SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8689.
|
|
|---|---|---|---|
| Author | Jim Fenton | ||
| Last updated | 2018-08-27 (Latest revision 2018-06-22) | ||
| Replaces | draft-fenton-smtp-require-tls | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Valery Smyslov | ||
| IESG | IESG state | Became RFC 8689 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Valery Smyslov <valery@smyslov.net> |
draft-ietf-uta-smtp-require-tls-03
Internet Engineering Task Force J. Fenton
Internet-Draft Altmode Networks
Intended status: Standards Track June 22, 2018
Expires: December 24, 2018
SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-03
Abstract
The SMTP STARTTLS option, used in negotiating transport-level
encryption of SMTP connections, is not as useful from a security
standpoint as it might be because of its opportunistic nature;
message delivery is, by default, prioritized over security. This
document describes an SMTP service extension, REQUIRETLS, and message
header field, RequireTLS. If the REQUIRETLS option or RequireTLS
message header field is used when sending a message, it asserts a
request on the part of the message sender to override the default
negotiation of TLS, either by requiring that TLS be negotiated when
the message is relayed, or by requesting that recipient-side policy
mechanisms such as MTA-STS and DANE be ignored when relaying a
message for which security is unimportant.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 24, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4
3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4
4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5
4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5
4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5
4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5
4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6
4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7
4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7
5. Non-delivery message handling . . . . . . . . . . . . . . . . 7
6. Mailing list considerations . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10
8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10
8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Changes since -02 Draft . . . . . . . . . . . . . . . . 11
10.2. Changes since -01 Draft . . . . . . . . . . . . . . . . 11
10.3. Changes since -00 Draft . . . . . . . . . . . . . . . . 11
10.4. Changes since fenton-03 Draft . . . . . . . . . . . . . 11
10.5. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12
10.6. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12
10.7. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
means by which an SMTP server and client can establish a Transport
Layer Security (TLS) protected session for the transmission of email
messages. By default, TLS is used only upon mutual agreement
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(successful negotiation) of STARTTLS between the client and server;
if this is not possible, the message is sent without transport
encryption. Furthermore, it is common practice for the client to
negotiate TLS even if the SMTP server's certificate is invalid.
Policy mechanisms such as DANE [RFC7672] and MTA-STS
[I-D.ietf-uta-mta-sts] may impose requirements for the use of TLS for
email destined for some domains. However, such policies do not allow
the sender to specify which messages are more sensitive and require
transport-level encryption, and which ones are less sensitive and
ought to be relayed even if TLS cannot be negotiated successfully.
The default opportunistic nature of SMTP TLS enables several "on the
wire" attacks on SMTP security between MTAs. These include passive
eavesdropping on connections for which TLS is not used, interference
in the SMTP protocol to prevent TLS from being negotiated (presumably
accompanied by eavesdropping), and insertion of a man-in-the-middle
attacker exploiting the lack of server authentication by the client.
Attacks are described in more detail in the Security Considerations
section of this document.
REQUIRETLS consists of two mechanisms: an SMTP service extension and
a message header field. The service extension is used to specify
that a given message sent during a particular session MUST be sent
over a TLS-protected session with specified security characteristics.
It also requires that the SMTP server advertise that it supports
REQUIRETLS, in effect promising that it will honor the requirement to
enforce TLS transmission and REQUIRETLS support for onward
transmission of those messages.
The RequireTLS message header field is used to convey a request to
ignore recipient-side policy mechanisms such as MTA-STS and DANE,
thereby prioritizing delivery over ability to negotiate TLS. Unlike
the service extension, the RequireTLS header field allows the message
to transit through one or more MTAs that do not support REQUIRETLS.
1.1. Requirements Language
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.
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2. The REQUIRETLS Service Extension
1. The textual name of the extension is "Require TLS".
2. The EHLO keyword value associated with this extension is
"REQUIRETLS".
3. One MAIL FROM option is defined by this extension.
4. Two new SMTP status codes are defined by this extension to convey
error conditions resulting from failure of the client to
negotiate a TLS connection with the required security and as a
result of an attempt to send to a server not also supporting the
REQUIRETLS extension.
In order to specify REQUIRETLS treatment for a given message, the
REQUIRETLS option is specified on the MAIL FROM command when that
message is transmitted. This option MUST only be specified in the
context of an SMTP session meeting the security requirements that
have been specified:
o The session itself MUST employ TLS transmission.
o The certificate presented by the SMTP server MUST either verify
successfully in a trust chain leading to a certificate trusted by
the SMTP client or it MUST verify succesfully using DANE as
specified in RFC 7672 [RFC7672]. For trust chains, the choice of
trusted (root) certificates is at the discretion of the SMTP
client.
o Following the negotiation of STARTTLS, the SMTP server MUST
advertise in the subsequent EHLO response that it supports
REQUIRETLS.
3. The RequireTLS Header Field
One new message header field, RequireTLS, is defined by this
specification. It is used for messages requesting that recipient TLS
policy (MTA-STS [I-D.ietf-uta-mta-sts] or DANE [RFC7672]) be ignored.
The RequireTLS header field has a single REQUIRED parameter:
o NO - The SMTP client SHOULD attempt to send the message regardless
of its ability to negotiate STARTTLS with the SMTP server,
ignoring policy-based mechanisms, if any, asserted by the
recipient domain. Nevertheless, the client SHOULD negotiate
STARTTLS with the server if available.
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More than one instance of the RequireTLS header field MUST NOT appear
in a given message.
4. REQUIRETLS Semantics
4.1. REQUIRETLS Receipt Requirements
Upon receipt of the REQUIRETLS option on a MAIL FROM command during
the receipt of a message, an SMTP server MUST tag that message as
needing REQUIRETLS handling.
Upon receipt of a message not specifying the REQUIRETLS option on its
MAIL FROM command but containing the RequireTLS header field in its
message header, an SMTP server implementing this specification MUST
tag that message with the option specified in the RequireTLS header
field. If the REQUIRETLS MAIL FROM parameter is specified, the
RequireTLS header field MUST be ignored but MAY be included in onward
relay of the message.
The manner in which the above tagging takes place is implementation-
dependent. If the message is being locally aliased and redistributed
to multiple addresses, all instances of the message MUST be tagged in
the same manner.
4.2. REQUIRETLS Sender Requirements
4.2.1. Sending with TLS Required
When sending a message tagged as requiring TLS, the sending (client)
MTA MUST:
1. Look up the SMTP server to which the message is to be sent as
described in [RFC5321] Section 5.1.
2. Open an SMTP session with the peer SMTP server using the EHLO
verb.
3. Establish a TLS-protected SMTP session with its peer SMTP server
and authenticate the server's certificate as specified in
[RFC6125] or [RFC6698] as applicable.
4. Ensure that the response to the subsequent EHLO following
establishment of the TLS protection advertises the REQUIRETLS
capability.
5. The SMTP client SHOULD also require that meaningfully secure
cipher algorithms and key lengths be negotiated with the server.
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The choices of key lengths and algorithms change over time, so a
specific requirement is not presented here.
If any of the above steps fail, the client MUST issue a QUIT to the
server and repeat steps 2-4 with each host on the recipient domain's
list of MX hosts in an attempt to find a mail path that meets the
sender's requirements. The client MAY send other, unprotected,
messages to that server if it has any prior to issuing the QUIT. If
there are no more MX hosts, the client MUST NOT transmit the message
to the domain.
Following such a failure, the SMTP client MUST send a non-delivery
notification to the reverse-path of the failed message as described
in section 3.6 of [RFC5321]. The following status codes [RFC5248]
SHOULD be used:
o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed
o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption
needed
Refer to Section 5 for further requirements regarding non-delivery
messages.
If all REQUIRETLS requirements have been met, transmit the message,
issuing the REQUIRETLS option on the MAIL FROM command with the
required option(s), if any.
4.2.2. Sending with TLS Optional
Messages tagged RequireTLS: NO are handled as follows. When sending
such a message, the sending (client) MTA MUST:
o Look up the SMTP server to which the message is to be sent as
described in [RFC5321] Section 5.1.
o Open an SMTP session with the peer SMTP server using the EHLO
verb. Attempt to negotiate STARTTLS if possible, and follow any
policy published by the recipient domain, but do not fail if this
is unsuccessful.
Some SMTP servers may be configured to require STARTTLS connections
as a matter of policy and not accept messages in the absence of
STARTTLS. A non-delivery notification MUST be returned to the sender
if message relay fails due to an inability to negotiate STARTTLS when
required by the server.
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Since messages tagged with RequireTLS: NO will sometimes be sent to
SMTP servers not supporting REQUIRETLS, that option will not be
uniformly observed by all SMTP relay hops.
4.3. REQUIRETLS Submission
An MUA or other agent making the initial introduction of a message
has authority to decide whether to require TLS. When TLS is to be
required, it MUST do so by negotiating STARTTLS and REQUIRETLS and
include the REQUIRETLS option on the MAIL FROM command, as is done
for message relay.
When TLS is not to be required, the sender MUST include the
RequireTLS header field in the message. SMTP servers implementing
this specification MUST interpret this header field as described in
Section 4.1.
In either case, the decision whether to specify REQUIRETLS MAY be
done based on a user interface selection or based on a ruleset or
other policy. The manner in which the decision to require TLS is
made is implementation-dependent and is beyond the scope of this
specification.
4.4. Delivery of REQUIRETLS messages
Messages are usually retrieved by end users using protocols other
than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems.
Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD
observe the guidelines in [RFC8314].
5. Non-delivery message handling
Non-delivery ("bounce") messages usually contain important metadata
about the message to which they refer, including the original message
header. They therefore MUST be protected in the same manner as the
original message. All non-delivery messages resulting from messages
with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS
error or some other, MUST also specify the REQUIRETLS SMTP option
unless redacted as described below.
The path from the origination of an error bounce message back to the
MAIL FROM address may not share the same REQUIRETLS support as the
forward path. Therefore, users requiring TLS are advised to make
sure that they are capable of receiving mail using REQUIRETLS as
well. Otherwise, such non-delivery messages will be lost.
If a REQUIRETLS message is bounced, the server MUST behave as if
RET=HDRS was present as described in [RFC3461]. If both RET=FULL and
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REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be
transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS
bounce message MUST use an empty MAIL FROM return-path as required by
[RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS
parameter SHOULD NOT cause a bounce message to be discarded even if
the next-hop relay does not advertise REQUIRETLS.
Senders of messages requiring TLS are advised to consider the
possibility that bounce messages will be lost as a result of
REQUIRETLS return path failure, and that some information could be
leaked if a bounce message is not able to be transmitted with
REQUIRETLS.
6. Mailing list considerations
Mailing lists, upon receipt of a message, originate new messages to
list addresses. This is distinct from an aliasing operation that
redirects the original message, in some cases to multiple recipients.
The requirement to preserve the REQUIRETLS tag therefore does not
necessarily extend to mailing lists, although the inclusion of the
RequireTLS header field MAY cause messages sent to mailing lists to
inherit this characteristic. REQUIRETLS users SHOULD be made aware
of this limitation so that they use caution when sending to mailing
lists and do not assume that REQUIRETLS applies to messages from the
list operator to list members.
Mailing list operators MAY apply REQUIRETLS requirements in incoming
messages to the resulting messages they originate. If this is done,
they SHOULD also apply these requirements to administrative traffic,
such as messages to moderators requesting approval of messages.
7. IANA Considerations
If published as an RFC, this draft requests the addition of the
following keyword to the SMTP Service Extensions Registry
[MailParams]:
Textual name: RequireTLS
EHLO keyword value: REQUIRETLS
Syntax and parameters: (no parameters)
Additional SMTP verbs: none
MAIL and RCPT parameters: REQUIRETLS parameter on MAIL
Behavior: Use of the REQUIRETLS parameter on the
MAIL verb causes that message to require
the use of TLS and tagging with REQUIRETLS
for all onward relay.
Command length increment: 11 characters
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If published as an RFC, this draft requests the addition of an entry
to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry [SMTPStatusCodes]:
Code: 5.7.YYY
Sample Text: REQUIRETLS support required
Associated basic status code: 530
Description: This indicates that the message was not
able to be forwarded because it was
received with a REQUIRETLS requirement
and none of the SMTP servers to which
the message should be forwarded provide
this support.
Reference: (this document)
Submitter: J. Fenton
Change controller: IESG
If published as an RFC, this draft requests the addition of an entry
to the Permanent Message Header Field Names Registry
[PermMessageHeaderFields]:
Header field name: RequireTLS
Applicable protocol: mail
Status: provisional
Author/change controller: IETF UTA Working Group
Specification document: (this document)
This section is to be updated for publication by the RFC Editor.
8. Security Considerations
The purpose of REQUIRETLS is to improve communications security for
email by giving the originator of a message an expectation that it
will be transmitted in an encrypted form "over the wire". When used,
REQUIRETLS changes the traditional behavior of email transmission,
which favors delivery over the ability to send email messages using
transport-layer security, to one in which requested security takes
precedence over delivery and domain-level policy.
The following considerations apply to the REQUIRETLS service
extension but not the RequireTLS header field, since messages
specifying the header field are less concerned with transport
security.
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8.1. Passive attacks
REQUIRETLS is generally effective against passive attackers who are
merely trying to eavesdrop on an SMTP exchange between an SMTP client
and server. This assumes, of course, the cryptographic integrity of
the TLS connection being used.
8.2. Active attacks
Active attacks against TLS encrypted SMTP connections can take many
forms. One such attack is to interfere in the negotiation by
changing the STARTTLS command to something illegal such as XXXXXXXX.
This causes TLS negotiation to fail and messages to be sent in the
clear, where they can be intercepted. REQUIRETLS detects the failure
of STARTTLS and declines to send the message rather than send it
insecurely.
A second form of attack is a man-in-the-middle attack where the
attacker terminates the TLS connection rather than the intended SMTP
server. This is possible when, as is commonly the case, the SMTP
client either does not verify the server's certificate or establishes
the connection even when the verification fails. REQUIRETLS requires
successful certificate validation before sending the message.
Another active attack involves the spoofing of DNS MX records of the
recipient domain. An attacker having this capability could cause the
message to be redirected to a mail server under the attacker's own
control, which would presumably have a valid certificate. REQUIRETLS
does not address this attack.
8.3. Bad Actor MTAs
A bad-actor MTA along the message transmission path could
misrepresent its support of REQUIRETLS and/or actively strip
REQUIRETLS tags from messages it handles. However, since
intermediate MTAs are already trusted with the cleartext of messages
they handle, and are not part of the threat model for transport-layer
security, they are also not part of the threat model for REQUIRETLS.
It should be reemphasized that since SMTP TLS is a transport-layer
security protocol, messages sent using REQUIRETLS are not encrypted
end-to-end and are visible to MTAs that are part of the message
delivery path. Messages containing sensitive information that MTAs
should not have access to MUST be sent using end-to-end content
encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751].
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9. Acknowledgements
The author would like to acknowledge many helpful suggestions on the
ietf-smtp and uta mailing lists, in particular those of Viktor
Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
10. Revision History
To be removed by RFC Editor upon publication as an RFC.
10.1. Changes since -02 Draft
o More complete documentation for IANA registration requests.
o Changed bounce handling to use RET parameters of RFC 3461, along
with slightly more liberal transmission of bounces even if
REQUIRETLS can't be negotiated.
10.2. Changes since -01 Draft
o Converted DEEP references to RFC 8314.
o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.
o Editorial corrections, notably making the header field name
consistent (RequireTLS rather than Require-TLS).
10.3. Changes since -00 Draft
o Created new header field, Require-TLS, for use by "NO" option.
o Removed "NO" option from SMTP service extension.
o Recommend DEEP requirements for delivery of messages requiring
TLS.
o Assorted copy edits
10.4. Changes since fenton-03 Draft
o Wording improvements from Rolf Sonneveld review 22 July 2017
o A few copy edits
o Conversion from individual to UTA WG draft
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10.5. Changes Since -02 Draft
o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
suggestion on UTA WG mailing list.
o Additional guidance on bounce messages
10.6. Changes Since -01 Draft
o Specified retries when multiple MX hosts exist for a given domain.
o Clarified generation of non-delivery messages
o Specified requirements for application of REQUIRETLS to mail
forwarders and mailing lists.
o Clarified DNSSEC requirements to include MX lookup only.
o Corrected terminology regarding message retrieval vs. delivery.
o Changed category to standards track.
10.7. Changes Since -00 Draft
o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
parameter to better associate REQUIRETLS requirements with
transmission of individual messages.
o Addition of an option to require DNSSEC lookup of the remote mail
server, since this affects the common name of the certificate that
is presented.
o Clarified the wording to more clearly state that TLS sessions must
be established and not simply that STARTTLS is negotiated.
o Introduced need for minimum encryption standards (key lengths and
algorithms)
o Substantially rewritten Security Considerations section
11. References
11.1. Normative References
[MailParams]
Internet Assigned Numbers Authority (IANA), "IANA Mail
Parameters", 2007,
<http://www.iana.org/assignments/mail-parameters>.
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[PermMessageHeaderFields]
Internet Assigned Numbers Authority (IANA), "Permanent
Message Header Field Names Registry", 2004,
<https://www.iana.org/assignments/message-headers/
message-headers.xhtml#perm-headers>.
[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>.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, DOI 10.17487/RFC3461, January 2003,
<https://www.rfc-editor.org/info/rfc3461>.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248,
DOI 10.17487/RFC5248, June 2008,
<https://www.rfc-editor.org/info/rfc5248>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[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>.
[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>.
[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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[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<https://www.rfc-editor.org/info/rfc8314>.
[SMTPStatusCodes]
Internet Assigned Numbers Authority (IANA), "Simple Mail
Transfer Protocol (SMTP) Enhanced Status Codes Registry",
2008, <http://www.iana.org/assignments/
smtp-enhanced-status-codes>.
11.2. Informative References
[I-D.ietf-uta-mta-sts]
Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", draft-ietf-uta-mta-sts-21 (work in progress), June
2018.
[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>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[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>.
Author's Address
Fenton Expires December 24, 2018 [Page 14]
Internet-Draft SMTP Require TLS Option June 2018
Jim Fenton
Altmode Networks
Los Altos, California 94024
USA
Email: fenton@bluepopcorn.net
Fenton Expires December 24, 2018 [Page 15]