;

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  TLS Server Certificate Requirements</span>

   MSPs MUST maintain valid server certificates for all servers.  See
   [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>] for the recommendations and requirements necessary to
   achieve this.

   If a protocol server provides service for more than one mail domain,
   it MAY use a separate IP address for each domain and/or a server
   certificate that advertises multiple domains.  This will generally be
   necessary unless and until it is acceptable to impose the constraint
   that the server and all clients support the Server Name Indication
   (SNI) extension to TLS [<a href="./rfc6066" title="&quot;Transport Layer Security (TLS) Extensions: Extension Definitions&quot;">RFC6066</a>].  Mail servers supporting the SNI
   need to support the post-SRV hostname to interoperate with MUAs that
   have not implemented [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>].  For more discussion of this problem,
   see <a href="./rfc7817#section-5.1">Section&nbsp;5.1 of [RFC7817]</a>.




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<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Recommended DNS Records for Mail Protocol Servers</span>

   This section discusses not only the DNS records that are recommended
   but also implications of DNS records for server configuration and TLS
   server certificates.

<span class="h4"><a class="selflink" id="section-4.5.1" href="#section-4.5.1">4.5.1</a>.  MX Records</span>

   It is recommended that MSPs advertise MX records for the handling of
   inbound mail (instead of relying entirely on A or AAAA records) and
   that those MX records be signed using DNSSEC [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>].  This is
   mentioned here only for completeness, as the handling of inbound mail
   is out of scope for this document.

<span class="h4"><a class="selflink" id="section-4.5.2" href="#section-4.5.2">4.5.2</a>.  SRV Records</span>

   MSPs SHOULD advertise SRV records to aid MUAs in determining the
   proper configuration of servers, per the instructions in [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>].

   MSPs SHOULD advertise servers that support Implicit TLS in preference
   to servers that support cleartext and/or STARTTLS operation.

<span class="h4"><a class="selflink" id="section-4.5.3" href="#section-4.5.3">4.5.3</a>.  DNSSEC</span>

   All DNS records advertised by an MSP as a means of aiding clients in
   communicating with the MSP's servers SHOULD be signed using DNSSEC if
   and when the parent DNS zone supports doing so.

<span class="h4"><a class="selflink" id="section-4.5.4" href="#section-4.5.4">4.5.4</a>.  TLSA Records</span>

   MSPs SHOULD advertise TLSA records to provide an additional trust
   anchor for public keys used in TLS server certificates.  However,
   TLSA records MUST NOT be advertised unless they are signed using
   DNSSEC.

<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Changes to Internet-Facing Servers</span>

   When an MSP changes the Internet-facing Mail Access Servers and Mail
   Submission Servers, including SMTP-based spam/virus filters, it is
   generally necessary to support the same and/or a newer version of TLS
   than the one previously used.










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<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Use of TLS by Mail User Agents</span>

   The following requirements and recommendations apply to MUAs:

   o  MUAs SHOULD be capable of using DNS SRV records to discover Mail
      Access Servers and Mail Submission Servers that are advertised by
      an MSP for an account being configured.  Other means of
      discovering server configuration information (e.g., a database
      maintained by the MUA vendor) MAY also be supported.  (See
      <a href="#section-5.1">Section 5.1</a> for more information.)

   o  MUAs SHOULD be configurable to require a minimum level of
      confidentiality for any particular Mail Account and refuse to
      exchange information via any service associated with that Mail
      Account if the session does not provide that minimum level of
      confidentiality.  (See <a href="#section-5.2">Section 5.2</a>.)

   o  MUAs MUST NOT treat a session as meeting a minimum level of
      confidentiality if the server's TLS certificate cannot be
      validated.  (See <a href="#section-5.3">Section 5.3</a>.)

   o  MUAs MAY impose other minimum confidentiality requirements in the
      future, e.g., in order to discourage the use of TLS versions or
      cryptographic algorithms in which weaknesses have been discovered.

   o  MUAs SHOULD provide a prominent indication of the level of
      confidentiality associated with an account configuration that is
      appropriate for the user interface (for example, a "lock" icon or
      changed background color for a visual interface, or some sort of
      audible indication for an audio user interface), at appropriate
      times and/or locations, in order to inform the user of the
      confidentiality of the communications associated with that
      account.  For example, this might be done whenever (a) the user is
      prompted for authentication credentials, (b) the user is composing
      mail that will be sent to a particular submission server, (c) a
      list of accounts is displayed (particularly if the user can select
      from that list to read mail), or (d) the user is asking to view or
      update any configuration data that will be stored on a remote
      server.  If, however, an MUA provides such an indication, it
      MUST NOT indicate confidentiality for any connection that does not
      at least use TLS 1.1 with certificate verification and also meet
      the minimum confidentiality requirements associated with that
      account.

   o  MUAs MUST implement TLS 1.2 [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>] or later.  Earlier TLS and
      SSL versions MAY also be supported, so long as the MUA requires at
      least TLS 1.1 [<a href="./rfc4346" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.1&quot;">RFC4346</a>] when accessing accounts that are
      configured to impose minimum confidentiality requirements.



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   o  All MUAs SHOULD implement the recommended TLS ciphersuites
      described in [<a href="./rfc7525" title="&quot;Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)&quot;">RFC7525</a>] or a future BCP or Standards Track revision
      of that document.

   o  MUAs that are configured to not require minimum confidentiality
      for one or more accounts SHOULD detect when TLS becomes available
      on those accounts (using [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>] or other means) and offer to
      upgrade the account to require TLS.

   Additional considerations and details appear below.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Use of SRV Records in Establishing Configuration</span>

   This document updates [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>] by changing the preference rules and
   adding a new SRV service label _submissions._tcp to refer to Message
   Submission with Implicit TLS.

   User-configurable MUAs SHOULD support the use of [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>] for
   account setup.  However, when using configuration information
   obtained via this method, MUAs SHOULD ignore advertised services that
   do not satisfy minimum confidentiality requirements, unless the user
   has explicitly requested reduced confidentiality.  This will have the
   effect of causing the MUA to default to ignoring advertised
   configurations that do not support TLS, even when those advertised
   configurations have a higher priority than other advertised
   configurations.

   When using configuration information per [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>], MUAs SHOULD NOT
   automatically establish new configurations that do not require TLS
   for all servers, unless there are no advertised configurations using
   TLS.  If such a configuration is chosen, prior to attempting to
   authenticate to the server or use the server for Message Submission,
   the MUA SHOULD warn the user that traffic to that server will not be
   encrypted and that it will therefore likely be intercepted by
   unauthorized parties.  The specific wording is to be determined by
   the implementation, but it should adequately capture the sense of
   risk, given the widespread incidence of mass surveillance of email
   traffic.

   Similarly, an MUA MUST NOT attempt to "test" a particular Mail
   Account configuration by submitting the user's authentication
   credentials to a server, unless a TLS session meeting minimum
   confidentiality levels has been established with that server.  If
   minimum confidentiality requirements have not been satisfied, the MUA
   must explicitly warn that the user's password may be exposed to
   attackers before testing the new configuration.





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   When establishing a new configuration for connecting to an IMAP, POP,
   or SMTP submission server, based on SRV records, an MUA SHOULD verify
   that either (a) the SRV records are signed using DNSSEC or (b) the
   target Fully Qualified Domain Name (FQDN) of the SRV record matches
   the original server FQDN for which the SRV queries were made.  If the
   target FQDN is not in the queried domain, the MUA SHOULD verify with
   the user that the SRV target FQDN is suitable for use, before
   executing any connections to the host.  (See <a href="./rfc6186#section-6">Section&nbsp;6 of [RFC6186]</a>.)

   An MUA MUST NOT consult SRV records to determine which servers to use
   on every connection attempt, unless those SRV records are signed by
   DNSSEC and have a valid signature.  However, an MUA MAY consult SRV
   records from time to time to determine if an MSP's server
   configuration has changed and alert the user if it appears that this
   has happened.  This can also serve as a means to encourage users to
   upgrade their configurations to require TLS if and when their MSPs
   support it.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Minimum Confidentiality Level</span>

   MUAs SHOULD, by default, require a minimum level of confidentiality
   for services accessed by each account.  For MUAs supporting the
   ability to access multiple Mail Accounts, this requirement SHOULD be
   configurable on a per-account basis.

   The default minimum expected level of confidentiality for all new
   accounts MUST require successful validation of the server's
   certificate and SHOULD require negotiation of TLS version 1.1 or
   greater.  (Future revisions to this specification may raise these
   requirements or impose additional requirements to address newly
   discovered weaknesses in protocols or cryptographic algorithms.)

   MUAs MAY permit the user to disable this minimum confidentiality
   requirement during initial account configuration or when subsequently
   editing an account configuration but MUST warn users that such a
   configuration will not assure privacy for either passwords or
   messages.

   An MUA that is configured to require a minimum level of
   confidentiality for a Mail Account MUST NOT attempt to perform any
   operation other than capability discovery, or STARTTLS for servers
   not using Implicit TLS, unless the minimum level of confidentiality
   is provided by that connection.

   MUAs SHOULD NOT allow users to easily access or send mail via a
   connection, or authenticate to any service using a password, if that
   account is configured to impose minimum confidentiality requirements
   and that connection does not meet all of those requirements.  An



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   example of "easy access" would be to display a dialog informing the
   user that the security requirements of the account were not met by
   the connection but allowing the user to "click through" to send mail
   or access the service anyway.  Experience indicates that users
   presented with such an option often "click through" without
   understanding the risks that they're accepting by doing so.
   Furthermore, users who frequently find the need to "click through" to
   use an insecure connection may become conditioned to do so as a
   matter of habit, before considering whether the risks are reasonable
   in each specific instance.

   An MUA that is not configured to require a minimum level of
   confidentiality for a Mail Account SHOULD still attempt to connect to
   the services associated with that account using the most secure means
   available, e.g., by using Implicit TLS or STARTTLS.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Certificate Validation</span>

   MUAs MUST validate TLS server certificates according to [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>] and
   PKIX [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>].

   MUAs MAY also support DNS-Based Authentication of Named Entities
   (DANE) [<a href="./rfc6698" title="&quot;The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA&quot;">RFC6698</a>] as a means of validating server certificates in
   order to meet minimum confidentiality requirements.

   MUAs MAY support the use of certificate pinning but MUST NOT consider
   a connection in which the server's authenticity relies on certificate
   pinning as providing the minimum level of confidentiality.  (See
   <a href="#section-5.4">Section 5.4</a>.)

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Certificate Pinning</span>

   During account setup, the MUA will identify servers that provide
   account services such as mail access and mail submission (<a href="#section-5.1">Section 5.1</a>
   describes one way to do this).  The certificates for these servers
   are verified using the rules described in [<a href="./rfc7817" title="&quot;Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols&quot;">RFC7817</a>] and PKIX
   [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>].  In the event that the certificate does not validate due
   to an expired certificate, a lack of an appropriate chain of trust,
   or a lack of an identifier match, the MUA MAY offer to create a
   persistent binding between that certificate and the saved hostname
   for the server, for use when accessing that account's servers.  This
   is called "certificate pinning".

   (Note: This use of the term "certificate pinning" means something
   subtly different than HTTP Public Key Pinning as described in
   [<a href="./rfc7469" title="&quot;Public Key Pinning Extension for HTTP&quot;">RFC7469</a>].  The dual use of the same term is confusing, but
   unfortunately both uses are well established.)




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   Certificate pinning is only appropriate during Mail Account setup and
   MUST NOT be offered as an option in response to a failed certificate
   validation for an existing Mail Account.  An MUA that allows
   certificate pinning MUST NOT allow a certificate pinned for one
   account to validate connections for other accounts.  An MUA that
   allows certificate pinning MUST also allow a user to undo the
   pinning, i.e., to revoke trust in a certificate that has previously
   been pinned.

   A pinned certificate is subject to a man-in-the-middle attack at
   account setup time and typically lacks a mechanism to automatically
   revoke or securely refresh the certificate.  Note also that a man-in-
   the-middle attack at account setup time will expose the user's
   password to the attacker (if a password is used).  Therefore, the use
   of a pinned certificate does not meet the requirement for a minimum
   confidentiality level, and an MUA MUST NOT indicate to the user that
   such confidentiality is provided.  Additional advice on certificate
   pinning is presented in [<a href="./rfc6125" title="&quot;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)&quot;">RFC6125</a>].

<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>.  Client Certificate Authentication</span>

   MUAs MAY implement client certificate authentication on the Implicit
   TLS port.  An MUA MUST NOT provide a client certificate during the
   TLS handshake unless the server requests one and the MUA has been
   authorized to use that client certificate with that account.  Having
   the end user explicitly configure a client certificate for use with a
   given account is sufficient to meet this requirement.  However,
   installing a client certificate for use with one account MUST NOT
   automatically authorize the use of that certificate with other
   accounts.  This is not intended to prohibit site-specific
   authorization mechanisms, such as (a) a site-administrator-controlled
   mechanism to authorize the use of a client certificate with a given
   account or (b) a domain-name-matching mechanism.

   Note: The requirement that the server request a certificate is just a
   restatement of the TLS protocol rules, e.g., <a href="./rfc5246#section-7.4.6">Section&nbsp;7.4.6 of
   [RFC5246]</a>.  The requirement that the client not send a certificate
   not known to be acceptable to the server is pragmatic in multiple
   ways: the current TLS protocol provides no way for the client to know
   which of the potentially multiple certificates it should use; also,
   when the client sends a certificate, it is potentially disclosing its
   identity (or its user's identity) to both the server and any party
   with access to the transmission medium, perhaps unnecessarily and for
   no useful purpose.







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   A client supporting client certificate authentication with Implicit
   TLS MUST implement the SASL EXTERNAL mechanism [<a href="./rfc4422" title="&quot;Simple Authentication and Security Layer (SASL)&quot;">RFC4422</a>], using the
   appropriate authentication command (AUTH for POP3 [<a href="./rfc5034" title="&quot;The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism&quot;">RFC5034</a>], AUTH for
   SMTP Submission [<a href="./rfc4954" title="&quot;SMTP Service Extension for Authentication&quot;">RFC4954</a>], or AUTHENTICATE for IMAP [<a href="./rfc3501" title="&quot;INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1&quot;">RFC3501</a>]).

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Considerations Related to Antivirus/Antispam Software and Services</span>

   There are multiple ways to connect an AVAS service (e.g., "Antivirus
   &amp; Antispam") to a mail server.  Some mechanisms, such as the de facto
   "milter" protocol, are out of scope for this specification.  However,
   some services use an SMTP relay proxy that intercepts mail at the
   application layer to perform a scan and proxy or forward to another
   Mail Transfer Agent (MTA).  Deploying AVAS services in this way can
   cause many problems [<a href="./rfc2979" title="&quot;Behavior of and Requirements for Internet Firewalls&quot;">RFC2979</a>], including direct interference with
   this specification, and other forms of confidentiality or security
   reduction.  An AVAS product or service is considered compatible with
   this specification if all IMAP, POP, and SMTP-related software
   (including proxies) it includes are compliant with this
   specification.

   Note that end-to-end email encryption prevents AVAS software and
   services from using email content as part of a spam or virus
   assessment.  Furthermore, although a minimum confidentiality level
   can prevent a man-in-the-middle from introducing spam or virus
   content between the MUA and Submission server, it does not prevent
   other forms of client or account compromise.  The use of AVAS
   services for submitted email therefore remains necessary.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  POP3S Port Registration Update</span>

   IANA has updated the registration of the TCP well-known port 995
   using the following template [<a href="./rfc6335" title="&quot;Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry&quot;">RFC6335</a>]:

     Service Name: pop3s
     Transport Protocol: TCP
     Assignee: IESG &lt;iesg@ietf.org&gt;
     Contact: IETF Chair &lt;chair@ietf.org&gt;
     Description: POP3 over TLS protocol
     Reference: <a href="./rfc8314">RFC 8314</a>
     Port Number: 995









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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  IMAPS Port Registration Update</span>

   IANA has updated the registration of the TCP well-known port 993
   using the following template [<a href="./rfc6335" title="&quot;Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry&quot;">RFC6335</a>]:

     Service Name: imaps
     Transport Protocol: TCP
     Assignee: IESG &lt;iesg@ietf.org&gt;
     Contact: IETF Chair &lt;chair@ietf.org&gt;
     Description: IMAP over TLS protocol
     Reference: <a href="./rfc8314">RFC 8314</a>
     Port Number: 993

   No changes to existing UDP port assignments for pop3s or imaps are
   being requested.

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Submissions Port Registration</span>

   IANA has assigned an alternate usage of TCP port 465 in addition to
   the current assignment using the following template [<a href="./rfc6335" title="&quot;Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry&quot;">RFC6335</a>]:

     Service Name: submissions
     Transport Protocol: TCP
     Assignee: IESG &lt;iesg@ietf.org&gt;
     Contact: IETF Chair &lt;chair@ietf.org&gt;
     Description: Message Submission over TLS protocol
     Reference: <a href="./rfc8314">RFC 8314</a>
     Port Number: 465

   This is a one-time procedural exception to the rules in [<a href="./rfc6335" title="&quot;Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry&quot;">RFC6335</a>].
   This requires explicit IESG approval and does not set a precedent.
   Note: Since the purpose of this alternate usage assignment is to
   align with widespread existing practice and there is no known usage
   of UDP port 465 for Message Submission over TLS, IANA has not
   assigned an alternate usage of UDP port 465.

   Historically, port 465 was briefly registered as the "smtps" port.
   This registration made no sense, as the SMTP transport MX
   infrastructure has no way to specify a port, so port 25 is always
   used.  As a result, the registration was revoked and was subsequently
   reassigned to a different service.  In hindsight, the "smtps"
   registration should have been renamed or reserved rather than
   revoked.  Unfortunately, some widely deployed mail software
   interpreted "smtps" as "submissions" [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>] and used that port for
   email submission by default when an end user requested security
   during account setup.  If a new port is assigned for the submissions
   service, either (a) email software will continue with unregistered
   use of port 465 (leaving the port registry inaccurate relative to



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   de facto practice and wasting a well-known port) or (b) confusion
   between the de facto and registered ports will cause harmful
   interoperability problems that will deter the use of TLS for Message
   Submission.  The authors of this document believe that both of these
   outcomes are less desirable than a "wart" in the registry documenting
   real-world usage of a port for two purposes.  Although STARTTLS on
   port 587 has been deployed, it has not replaced the deployed use of
   Implicit TLS submission on port 465.

<span class="h3"><a class="selflink" id="section-7.4" href="#section-7.4">7.4</a>.  Additional Registered Clauses for "Received" Fields</span>

   Per the provisions in [<a href="./rfc5321" title="&quot;Simple Mail Transfer Protocol&quot;">RFC5321</a>], IANA has added two additional-
   registered-clauses for Received fields as defined in <a href="#section-4.3">Section 4.3</a> of
   this document:

   o  "tls": Indicates the TLS cipher used (if applicable)

   o  "group": Indicates the Diffie-Hellman group used with the TLS
      cipher (if applicable)

   The descriptions and syntax of these additional clauses are provided
   in <a href="#section-4.3">Section 4.3</a> of this document.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   This entire document is about security considerations.  In general,
   this is targeted to improve mail confidentiality and to mitigate
   threats external to the email system such as network-level snooping
   or interception; this is not intended to mitigate active attackers
   who have compromised service provider systems.

   Implementers should be aware that the use of client certificates with
   TLS 1.2 reveals the user's identity to any party with the ability to
   read packets from the transmission medium and therefore may
   compromise the user's privacy.  There seems to be no easy fix with
   TLS 1.2 or earlier versions, other than to avoid presenting client
   certificates except when there is explicit authorization to do so.
   TLS 1.3 [<a href="#ref-TLS-1.3" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.3&quot;">TLS-1.3</a>] appears to reduce this privacy risk somewhat.













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<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-RFC793">RFC793</a>]   Postel, J., "Transmission Control Protocol", STD 7,
              <a href="./rfc793">RFC 793</a>, DOI 10.17487/RFC0793, September 1981,
              &lt;<a href="https://www.rfc-editor.org/info/rfc793">https://www.rfc-editor.org/info/rfc793</a>&gt;.

   [<a id="ref-RFC1939">RFC1939</a>]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, <a href="./rfc1939">RFC 1939</a>, DOI 10.17487/RFC1939, May 1996,
              &lt;<a href="https://www.rfc-editor.org/info/rfc1939">https://www.rfc-editor.org/info/rfc1939</a>&gt;.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3207">RFC3207</a>]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", <a href="./rfc3207">RFC 3207</a>, DOI 10.17487/RFC3207,
              February 2002, &lt;<a href="https://www.rfc-editor.org/info/rfc3207">https://www.rfc-editor.org/info/rfc3207</a>&gt;.

   [<a id="ref-RFC3501">RFC3501</a>]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
              VERSION 4rev1", <a href="./rfc3501">RFC 3501</a>, DOI 10.17487/RFC3501,
              March 2003, &lt;<a href="https://www.rfc-editor.org/info/rfc3501">https://www.rfc-editor.org/info/rfc3501</a>&gt;.

   [<a id="ref-RFC4033">RFC4033</a>]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              <a href="./rfc4033">RFC 4033</a>, DOI 10.17487/RFC4033, March 2005,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4033">https://www.rfc-editor.org/info/rfc4033</a>&gt;.

   [<a id="ref-RFC5034">RFC5034</a>]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", <a href="./rfc5034">RFC 5034</a>, DOI 10.17487/RFC5034,
              July 2007, &lt;<a href="https://www.rfc-editor.org/info/rfc5034">https://www.rfc-editor.org/info/rfc5034</a>&gt;.

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

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







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   [<a id="ref-RFC5280">RFC5280</a>]  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", <a href="./rfc5280">RFC 5280</a>, DOI 10.17487/RFC5280, May 2008,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5280">https://www.rfc-editor.org/info/rfc5280</a>&gt;.

   [<a id="ref-RFC5322">RFC5322</a>]  Resnick, P., Ed., "Internet Message Format", <a href="./rfc5322">RFC 5322</a>,
              DOI 10.17487/RFC5322, October 2008,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5322">https://www.rfc-editor.org/info/rfc5322</a>&gt;.

   [<a id="ref-RFC6186">RFC6186</a>]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", <a href="./rfc6186">RFC 6186</a>,
              DOI 10.17487/RFC6186, March 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6186">https://www.rfc-editor.org/info/rfc6186</a>&gt;.

   [<a id="ref-RFC6409">RFC6409</a>]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, <a href="./rfc6409">RFC 6409</a>, DOI 10.17487/RFC6409, November 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6409">https://www.rfc-editor.org/info/rfc6409</a>&gt;.

   [<a id="ref-RFC6698">RFC6698</a>]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", <a href="./rfc6698">RFC 6698</a>, DOI 10.17487/RFC6698,
              August 2012, &lt;<a href="https://www.rfc-editor.org/info/rfc6698">https://www.rfc-editor.org/info/rfc6698</a>&gt;.

   [<a id="ref-RFC7525">RFC7525</a>]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", <a href="https://www.rfc-editor.org/bcp/bcp195">BCP 195</a>, <a href="./rfc7525">RFC 7525</a>, DOI 10.17487/RFC7525,
              May 2015, &lt;<a href="https://www.rfc-editor.org/info/rfc7525">https://www.rfc-editor.org/info/rfc7525</a>&gt;.

   [<a id="ref-RFC7672">RFC7672</a>]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", <a href="./rfc7672">RFC 7672</a>,
              DOI 10.17487/RFC7672, October 2015,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7672">https://www.rfc-editor.org/info/rfc7672</a>&gt;.

   [<a id="ref-RFC7817">RFC7817</a>]  Melnikov, A., "Updated Transport Layer Security (TLS)
              Server Identity Check Procedure for Email-Related
              Protocols", <a href="./rfc7817">RFC 7817</a>, DOI 10.17487/RFC7817, March 2016,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7817">https://www.rfc-editor.org/info/rfc7817</a>&gt;.

   [<a id="ref-RFC8174">RFC8174</a>]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              <a href="./rfc2119">RFC 2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>,
              DOI 10.17487/RFC8174, May 2017,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;.






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<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-CERT-555316">CERT-555316</a>]
              CERT, "Vulnerability Note VU#555316: STARTTLS plaintext
              command injection vulnerability", Carnegie Mellon
              University Software Engineering Institute, September 2011,
              &lt;<a href="https://www.kb.cert.org/vuls/id/555316">https://www.kb.cert.org/vuls/id/555316</a>&gt;.

   [<a id="ref-Email-TLS">Email-TLS</a>]
              Moore, K., "Recommendations for use of TLS by Electronic
              Mail Access Protocols", Work in Progress, <a href="./draft-moore-email-tls-00">draft-moore-</a>
              <a href="./draft-moore-email-tls-00">email-tls-00</a>, October 2013.

   [<a id="ref-MTA-STS">MTA-STS</a>]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security
              (MTA-STS)", Work in Progress, <a href="./draft-ietf-uta-mta-sts-14">draft-ietf-uta-mta-sts-14</a>,
              January 2018.

   [<a id="ref-POP3-over-TLS">POP3-over-TLS</a>]
              Melnikov, A., Newman, C., and M. Yevstifeyev, Ed., "POP3
              over TLS", Work in Progress, <a href="./draft-melnikov-pop3-over-tls-02">draft-melnikov-pop3-</a>
              <a href="./draft-melnikov-pop3-over-tls-02">over-tls-02</a>, August 2011.

   [<a id="ref-RFC2595">RFC2595</a>]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              <a href="./rfc2595">RFC 2595</a>, DOI 10.17487/RFC2595, June 1999,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2595">https://www.rfc-editor.org/info/rfc2595</a>&gt;.

   [<a id="ref-RFC2979">RFC2979</a>]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", <a href="./rfc2979">RFC 2979</a>, DOI 10.17487/RFC2979, October 2000,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2979">https://www.rfc-editor.org/info/rfc2979</a>&gt;.

   [<a id="ref-RFC3848">RFC3848</a>]  Newman, C., "ESMTP and LMTP Transmission Types
              Registration", <a href="./rfc3848">RFC 3848</a>, DOI 10.17487/RFC3848, July 2004,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3848">https://www.rfc-editor.org/info/rfc3848</a>&gt;.

   [<a id="ref-RFC4346">RFC4346</a>]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", <a href="./rfc4346">RFC 4346</a>,
              DOI 10.17487/RFC4346, April 2006,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4346">https://www.rfc-editor.org/info/rfc4346</a>&gt;.

   [<a id="ref-RFC4422">RFC4422</a>]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", <a href="./rfc4422">RFC 4422</a>,
              DOI 10.17487/RFC4422, June 2006,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4422">https://www.rfc-editor.org/info/rfc4422</a>&gt;.







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   [<a id="ref-RFC4954">RFC4954</a>]  Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", <a href="./rfc4954">RFC 4954</a>,
              DOI 10.17487/RFC4954, July 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4954">https://www.rfc-editor.org/info/rfc4954</a>&gt;.

   [<a id="ref-RFC5068">RFC5068</a>]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", <a href="https://www.rfc-editor.org/bcp/bcp134">BCP 134</a>, <a href="./rfc5068">RFC 5068</a>,
              DOI 10.17487/RFC5068, November 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5068">https://www.rfc-editor.org/info/rfc5068</a>&gt;.

   [<a id="ref-RFC5321">RFC5321</a>]  Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc5321">RFC 5321</a>,
              DOI 10.17487/RFC5321, October 2008,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5321">https://www.rfc-editor.org/info/rfc5321</a>&gt;.

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

   [<a id="ref-RFC6125">RFC6125</a>]  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)", <a href="./rfc6125">RFC 6125</a>, DOI 10.17487/RFC6125,
              March 2011, &lt;<a href="https://www.rfc-editor.org/info/rfc6125">https://www.rfc-editor.org/info/rfc6125</a>&gt;.

   [<a id="ref-RFC6335">RFC6335</a>]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", <a href="https://www.rfc-editor.org/bcp/bcp165">BCP 165</a>,
              <a href="./rfc6335">RFC 6335</a>, DOI 10.17487/RFC6335, August 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6335">https://www.rfc-editor.org/info/rfc6335</a>&gt;.

   [<a id="ref-RFC7469">RFC7469</a>]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", <a href="./rfc7469">RFC 7469</a>, DOI 10.17487/RFC7469,
              April 2015, &lt;<a href="https://www.rfc-editor.org/info/rfc7469">https://www.rfc-editor.org/info/rfc7469</a>&gt;.

   [<a id="ref-TLS-1.3">TLS-1.3</a>]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, <a href="./draft-ietf-tls-tls13-23">draft-ietf-tls-tls13-23</a>,
              January 2018.










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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Design Considerations</span>

   This section is not normative.

   The first version of this document was written independently from the
   October 2013 version of [<a href="#ref-Email-TLS">Email-TLS</a>] ("Recommendations for use of TLS
   by Electronic Mail Access Protocols").  Subsequent versions merge
   ideas from both documents.

   One author of this document was also the author of <a href="./rfc2595">RFC 2595</a>, which
   became the standard for TLS usage with POP and IMAP, and the other
   author was perhaps the first to propose that idea.  In hindsight,
   both authors now believe that that approach was a mistake.  At this
   point, the authors believe that while anything that makes it easier
   to deploy TLS is good, the desirable end state is that these
   protocols always use TLS, leaving no need for a separate port for
   cleartext operation except to support legacy clients while they
   continue to be used.  The separate-port model for TLS is inherently
   simpler to implement, debug, and deploy.  It also enables a "generic
   TLS load-balancer" that accepts secure client connections for
   arbitrary foo-over-TLS protocols and forwards them to a server that
   may or may not support TLS.  Such load-balancers cause many problems
   because they violate the end-to-end principle and the server loses
   the ability to log security-relevant information about the client
   unless the protocol is designed to forward that information (as this
   specification does for the ciphersuite).  However, they can result in
   TLS deployment where it would not otherwise happen, which is a
   sufficiently important goal that it overrides any problems.

   Although STARTTLS appears only slightly more complex than
   separate-port TLS, we again learned the lesson that complexity is the
   enemy of security in the form of the STARTTLS command injection
   vulnerability (Computer Emergency Readiness Team (CERT) vulnerability
   ID #555316 [<a href="#ref-CERT-555316">CERT-555316</a>]).  Although there's nothing inherently wrong
   with STARTTLS, the fact that it resulted in a common implementation
   error (made independently by multiple implementers) suggests that it
   is a less secure architecture than Implicit TLS.

   <a href="./rfc2595#section-7">Section&nbsp;7 of RFC 2595</a> critiques the separate-port approach to TLS.
   The first bullet was a correct critique.  There are proposals in the
   HTTP community to address that, and the use of SRV records as
   described in <a href="./rfc6186">RFC 6186</a> resolves that critique for email.  The second
   bullet is correct as well but is not very important because useful
   deployment of security layers other than TLS in email is small enough
   to be effectively irrelevant.  (Also, it's less correct than it used
   to be because "export" ciphersuites are no longer supported in modern
   versions of TLS.)  The third bullet is incorrect because it misses
   the desirable option of "use TLS for all subsequent connections to



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   this server once TLS is successfully negotiated".  The fourth bullet
   may be correct, but it is not a problem yet with current port
   consumption rates.  The fundamental error was prioritizing a
   perceived better design based on a mostly valid critique over
   real-world deployability.  But getting security and confidentiality
   facilities actually deployed is so important that it should trump
   design purity considerations.

   Port 465 is presently used for two purposes: for submissions by a
   large number of clients and service providers and for the "urd"
   protocol by one vendor.  Actually documenting this current state is
   controversial, as discussed in the IANA Considerations section.
   However, there is no good alternative.  Registering a new port for
   submissions when port 465 is already widely used for that purpose
   will just create interoperability problems.  Registering a port
   that's only used if advertised by an SRV record (<a href="./rfc6186">RFC 6186</a>) would not
   create interoperability problems but would require all client
   deployments, server deployments, and software to change
   significantly, which is contrary to the goal of promoting the
   increased use of TLS.  Encouraging the use of STARTTLS on port 587
   would not create interoperability problems, but it is unlikely to
   have any impact on the current undocumented use of port 465 and makes
   the guidance in this document less consistent.  The remaining option
   is to document the current state of the world and support future use
   of port 465 for submission, as this increases consistency and ease of
   deployment for TLS email submission.

























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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc8314">RFC 8314</a>         Use of TLS for Email Submission/Access     January 2018</span>


Acknowledgements

   Thanks to Ned Freed for discussion of the initial concepts in this
   document.  Thanks to Alexey Melnikov for [<a href="#ref-POP3-over-TLS">POP3-over-TLS</a>], which was
   the basis of the POP3 Implicit TLS text.  Thanks to Russ Housley,
   Alexey Melnikov, and Dan Newman for review feedback.  Thanks to
   Paul Hoffman for interesting feedback in initial conversations about
   this idea.

Authors' Addresses

   Keith Moore
   Windrock, Inc.
   PO Box 1934
   Knoxville, TN  37901
   United States of America

   Email: moore@network-heretics.com


   Chris Newman
   Oracle
   440 E. Huntington Dr., Suite 400
   Arcadia, CA  91006
   United States of America

   Email: chris.newman@oracle.com
























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</pre>