draft-ietf-uta-smtp-require-tls-02.txt		draft-ietf-uta-smtp-require-tls-03.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 April 6, 2018		Intended status: Standards Track June 22, 2018
	Expires: October 8, 2018		Expires: December 24, 2018
	SMTP Require TLS Option		SMTP Require TLS Option
	draft-ietf-uta-smtp-require-tls-02		draft-ietf-uta-smtp-require-tls-03
	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 October 8, 2018.		This Internet-Draft will expire on December 24, 2018.
	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
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
	2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 3		2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4
	3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4		3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4
	4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5		4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5
	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 . . . . . . . . . . . . . . . . . . . . . 9		8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10
	8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 9		8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10
	8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10		8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
	10. Revision History . . . . . . . . . . . . . . . . . . . . . . 10		10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
	10.1. Changes since -01 Draft . . . . . . . . . . . . . . . . 10		10.1. Changes since -02 Draft . . . . . . . . . . . . . . . . 11
	10.2. Changes since -00 Draft . . . . . . . . . . . . . . . . 10		10.2. Changes since -01 Draft . . . . . . . . . . . . . . . . 11
	10.3. Changes since fenton-03 Draft . . . . . . . . . . . . . 11		10.3. Changes since -00 Draft . . . . . . . . . . . . . . . . 11
	10.4. Changes Since -02 Draft . . . . . . . . . . . . . . . . 11		10.4. Changes since fenton-03 Draft . . . . . . . . . . . . . 11
	10.5. Changes Since -01 Draft . . . . . . . . . . . . . . . . 11		10.5. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12
	10.6. Changes Since -00 Draft . . . . . . . . . . . . . . . . 11		10.6. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12
			10.7. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
	11.1. Normative References . . . . . . . . . . . . . . . . . . 12		11.1. Normative References . . . . . . . . . . . . . . . . . . 12
	11.2. Informative References . . . . . . . . . . . . . . . . . 13		11.2. Informative References . . . . . . . . . . . . . . . . . 14
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13		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
	skipping to change at page 7, line 47 ¶		skipping to change at page 7, line 51 ¶
	with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS		with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS
	error or some other, MUST also specify the REQUIRETLS SMTP option		error or some other, MUST also specify the REQUIRETLS SMTP option
	unless redacted as described below.		unless redacted as described below.
	The path from the origination of an error bounce message back to the		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		MAIL FROM address may not share the same REQUIRETLS support as the
	forward path. Therefore, users requiring TLS are advised to make		forward path. Therefore, users requiring TLS are advised to make
	sure that they are capable of receiving mail using REQUIRETLS as		sure that they are capable of receiving mail using REQUIRETLS as
	well. Otherwise, such non-delivery messages will be lost.		well. Otherwise, such non-delivery messages will be lost.
	If unable to send a bounce message due to a REQUIRETLS failure (the		If a REQUIRETLS message is bounced, the server MUST behave as if
	return path not supporting the TLS requirements in the original		RET=HDRS was present as described in [RFC3461]. If both RET=FULL and
	message), the MTA sending the bounce message MAY send a redacted non-		REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be
	delivery message to the postmaster of the domain identified in the		transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS
	envelope-From address identifying the message only by Message-ID and		bounce message MUST use an empty MAIL FROM return-path as required by
	indicating the type of failure. The original From, Return-path, To,		[RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS
	Sender, Cc, and related header fields MUST NOT be included in this		parameter SHOULD NOT cause a bounce message to be discarded even if
	message.		the next-hop relay does not advertise REQUIRETLS.
	Senders of messages requiring TLS are advised to consider the		Senders of messages requiring TLS are advised to consider the
	increased likelihood that bounce messages will be lost as a result of		possibility that bounce messages will be lost as a result of
	REQUIRETLS return path failure.		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		6. Mailing list considerations
	Mailing lists, upon receipt of a message, originate new messages to		Mailing lists, upon receipt of a message, originate new messages to
	list addresses. This is distinct from an aliasing operation that		list addresses. This is distinct from an aliasing operation that
	redirects the original message, in some cases to multiple recipients.		redirects the original message, in some cases to multiple recipients.
	The requirement to preserve the REQUIRETLS tag therefore does not		The requirement to preserve the REQUIRETLS tag therefore does not
	necessarily extend to mailing lists, although the inclusion of the		necessarily extend to mailing lists, although the inclusion of the
	RequireTLS header field MAY cause messages sent to mailing lists to		RequireTLS header field MAY cause messages sent to mailing lists to
	inherit this characteristic. REQUIRETLS users SHOULD be made aware		inherit this characteristic. REQUIRETLS users SHOULD be made aware
	skipping to change at page 8, line 32 ¶		skipping to change at page 8, line 38 ¶
	list operator to list members.		list operator to list members.
	Mailing list operators MAY apply REQUIRETLS requirements in incoming		Mailing list operators MAY apply REQUIRETLS requirements in incoming
	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
	keyword REQUIRETLS to the SMTP Service Extensions Registry		following keyword to the SMTP Service Extensions Registry
	[MailParams].		[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
	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] in the 5.7.YYY range to indicate lack of		Registry [SMTPStatusCodes]:
	REQUIRETLS support by an SMTP server to which a message is being
	routed.
	If published as an RFC, this draft requests the addition of the		Code: 5.7.YYY
	header field name RequireTLS to the Permanent Message Header Field		Sample Text: REQUIRETLS support required
	Names Registry [PermMessageHeaderFields].		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
	This section is to be removed during conversion into an RFC by the		If published as an RFC, this draft requests the addition of an entry
	RFC Editor.		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		8. Security Considerations
	The purpose of REQUIRETLS is to improve communications security for		The purpose of REQUIRETLS is to improve communications security for
	email by giving the originator of a message an expectation that it		email by giving the originator of a message an expectation that it
	will be transmitted in an encrypted form "over the wire". When used,		will be transmitted in an encrypted form "over the wire". When used,
	REQUIRETLS changes the traditional behavior of email transmission,		REQUIRETLS changes the traditional behavior of email transmission,
	which favors delivery over the ability to send email messages using		which favors delivery over the ability to send email messages using
	transport-layer security, to one in which requested security takes		transport-layer security, to one in which requested security takes
	precedence over delivery and domain-level policy.		precedence over delivery and domain-level policy.
	skipping to change at page 10, line 25 ¶		skipping to change at page 11, line 9 ¶
	security protocol, messages sent using REQUIRETLS are not encrypted		security protocol, messages sent using REQUIRETLS are not encrypted
	end-to-end and are visible to MTAs that are part of the message		end-to-end and are visible to MTAs that are part of the message
	delivery path. Messages containing sensitive information that MTAs		delivery path. Messages containing sensitive information that MTAs
	should not have access to MUST be sent using end-to-end content		should not have access to MUST be sent using end-to-end content
	encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751].		encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751].
	9. Acknowledgements		9. Acknowledgements
	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, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin,		Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
	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 -01 Draft		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 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).
	10.2. Changes since -00 Draft		10.3. 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
	10.3. Changes since fenton-03 Draft		10.4. 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
	10.4. Changes Since -02 Draft		10.5. 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
	10.5. Changes Since -01 Draft		10.6. 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.
	10.6. Changes Since -00 Draft		10.7. 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 12, line 29 ¶		skipping to change at page 13, line 20 ¶
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over		[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
	Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,		Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
	February 2002, <https://www.rfc-editor.org/info/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		[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
	Mail System Status Codes", BCP 138, RFC 5248,		Mail System Status Codes", BCP 138, RFC 5248,
	DOI 10.17487/RFC5248, June 2008,		DOI 10.17487/RFC5248, June 2008,
	<https://www.rfc-editor.org/info/rfc5248>.		<https://www.rfc-editor.org/info/rfc5248>.
	[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,		[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
	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
	skipping to change at page 13, line 21 ¶		skipping to change at page 14, line 21 ¶
	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]		[I-D.ietf-uta-mta-sts]
	Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,		Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
	and J. Jones, "SMTP MTA Strict Transport Security (MTA-		and J. Jones, "SMTP MTA Strict Transport Security (MTA-
	STS)", draft-ietf-uta-mta-sts-15 (work in progress), April		STS)", draft-ietf-uta-mta-sts-21 (work in progress), June
	2018.		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>.
End of changes. 23 change blocks.
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