Internet Engineering Task Force (IETF) K. Moore
Request for Comments: 8314 Windrock, Inc.
Updates: 1939, 2595, 3501, 5068, 6186, 6409 C. Newman
Category: Standards Track Oracle
ISSN: 2070-1721 January 2018
Cleartext Considered Obsolete: Use of Transport Layer Security (TLS)
for Email Submission and Access
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 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/rfc8314.
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.
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RFC 8314 Use of TLS for Email Submission/Access January 2018
Table of Contents
1. Introduction ....................................................3
1.1. How This Document Updates Previous RFCs ....................3
2. Conventions and Terminology Used in This Document ...............4
3. Implicit TLS ....................................................5
3.1. Implicit TLS for POP .......................................5
3.2. Implicit TLS for IMAP ......................................5
3.3. Implicit TLS for SMTP Submission ...........................6
3.4. Implicit TLS Connection Closure for POP, IMAP, and
SMTP Submission ............................................7
4. Use of TLS by Mail Access Servers and Message Submission
Servers .........................................................7
4.1. Deprecation of Services Using Cleartext and TLS Versions
Less Than 1.1 ..............................................8
4.2. Mail Server Use of Client Certificate Authentication .......9
4.3. Recording TLS Ciphersuite in "Received" Header Field .......9
4.4. TLS Server Certificate Requirements .......................10
4.5. Recommended DNS Records for Mail Protocol Servers .........11
4.5.1. MX Records .........................................11
4.5.2. SRV Records ........................................11
4.5.3. DNSSEC .............................................11
4.5.4. TLSA Records .......................................11
4.6. Changes to Internet-Facing Servers ........................11
5. Use of TLS by Mail User Agents .................................12
5.1. Use of SRV Records in Establishing Configuration ..........13
5.2. Minimum Confidentiality Level .............................14
5.3. Certificate Validation ....................................15
5.4. Certificate Pinning .......................................15
5.5. Client Certificate Authentication .........................16
6. Considerations Related to Antivirus/Antispam Software
and Services ...................................................17
7. IANA Considerations ............................................17
7.1. POP3S Port Registration Update ............................17
7.2. IMAPS Port Registration Update ............................18
7.3. Submissions Port Registration .............................18
7.4. Additional Registered Clauses for "Received" Fields .......19
8. Security Considerations ........................................19
9. References .....................................................20
9.1. Normative References ......................................20
9.2. Informative References ....................................22
Appendix A. Design Considerations .................................24
Acknowledgements ..................................................26
Authors' Addresses ................................................26
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RFC 8314 Use of TLS for Email Submission/Access January 2018
1. Introduction
Software that provides email service via the Internet Message Access
Protocol (IMAP) [RFC3501], the Post Office Protocol (POP) [RFC1939],
and/or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409]
usually has Transport Layer Security (TLS) [RFC5246] 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 [RFC6409] 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 [RFC7672]; another
is provided in [MTA-STS].
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.
1.1. How This Document Updates Previous RFCs
This document updates POP (RFC 1939), IMAP (RFC 3501), and Submission
(RFC 6409, RFC 5068) in two ways:
1. By adding Implicit TLS ports as Standards Track ports for these
protocols as described in Section 3.
2. By updating TLS best practices that apply to these protocols as
described in Sections 4 and 5.
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This document updates RFC 2595 by replacing Section 7 of RFC 2595
with the preference for Implicit TLS as described in Sections 1 and 3
of this document, as well as by updating TLS best practices that
apply to the protocols in RFC 2595 as described in Sections 4 and 5
of this document.
This document updates RFC 6186 as described herein, in Section 5.1.
2. Conventions and Terminology Used in This Document
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.
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 [RFC6409] (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 [RFC5234], including the core rules
provided in Appendix B of [RFC5234] and the rules provided in
[RFC5322].
3. Implicit TLS
Previous standards for the use of email protocols with TLS used the
STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. 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.
3.1. Implicit TLS for POP
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
[RFC7817]. Once the TLS session is established, POP3 [RFC1939]
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 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.1 for port registration
information.
3.2. Implicit TLS for IMAP
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
[RFC7817]. Once the TLS session is established, IMAP [RFC3501]
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 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.2 for port registration
information.
3.3. Implicit TLS for SMTP Submission
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
[RFC7817]. Once the TLS session is established, Message Submission
protocol data [RFC6409] is exchanged as TLS application data for the
remainder of the TCP connection. (Note: The "submissions" service
name is defined in Section 7.3 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 Section 7.3). 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
Appendix 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 [RFC6409], so requirements and
recommendations for MSAs in that document, including the requirement
to implement SMTP AUTH [RFC4954] and the requirements of Email
Submission Operations [RFC5068], also apply to the submissions port.
See Sections 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.3 for port registration
information.
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3.4. Implicit TLS Connection Closure for POP, IMAP, and SMTP Submission
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 ([RFC793],
Section 3.5)) without waiting for a TLS response from the server.
4. Use of TLS by Mail Access Servers and Message Submission Servers
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 Section 3.)
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 [RFC6186].
(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
Section 4.5.)
o MSPs SHOULD deprecate the use of cleartext Mail Access Servers and
Mail Submission Servers as soon as practicable. (See
Section 4.1.)
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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 [RFC7525]
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 Section 4.3.)
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.
4.1. Deprecation of Services Using Cleartext and TLS Versions
Less Than 1.1
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.
4.2. Mail Server Use of Client Certificate Authentication
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 [RFC4422]. An IMAPS server MAY issue
a PREAUTH greeting instead of enabling SASL EXTERNAL.
4.3. Recording TLS Ciphersuite in "Received" Header Field
The ESMTPS transmission type [RFC3848] 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"
; <https://www.iana.org/assignments/tls-parameters>
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"
; <https://www.iana.org/assignments/tls-parameters>
4.4. TLS Server Certificate Requirements
MSPs MUST maintain valid server certificates for all servers. See
[RFC7817] 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 [RFC6066]. Mail servers supporting the SNI
need to support the post-SRV hostname to interoperate with MUAs that
have not implemented [RFC6186]. For more discussion of this problem,
see Section 5.1 of [RFC7817].
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4.5. Recommended DNS Records for Mail Protocol Servers
This section discusses not only the DNS records that are recommended
but also implications of DNS records for server configuration and TLS
server certificates.
4.5.1. MX Records
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 [RFC4033]. This is
mentioned here only for completeness, as the handling of inbound mail
is out of scope for this document.
4.5.2. SRV Records
MSPs SHOULD advertise SRV records to aid MUAs in determining the
proper configuration of servers, per the instructions in [RFC6186].
MSPs SHOULD advertise servers that support Implicit TLS in preference
to servers that support cleartext and/or STARTTLS operation.
4.5.3. DNSSEC
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.
4.5.4. TLSA Records
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.
4.6. Changes to Internet-Facing Servers
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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5. Use of TLS by Mail User Agents
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
Section 5.1 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 Section 5.2.)
o MUAs MUST NOT treat a session as meeting a minimum level of
confidentiality if the server's TLS certificate cannot be
validated. (See Section 5.3.)
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 [RFC5246] or later. Earlier TLS and
SSL versions MAY also be supported, so long as the MUA requires at
least TLS 1.1 [RFC4346] when accessing accounts that are
configured to impose minimum confidentiality requirements.
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RFC 8314 Use of TLS for Email Submission/Access January 2018
o All MUAs SHOULD implement the recommended TLS ciphersuites
described in [RFC7525] 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 [RFC6186] or other means) and offer to
upgrade the account to require TLS.
Additional considerations and details appear below.
5.1. Use of SRV Records in Establishing Configuration
This document updates [RFC6186] 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 [RFC6186] 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 [RFC6186], 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 Section 6 of [RFC6186].)
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.
5.2. Minimum Confidentiality Level
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.
5.3. Certificate Validation
MUAs MUST validate TLS server certificates according to [RFC7817] and
PKIX [RFC5280].
MUAs MAY also support DNS-Based Authentication of Named Entities
(DANE) [RFC6698] 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
Section 5.4.)
5.4. Certificate Pinning
During account setup, the MUA will identify servers that provide
account services such as mail access and mail submission (Section 5.1
describes one way to do this). The certificates for these servers
are verified using the rules described in [RFC7817] and PKIX
[RFC5280]. 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
[RFC7469]. 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 [RFC6125].
5.5. Client Certificate Authentication
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., Section 7.4.6 of
[RFC5246]. 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 [RFC4422], using the
appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]).
6. Considerations Related to Antivirus/Antispam Software and Services
There are multiple ways to connect an AVAS service (e.g., "Antivirus
& 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 [RFC2979], 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.
7. IANA Considerations
7.1. POP3S Port Registration Update
IANA has updated the registration of the TCP well-known port 995
using the following template [RFC6335]:
Service Name: pop3s
Transport Protocol: TCP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: POP3 over TLS protocol
Reference: RFC 8314
Port Number: 995
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7.2. IMAPS Port Registration Update
IANA has updated the registration of the TCP well-known port 993
using the following template [RFC6335]:
Service Name: imaps
Transport Protocol: TCP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: IMAP over TLS protocol
Reference: RFC 8314
Port Number: 993
No changes to existing UDP port assignments for pop3s or imaps are
being requested.
7.3. Submissions Port Registration
IANA has assigned an alternate usage of TCP port 465 in addition to
the current assignment using the following template [RFC6335]:
Service Name: submissions
Transport Protocol: TCP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: Message Submission over TLS protocol
Reference: RFC 8314
Port Number: 465
This is a one-time procedural exception to the rules in [RFC6335].
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" [RFC6409] 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.
7.4. Additional Registered Clauses for "Received" Fields
Per the provisions in [RFC5321], IANA has added two additional-
registered-clauses for Received fields as defined in Section 4.3 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 Section 4.3 of this document.
8. Security Considerations
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 [TLS-1.3] appears to reduce this privacy risk somewhat.
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9. References
9.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>.
[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>.
[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>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501,
March 2003, <https://www.rfc-editor.org/info/rfc3501>.
[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>.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL)
Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
July 2007, <https://www.rfc-editor.org/info/rfc5034>.
[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>.
[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>.
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[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>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
August 2012, <https://www.rfc-editor.org/info/rfc6698>.
[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>.
[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>.
[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS)
Server Identity Check Procedure for Email-Related
Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
<https://www.rfc-editor.org/info/rfc7817>.
[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>.
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9.2. Informative References
[CERT-555316]
CERT, "Vulnerability Note VU#555316: STARTTLS plaintext
command injection vulnerability", Carnegie Mellon
University Software Engineering Institute, September 2011,
<https://www.kb.cert.org/vuls/id/555316>.
[Email-TLS]
Moore, K., "Recommendations for use of TLS by Electronic
Mail Access Protocols", Work in Progress, draft-moore-
email-tls-00, October 2013.
[MTA-STS] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security
(MTA-STS)", Work in Progress, draft-ietf-uta-mta-sts-14,
January 2018.
[POP3-over-TLS]
Melnikov, A., Newman, C., and M. Yevstifeyev, Ed., "POP3
over TLS", Work in Progress, draft-melnikov-pop3-
over-tls-02, August 2011.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
<https://www.rfc-editor.org/info/rfc2979>.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
<https://www.rfc-editor.org/info/rfc3848>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.
[RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
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[RFC4954] Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/info/rfc4954>.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068,
DOI 10.17487/RFC5068, November 2007,
<https://www.rfc-editor.org/info/rfc5068>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[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>.
[RFC6335] 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", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469,
April 2015, <https://www.rfc-editor.org/info/rfc7469>.
[TLS-1.3] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, draft-ietf-tls-tls13-23,
January 2018.
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Appendix A. Design Considerations
This section is not normative.
The first version of this document was written independently from the
October 2013 version of [Email-TLS] ("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 RFC 2595, 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 [CERT-555316]). 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.
Section 7 of RFC 2595 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 RFC 6186 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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RFC 8314 Use of TLS for Email Submission/Access January 2018
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 (RFC 6186) 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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Acknowledgements
Thanks to Ned Freed for discussion of the initial concepts in this
document. Thanks to Alexey Melnikov for [POP3-over-TLS], 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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