draft-ietf-uta-smtp-require-tls-03.txt   draft-ietf-uta-smtp-require-tls-04.txt 
Internet Engineering Task Force J. Fenton Internet Engineering Task Force J. Fenton
Internet-Draft Altmode Networks Internet-Draft Altmode Networks
Intended status: Standards Track June 22, 2018 Intended status: Standards Track September 26, 2018
Expires: December 24, 2018 Expires: March 30, 2019
SMTP Require TLS Option SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-03 draft-ietf-uta-smtp-require-tls-04
Abstract Abstract
The SMTP STARTTLS option, used in negotiating transport-level The SMTP STARTTLS option, used in negotiating transport-level
encryption of SMTP connections, is not as useful from a security encryption of SMTP connections, is not as useful from a security
standpoint as it might be because of its opportunistic nature; standpoint as it might be because of its opportunistic nature;
message delivery is, by default, prioritized over security. This message delivery is, by default, prioritized over security. This
document describes an SMTP service extension, REQUIRETLS, and message document describes an SMTP service extension, REQUIRETLS, and message
header field, RequireTLS. If the REQUIRETLS option or RequireTLS header field, RequireTLS. If the REQUIRETLS option or RequireTLS
message header field is used when sending a message, it asserts a message header field is used when sending a message, it asserts a
skipping to change at page 1, line 41 skipping to change at page 1, line 41
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 24, 2018. This Internet-Draft will expire on March 30, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 29 skipping to change at page 2, line 29
4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5
4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5
4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5
4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6
4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7
4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7
5. Non-delivery message handling . . . . . . . . . . . . . . . . 7 5. Non-delivery message handling . . . . . . . . . . . . . . . . 7
6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 9
8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10
8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 10.1. Changes since -03 Draft . . . . . . . . . . . . . . . . 11
10.2. Changes since -01 Draft . . . . . . . . . . . . . . . . 11 10.2. Changes since -02 Draft . . . . . . . . . . . . . . . . 11
10.3. Changes since -00 Draft . . . . . . . . . . . . . . . . 11 10.3. Changes since -01 Draft . . . . . . . . . . . . . . . . 11
10.4. Changes since fenton-03 Draft . . . . . . . . . . . . . 11 10.4. Changes since -00 Draft . . . . . . . . . . . . . . . . 11
10.5. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 10.5. Changes since fenton-03 Draft . . . . . . . . . . . . . 12
10.6. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 10.6. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12
10.7. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12 10.7. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.8. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 14 11.2. Informative References . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
means by which an SMTP server and client can establish a Transport means by which an SMTP server and client can establish a Transport
Layer Security (TLS) protected session for the transmission of email Layer Security (TLS) protected session for the transmission of email
messages. By default, TLS is used only upon mutual agreement messages. By default, TLS is used only upon mutual agreement
(successful negotiation) of STARTTLS between the client and server; (successful negotiation) of STARTTLS between the client and server;
if this is not possible, the message is sent without transport if this is not possible, the message is sent without transport
encryption. Furthermore, it is common practice for the client to encryption. Furthermore, it is common practice for the client to
negotiate TLS even if the SMTP server's certificate is invalid. negotiate TLS even if the SMTP server's certificate is invalid.
Policy mechanisms such as DANE [RFC7672] and MTA-STS Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may
[I-D.ietf-uta-mta-sts] may impose requirements for the use of TLS for impose requirements for the use of TLS for email destined for some
email destined for some domains. However, such policies do not allow domains. However, such policies do not allow the sender to specify
the sender to specify which messages are more sensitive and require which messages are more sensitive and require transport-level
transport-level encryption, and which ones are less sensitive and encryption, and which ones are less sensitive and ought to be relayed
ought to be relayed even if TLS cannot be negotiated successfully. even if TLS cannot be negotiated successfully.
The default opportunistic nature of SMTP TLS enables several "on the The default opportunistic nature of SMTP TLS enables several "on the
wire" attacks on SMTP security between MTAs. These include passive wire" attacks on SMTP security between MTAs. These include passive
eavesdropping on connections for which TLS is not used, interference eavesdropping on connections for which TLS is not used, interference
in the SMTP protocol to prevent TLS from being negotiated (presumably in the SMTP protocol to prevent TLS from being negotiated (presumably
accompanied by eavesdropping), and insertion of a man-in-the-middle accompanied by eavesdropping), and insertion of a man-in-the-middle
attacker exploiting the lack of server authentication by the client. attacker exploiting the lack of server authentication by the client.
Attacks are described in more detail in the Security Considerations Attacks are described in more detail in the Security Considerations
section of this document. section of this document.
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2. The REQUIRETLS Service Extension 2. The REQUIRETLS Service Extension
1. The textual name of the extension is "Require TLS". 1. The textual name of the extension is "Require TLS".
2. The EHLO keyword value associated with this extension is 2. The EHLO keyword value associated with this extension is
"REQUIRETLS". "REQUIRETLS".
3. One MAIL FROM option is defined by this extension. 3. One MAIL FROM option is defined by this extension.
4. Two new SMTP status codes are defined by this extension to convey 4. One new SMTP status code is defined by this extension to convey
error conditions resulting from failure of the client to an error condition resulting from failure of the client to send
negotiate a TLS connection with the required security and as a to a server not also supporting the REQUIRETLS extension.
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 In order to specify REQUIRETLS treatment for a given message, the
REQUIRETLS option is specified on the MAIL FROM command when that REQUIRETLS option is specified on the MAIL FROM command when that
message is transmitted. This option MUST only be specified in the message is transmitted. This option MUST only be specified in the
context of an SMTP session meeting the security requirements that context of an SMTP session meeting the security requirements that
have been specified: have been specified:
o The session itself MUST employ TLS transmission. o The session itself MUST employ TLS transmission.
o The certificate presented by the SMTP server MUST either verify o The certificate presented by the SMTP server MUST either verify
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client. client.
o Following the negotiation of STARTTLS, the SMTP server MUST o Following the negotiation of STARTTLS, the SMTP server MUST
advertise in the subsequent EHLO response that it supports advertise in the subsequent EHLO response that it supports
REQUIRETLS. REQUIRETLS.
3. The RequireTLS Header Field 3. The RequireTLS Header Field
One new message header field, RequireTLS, is defined by this One new message header field, RequireTLS, is defined by this
specification. It is used for messages requesting that recipient TLS specification. It is used for messages requesting that recipient TLS
policy (MTA-STS [I-D.ietf-uta-mta-sts] or DANE [RFC7672]) be ignored. policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored.
The RequireTLS header field has a single REQUIRED parameter: The RequireTLS header field has a single REQUIRED parameter:
o NO - The SMTP client SHOULD attempt to send the message regardless o NO - The SMTP client SHOULD attempt to send the message regardless
of its ability to negotiate STARTTLS with the SMTP server, of its ability to negotiate STARTTLS with the SMTP server,
ignoring policy-based mechanisms, if any, asserted by the ignoring policy-based mechanisms (including MTA-STS and DANE), if
recipient domain. Nevertheless, the client SHOULD negotiate any, asserted by the recipient domain. Nevertheless, the client
STARTTLS with the server if available. SHOULD negotiate STARTTLS with the server if available.
More than one instance of the RequireTLS header field MUST NOT appear More than one instance of the RequireTLS header field MUST NOT appear
in a given message. in a given message.
4. REQUIRETLS Semantics 4. REQUIRETLS Semantics
4.1. REQUIRETLS Receipt Requirements 4.1. REQUIRETLS Receipt Requirements
Upon receipt of the REQUIRETLS option on a MAIL FROM command during 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 the receipt of a message, an SMTP server MUST tag that message as
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MTA MUST: MTA MUST:
1. Look up the SMTP server to which the message is to be sent as 1. Look up the SMTP server to which the message is to be sent as
described in [RFC5321] Section 5.1. described in [RFC5321] Section 5.1.
2. Open an SMTP session with the peer SMTP server using the EHLO 2. Open an SMTP session with the peer SMTP server using the EHLO
verb. verb.
3. Establish a TLS-protected SMTP session with its peer SMTP server 3. Establish a TLS-protected SMTP session with its peer SMTP server
and authenticate the server's certificate as specified in and authenticate the server's certificate as specified in
[RFC6125] or [RFC6698] as applicable. [RFC6125] or [RFC7672] as applicable.
4. Ensure that the response to the subsequent EHLO following 4. Ensure that the response to the subsequent EHLO following
establishment of the TLS protection advertises the REQUIRETLS establishment of the TLS protection advertises the REQUIRETLS
capability. capability.
5. The SMTP client SHOULD also require that meaningfully secure 5. The SMTP client SHOULD also require that meaningfully secure
cipher algorithms and key lengths be negotiated with the server. cipher algorithms and key lengths be negotiated with the server.
The choices of key lengths and algorithms change over time, so a The choices of key lengths and algorithms change over time, so a
specific requirement is not presented here. specific requirement is not presented here.
If any of the above steps fail, the client MUST issue a QUIT to the 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 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 list of MX hosts in an attempt to find a mail path that meets the
sender's requirements. The client MAY send other, unprotected, sender's requirements. The client MAY send other, unprotected,
messages to that server if it has any prior to issuing the QUIT. If 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 there are no more MX hosts, the client MUST NOT transmit the message
to the domain. to the domain.
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messages to the resulting messages they originate. If this is done, messages to the resulting messages they originate. If this is done,
they SHOULD also apply these requirements to administrative traffic, they SHOULD also apply these requirements to administrative traffic,
such as messages to moderators requesting approval of messages. such as messages to moderators requesting approval of messages.
7. IANA Considerations 7. IANA Considerations
If published as an RFC, this draft requests the addition of the If published as an RFC, this draft requests the addition of the
following keyword to the SMTP Service Extensions Registry following keyword to the SMTP Service Extensions Registry
[MailParams]: [MailParams]:
Textual name: RequireTLS Textual name: RequireTLS
EHLO keyword value: REQUIRETLS EHLO keyword value: REQUIRETLS
Syntax and parameters: (no parameters) Syntax and parameters: (no parameters)
Additional SMTP verbs: none Additional SMTP verbs: none
MAIL and RCPT parameters: REQUIRETLS parameter on MAIL MAIL and RCPT parameters: REQUIRETLS parameter on MAIL
Behavior: Use of the REQUIRETLS parameter on the Behavior: Use of the REQUIRETLS parameter on the
MAIL verb causes that message to require MAIL verb causes that message to require
the use of TLS and tagging with REQUIRETLS the use of TLS and tagging with
for all onward relay. REQUIRETLS for all onward relay.
Command length increment: 11 characters Command length increment: 11 characters
If published as an RFC, this draft requests the addition of an entry If published as an RFC, this draft requests the addition of an entry
to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry [SMTPStatusCodes]: Registry [SMTPStatusCodes]:
Code: 5.7.YYY Code: 5.7.YYY
Sample Text: REQUIRETLS support required Sample Text: REQUIRETLS support required
Associated basic status code: 530 Associated basic status code: 530
Description: This indicates that the message was not Description: This indicates that the message was not
able to be forwarded because it was able to be forwarded because it was
received with a REQUIRETLS requirement received with a REQUIRETLS requirement
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The author would like to acknowledge many helpful suggestions on the The author would like to acknowledge many helpful suggestions on the
ietf-smtp and uta mailing lists, in particular those of Viktor ietf-smtp and uta mailing lists, in particular those of Viktor
Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
10. Revision History 10. Revision History
To be removed by RFC Editor upon publication as an RFC. To be removed by RFC Editor upon publication as an RFC.
10.1. Changes since -02 Draft 10.1. Changes since -03 Draft
Working Group Last Call changes, including:
o Correct reference for SMTP DANE
o Clarify that RequireTLS: NO applies to both MTA-STS and DANE
policies
o Correct newly-defined status codes
o Update MTA-STS references to RFC
10.2. Changes since -02 Draft
o More complete documentation for IANA registration requests. o More complete documentation for IANA registration requests.
o Changed bounce handling to use RET parameters of RFC 3461, along o Changed bounce handling to use RET parameters of RFC 3461, along
with slightly more liberal transmission of bounces even if with slightly more liberal transmission of bounces even if
REQUIRETLS can't be negotiated. REQUIRETLS can't be negotiated.
10.2. Changes since -01 Draft 10.3. Changes since -01 Draft
o Converted DEEP references to RFC 8314. o Converted DEEP references to RFC 8314.
o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.
o Editorial corrections, notably making the header field name o Editorial corrections, notably making the header field name
consistent (RequireTLS rather than Require-TLS). consistent (RequireTLS rather than Require-TLS).
10.3. Changes since -00 Draft 10.4. Changes since -00 Draft
o Created new header field, Require-TLS, for use by "NO" option. o Created new header field, Require-TLS, for use by "NO" option.
o Removed "NO" option from SMTP service extension. o Removed "NO" option from SMTP service extension.
o Recommend DEEP requirements for delivery of messages requiring o Recommend DEEP requirements for delivery of messages requiring
TLS. TLS.
o Assorted copy edits o Assorted copy edits
10.4. Changes since fenton-03 Draft 10.5. Changes since fenton-03 Draft
o Wording improvements from Rolf Sonneveld review 22 July 2017 o Wording improvements from Rolf Sonneveld review 22 July 2017
o A few copy edits o A few copy edits
o Conversion from individual to UTA WG draft o Conversion from individual to UTA WG draft
10.5. Changes Since -02 Draft 10.6. Changes Since -02 Draft
o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
suggestion on UTA WG mailing list. suggestion on UTA WG mailing list.
o Additional guidance on bounce messages o Additional guidance on bounce messages
10.6. Changes Since -01 Draft 10.7. Changes Since -01 Draft
o Specified retries when multiple MX hosts exist for a given domain. o Specified retries when multiple MX hosts exist for a given domain.
o Clarified generation of non-delivery messages o Clarified generation of non-delivery messages
o Specified requirements for application of REQUIRETLS to mail o Specified requirements for application of REQUIRETLS to mail
forwarders and mailing lists. forwarders and mailing lists.
o Clarified DNSSEC requirements to include MX lookup only. o Clarified DNSSEC requirements to include MX lookup only.
o Corrected terminology regarding message retrieval vs. delivery. o Corrected terminology regarding message retrieval vs. delivery.
o Changed category to standards track. o Changed category to standards track.
10.7. Changes Since -00 Draft 10.8. Changes Since -00 Draft
o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
parameter to better associate REQUIRETLS requirements with parameter to better associate REQUIRETLS requirements with
transmission of individual messages. transmission of individual messages.
o Addition of an option to require DNSSEC lookup of the remote mail o Addition of an option to require DNSSEC lookup of the remote mail
server, since this affects the common name of the certificate that server, since this affects the common name of the certificate that
is presented. is presented.
o Clarified the wording to more clearly state that TLS sessions must o Clarified the wording to more clearly state that TLS sessions must
skipping to change at page 13, line 41 skipping to change at page 13, line 50
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>. 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 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<https://www.rfc-editor.org/info/rfc8314>. <https://www.rfc-editor.org/info/rfc8314>.
[SMTPStatusCodes] [SMTPStatusCodes]
Internet Assigned Numbers Authority (IANA), "Simple Mail Internet Assigned Numbers Authority (IANA), "Simple Mail
Transfer Protocol (SMTP) Enhanced Status Codes Registry", Transfer Protocol (SMTP) Enhanced Status Codes Registry",
2008, <http://www.iana.org/assignments/ 2008, <http://www.iana.org/assignments/
smtp-enhanced-status-codes>. smtp-enhanced-status-codes>.
11.2. Informative References 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", [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>. <https://www.rfc-editor.org/info/rfc1939>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>. <https://www.rfc-editor.org/info/rfc3501>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, Thayer, "OpenPGP Message Format", RFC 4880,
skipping to change at page 14, line 48 skipping to change at page 14, line 42
Mail Extensions (S/MIME) Version 3.2 Message Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>. 2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>. <https://www.rfc-editor.org/info/rfc7672>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
Author's Address Author's Address
Jim Fenton Jim Fenton
Altmode Networks Altmode Networks
Los Altos, California 94024 Los Altos, California 94024
USA USA
Email: fenton@bluepopcorn.net Email: fenton@bluepopcorn.net
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