Diff: rfc8689xml2.original.xml

	rfc8689xml2.original.xml	rfc8689.xml

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	<!-- *** FRONT MATTER *** -->

	<front>

	<title>SMTP Require TLS Option</title>

	<!-- add 'role="editor"' below for the editors if appropriate -->

	<author fullname="Jim Fenton" initials="J.L."
	surname="Fenton">

	<organization>Altmode Networks</organization>

	<address>
	<postal>
	<street> </street>
	<city>Los Altos</city>
	<region>California</region>
	<code>94024</code>
	<country>USA</country>
	</postal>

	<email>fenton@bluepopcorn.net</email>

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	</address>
	</author>

	<date year="2019" month="August" />

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	<area>General</area>

	<workgroup>Internet Engineering Task Force</workgroup>

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	<keyword>SMTP</keyword>	<rfc xmlns:xi="http://www.w3.org/2001/XInclude" submissionType="IETF"
		category="std" consensus="true" number="8689" ipr="trust200902"
		obsoletes="" updates="" xml:lang="en" tocInclude="true" symRefs="true"
		sortRefs="true" version="3" docName="draft-ietf-uta-smtp-require-tls-09">


	<!-- Keywords will be incorporated into HTML output	<!-- xml2rfc v2v3 conversion 2.30.0 -->
	files in a meta tag but they have no effect on text or nroff	<front>
	output. If you submit your draft to the RFC Editor, the	<title>SMTP Require TLS Option</title>
	keywords will be used for the search engine. -->	<seriesInfo name="RFC" value="8689"/>


	<abstract>	<author fullname="Jim Fenton" initials="J." surname="Fenton">
	<t>The SMTP STARTTLS option, used in negotiating transport-level	<organization>Altmode Networks</organization>
		<address>
		<postal>
		<street> </street>
		<city>Los Altos</city>
		<region>California</region>
		<code>94024</code>
		<country>United States of America</country>
		</postal>
		<email>fenton@bluepopcorn.net</email>
		</address>
		</author>
		<date year="2019" month="November"/>
		<area>General</area>
		<workgroup>Internet Engineering Task Force</workgroup>
		<keyword>SMTP</keyword>
		<abstract>
		<t>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	document describes an SMTP service extension, REQUIRETLS, and

	message header field, TLS-Required. If the REQUIRETLS option or	a message header field, TLS-Required. If the REQUIRETLS option or
	TLS-Required message header field is used when sending a message,	TLS-Required message header field is used when sending a message,
	it asserts a request on the part of the message sender to	it asserts a request on the part of the message sender to
	override the default negotiation of TLS, either by requiring that	override the default negotiation of TLS, either by requiring that

	TLS be negotiated when the message is relayed, or by requesting	TLS be negotiated when the message is relayed or by requesting
	that recipient-side policy mechanisms such as MTA-STS and DANE be	that recipient-side policy mechanisms such as MTA-STS and DNS-Based
		Authentication of Named Entities (DANE) be
	ignored when relaying a message for which security is	ignored when relaying a message for which security is
	unimportant.</t>	unimportant.</t>

	</abstract>	</abstract>
		</front>
	</front>	<middle>
		<section numbered="true" toc="default">
	<middle>	<name>Introduction</name>
	<section title="Introduction">	<t>The <xref target="RFC5321" format="default">SMTP</xref> <xref target="R
	<t>The <xref target="RFC5321">SMTP</xref> <xref	FC3207" format="default">STARTTLS service extension</xref> provides a
	target="RFC3207">STARTTLS service extension</xref> provides a
	means by which an SMTP server and client can establish a	means by which an SMTP server and client can establish a
	Transport Layer Security (TLS) protected session for the	Transport Layer Security (TLS) protected session for the
	transmission of email messages. By default, TLS is used only upon	transmission of email messages. By default, TLS is used only upon
	mutual agreement (successful negotiation) of STARTTLS between the	mutual agreement (successful negotiation) of STARTTLS between the
	client and server; if this is not possible, the message is sent	client and server; if this is not possible, the message is sent
	without transport encryption. Furthermore, it is common practice	without transport encryption. Furthermore, it is common practice
	for the client to negotiate TLS even if the SMTP server's	for the client to negotiate TLS even if the SMTP server's
	certificate is invalid.</t>	certificate is invalid.</t>

		<t>Policy mechanisms such as <xref target="RFC7672" format="default">DANE<
	<t>Policy mechanisms such as <xref target="RFC7672">DANE</xref>	/xref>
	and <xref target="RFC8461">MTA-STS</xref> may	and <xref target="RFC8461" format="default">MTA-STS</xref> may
	impose requirements for the use of TLS for email destined for	impose requirements for the use of TLS for email destined for
	some domains. However, such policies do not allow the sender to	some domains. However, such policies do not allow the sender to
	specify which messages are more sensitive and require	specify which messages are more sensitive and require

	transport-level encryption, and which ones are less sensitive and	transport-level encryption and which ones are less sensitive and
	ought to be relayed even if TLS cannot be negotiated	ought to be relayed even if TLS cannot be negotiated
	successfully.</t>	successfully.</t>


	<t>The default opportunistic nature of SMTP TLS enables several	<t>The default opportunistic nature of SMTP TLS enables several

	"on the wire" attacks on SMTP security between MTAs. These	on-the-wire attacks on SMTP security between MTAs. These
	include passive eavesdropping on connections for which TLS is not	include passive eavesdropping on connections for which TLS is not
	used, interference in the SMTP protocol to prevent TLS from being	used, interference in the SMTP protocol to prevent TLS from being
	negotiated (presumably accompanied by eavesdropping), and insertion	negotiated (presumably accompanied by eavesdropping), and insertion
	of a man-in-the-middle attacker exploiting the lack of	of a man-in-the-middle attacker exploiting the lack of
	server authentication by the client. Attacks are described	server authentication by the client. Attacks are described

	in more detail in the Security Considerations section of this	in more detail in the <xref target="Security" format="title" /> section
		of this
	document.</t>	document.</t>

		<t>REQUIRETLS consists of two mechanisms: an SMTP service
	<t>REQUIRETLS consists of two mechanisms: an SMTP service
	extension and a message header field. The service extension is	extension and a message header field. The service extension is
	used to specify that a given message sent during a particular session	used to specify that a given message sent during a particular session

	MUST be sent over a TLS-protected session with specified security	<bcp14>MUST</bcp14> be sent over a TLS-protected session with specified sec urity
	characteristics. It also requires that the SMTP server advertise	characteristics. It also requires that the SMTP server advertise
	that it supports REQUIRETLS, in effect promising that it	that it supports REQUIRETLS, in effect promising that it
	will honor the requirement to enforce TLS transmission and	will honor the requirement to enforce TLS transmission and
	REQUIRETLS support for onward transmission of those messages.</t>	REQUIRETLS support for onward transmission of those messages.</t>

		<t>The TLS-Required message header field is used to convey a
	<t>The TLS-Required message header field is used to convey a
	request to ignore recipient-side policy mechanisms such as	request to ignore recipient-side policy mechanisms such as
	MTA-STS and DANE, thereby prioritizing delivery over ability to	MTA-STS and DANE, thereby prioritizing delivery over ability to
	negotiate TLS. Unlike the service extension, the TLS-Required	negotiate TLS. Unlike the service extension, the TLS-Required
	header field allows the message to transit through one or more	header field allows the message to transit through one or more
	MTAs that do not support REQUIRETLS.</t>	MTAs that do not support REQUIRETLS.</t>

		<section numbered="true" toc="default">
	<section title="Requirements Language">	<name>Requirements Language</name>
	<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",	<t>
	"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT	The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
	RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be	"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
	interpreted as described in BCP 14 <xref target="RFC2119" />	NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
	<xref target="RFC8174" /> when, and only when, they appear in	"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
	all capitals, as shown here.</t>	"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are
		to be interpreted as described in BCP 14 <xref target="RFC2119"/>
	<t>The formal syntax uses the Augmented Backus-Naur Form (ABNF)	<xref target="RFC8174"/> when, and only when, they appear in all capitals,
	<xref target="RFC5234" /> including the core rules defined in	as shown here.
		</t>
		<t>The formal syntax uses the Augmented Backus-Naur Form (ABNF)
		<xref target="RFC5234" format="default"/>, including the core rules defin
		ed in
	Appendix B of that document.</t>	Appendix B of that document.</t>

	</section>	</section>
	</section>	</section>
		<section anchor="service_extension" numbered="true" toc="default">
	<section anchor="service_extension" title="The REQUIRETLS Service Extension">	<name>The REQUIRETLS Service Extension</name>
	<t><list style="numbers">
	<t>The textual name of the extension is "Require TLS".</t>
	<t>The EHLO keyword value associated with this extension is
	"REQUIRETLS".</t>


	<t>No additional SMTP verbs are defined by this extension.</t>	<t>The REQUIRETLS SMTP service extension has the following characteristics:</t>


	<t>One optional parameter ("REQUIRETLS") is added to the MAIL	<ol spacing="normal" type="1">
		<li>The textual name of the extension is "Require TLS".</li>
		<li>The EHLO keyword value associated with this extension is
		"REQUIRETLS".</li>
		<li>No additional SMTP verbs are defined by this extension.</li>
		<li>One optional parameter ("REQUIRETLS") is added to the MAIL
	FROM command by this extension. No value is associated with	FROM command by this extension. No value is associated with

	this parameter.</t>	this parameter.</li>
		<li>The maximum length of a MAIL FROM command line is increased
	<t>The maximum length of a MAIL FROM command line is increased
	by 11 octets by the possible addition of a space and the	by 11 octets by the possible addition of a space and the

	REQUIRETLS keyword.</t>	REQUIRETLS keyword.</li>
		<li>One new SMTP status code is defined by this extension to
	<t>One new SMTP status code is defined by this extension to
	convey an error condition resulting from failure of the client to	convey an error condition resulting from failure of the client to

	send to a server not also supporting	send data to a server that does not also support
	the REQUIRETLS extension.</t>	the REQUIRETLS extension.</li>
		<li>The REQUIRETLS extension is valid for message relay <xref target="RF
	<t>The REQUIRETLS extension is valid for message relay <xref	C5321" format="default"/>, submission <xref target="RFC6409" format="default"/>,
	target="RFC5321"></xref>, submission <xref	and the Local Mail Transfer Protocol
	target="RFC6409"></xref>, and the Local Mail Transfer Protocol	(LMTP) <xref target="RFC2033" format="default"/>.</li>
	(LMTP) <xref target="RFC2033"></xref></t>	<li>
		<t>The ABNF syntax for the MAIL FROM parameter is as follows:
	<t>The ABNF syntax for the MAIL FROM parameter is as follows:	</t>
	<figure>	<sourcecode type="abnf"><![CDATA[
	<artwork>	requiretls-param = "REQUIRETLS"
	requiretls-param = "REQUIRETLS"	; where requiretls-param is an instance of an
	; where requiretls-param is an instance of an	; esmtp-param used in Mail-parameters in
	; esmtp-param used in Mail-parameters in	; RFC 5321, Section 4.1.2. There is no esmtp-value
	; RFC 5321 Section 4.1.2. There is no esmtp-value	; associated with requiretls-param. ]]></sourcecode>
	; associated with requiretls-param.	</li>
	</artwork>	</ol>
	</figure>	<t>In order to specify REQUIRETLS treatment for a given message,
	</t>	the REQUIRETLS option is specified in the MAIL FROM command when
	</list></t>	that message is transmitted. This option <bcp14>MUST</bcp14> only be specif
		ied in the
	<t>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 of	context of an SMTP session meeting the security requirements of
	REQUIRETLS:	REQUIRETLS:

	<list style="symbols">	</t>
	<t>The session itself MUST employ TLS transmission.</t>	<ul spacing="normal">
	<t>If the SMTP server to which the message is being transmitted	<li>The session itself <bcp14>MUST</bcp14> employ TLS transmission.</li>
		<li>If the SMTP server to which the message is being transmitted
	is identified through an MX record lookup, its name	is identified through an MX record lookup, its name

	MUST be validated via a DNSSEC signature on the recipient	<bcp14>MUST</bcp14> be validated via a DNSSEC signature on the recipient
	domain's MX record, or the MX hostname MUST be	domain's MX record, or the MX hostname <bcp14>MUST</bcp14> be
	validated by an MTA-STS policy as described in Section 4.1 of	validated by an MTA-STS policy as described in <xref target="RFC8461"
	<xref target="RFC8461">RFC 8461</xref>. DNSSEC is defined	sectionFormat="of" section="4.1" />. DNSSEC is defined
	in <xref target="RFC4033">RFC 4033</xref>,	in <xref target="RFC4033" format="default" />,
	<xref target="RFC4034">RFC 4034</xref>, and	<xref target="RFC4034" format="default" />, and
	<xref target="RFC4035">RFC 4035</xref>.</t>	<xref target="RFC4035" format="default" />.</li>
	<t>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 successfully using
	DANE as specified in <xref
	target="RFC7672">RFC 7672</xref>. For trust chains, the
	choice of trusted (root) certificates is at the discretion of
	the SMTP client.</t>
	<t>Following the negotiation of STARTTLS, the SMTP server MUST
	advertise in the subsequent EHLO response that it supports REQUIRETLS.</t
	>
	</list>
	</t>

	</section>

	<section anchor="header_field" title="The TLS-Required Header Field">


	<t>One new message header field <xref target="RFC5322" />,	<li>The certificate presented by the SMTP server either <bcp14>MUST</bcp
		14>
		be verified successfully by a trust chain leading to a certificate
		trusted by the SMTP client, or it <bcp14>MUST</bcp14> be verified success
		fully using
		DANE, as specified in <xref target="RFC7672" format="default" />. For tru
		st chains, the
		choice of trusted (root) certificates is at the discretion of
		the SMTP client.</li>
		<li>Following the negotiation of STARTTLS, the SMTP server <bcp14>MUST</
		bcp14>
		advertise in the subsequent EHLO response that it supports REQUIRETLS.</l
		i>
		</ul>
		</section>
		<section anchor="header_field" numbered="true" toc="default">
		<name>The TLS-Required Header Field</name>
		<t>One new message header field <xref target="RFC5322" format="default"/>,
	TLS-Required, is defined by this	TLS-Required, is defined by this
	specification. It is used for messages for which the originator	specification. It is used for messages for which the originator

	requests that recipient	requests that the recipient
	TLS policy (including <xref target="RFC8461">MTA-STS</xref> and	TLS policy (including <xref target="RFC8461" format="default">MTA-STS</xref
	<xref target="RFC7672">DANE</xref>) be ignored. This might be	> and
		<xref target="RFC7672" format="default">DANE</xref>) be ignored. This might
		be
	done, for example, to report a misconfigured mail server, such as	done, for example, to report a misconfigured mail server, such as
	an expired TLS certificate.</t>	an expired TLS certificate.</t>

		<t>The TLS-Required header field has a single <bcp14>REQUIRED</bcp14> para
	<t>The TLS-Required header field has a single REQUIRED parameter:</t>	meter:</t>
		<ul spacing="normal">
	<t><list style="symbols">	<li>No - The SMTP client <bcp14>SHOULD</bcp14> attempt to send the messa
	<t>No - The SMTP client SHOULD attempt to send the message	ge
	regardless of its ability to negotiate STARTTLS with the SMTP	regardless of its ability to negotiate STARTTLS with the SMTP
	server, ignoring policy-based mechanisms (including MTA-STS and	server, ignoring policy-based mechanisms (including MTA-STS and
	DANE), if any, asserted by	DANE), if any, asserted by

	the recipient domain. Nevertheless, the client SHOULD negotiate	the recipient domain. Nevertheless, the client <bcp14>SHOULD</bcp14> negot
	STARTTLS with the server if available.</t>	iate
	</list></t>	STARTTLS with the server if available.</li>
		</ul>
	<t>More than one instance of the TLS-Required header field MUST	<t>More than one instance of the TLS-Required header field <bcp14>MUST
	NOT appear in a given message.</t>	NOT</bcp14> appear in a given message.</t>
		<t>The ABNF syntax for the TLS-Required header field is as
	<t>The ABNF syntax for the TLS-Required header field is as
	follows:</t>	follows:</t>

	<figure>	<sourcecode type="abnf"><![CDATA[
	<artwork>	requiretls-field = "TLS-Required:" [FWS] "No" CRLF
	requiretls-field = "TLS-Required:" [FWS] "No" CRLF	; where requiretls-field in an instance of an
	; where requiretls-field in an instance of an	; optional-field defined in RFC 5322, Section 3.6.8.
	; optional-field defined in RFC 5322 Section	FWS = <as defined in RFC 5322>
	; 3.6.8.	CRLF = <as defined in RFC 5322> ]]></sourcecode>
	FWS = <as defined in RFC 5322>	</section>
	CRLF = <as defined in RFC 5322>	<section anchor="semantics" numbered="true" toc="default">
	</artwork>	<name>REQUIRETLS Semantics</name>
	</figure>	<section anchor="receipt" numbered="true" toc="default">
		<name>REQUIRETLS Receipt Requirements</name>
	</section>	<t>Upon receipt of the REQUIRETLS option on a MAIL FROM command
		during the receipt of a message, an SMTP server <bcp14>MUST</bcp14> tag t
	<section anchor="semantics" title="REQUIRETLS Semantics">	hat
	<section anchor="receipt" title="REQUIRETLS Receipt Requirements">
	<t>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.</t>	message as needing REQUIRETLS handling.</t>

		<t>Upon receipt of a message not specifying the REQUIRETLS
	<t>Upon receipt of a message not specifying the REQUIRETLS
	option on its MAIL FROM command but containing the TLS-Required	option on its MAIL FROM command but containing the TLS-Required
	header field in its message header, an SMTP server implementing	header field in its message header, an SMTP server implementing

	this specification MUST tag that message with the option	this specification <bcp14>MUST</bcp14> tag that message with the option
	specified in the TLS-Required header field. If the REQUIRETLS	specified in the TLS-Required header field. If the REQUIRETLS
	MAIL FROM parameter is specified, the TLS-Required header field	MAIL FROM parameter is specified, the TLS-Required header field

	MUST be ignored but MAY be included in onward relay of the	<bcp14>MUST</bcp14> be ignored but <bcp14>MAY</bcp14> be included in the onward relay of the
	message.</t>	message.</t>

		<t>The manner in which the above tagging takes place is
	<t>The manner in which the above tagging takes place is	implementation dependent. If the message is being locally
	implementation-dependent. If the message is being locally
	aliased and redistributed to multiple addresses, all instances	aliased and redistributed to multiple addresses, all instances

	of the message MUST be tagged in the same manner.</t>	of the message <bcp14>MUST</bcp14> be tagged in the same manner.</t>
	</section>	</section>
		<section anchor="sender" numbered="true" toc="default">
	<section anchor="sender" title="REQUIRETLS Sender Requirements">	<name>REQUIRETLS Sender Requirements</name>
	<section anchor="yestls" title="Sending with TLS Required">	<section anchor="yestls" numbered="true" toc="default">
	<t>When sending a message tagged as requiring TLS for which the	<name>Sending with TLS Required</name>
		<t>When sending a message tagged as requiring TLS for which the
	MAIL FROM return-path is not empty (an empty MAIL FROM	MAIL FROM return-path is not empty (an empty MAIL FROM
	return-path indicating a bounce message), the sending	return-path indicating a bounce message), the sending

	(client) MTA MUST:	(client) MTA <bcp14>MUST</bcp14>:

	<list style="numbers">

	<t>Look up the SMTP server to which the message is to be sent
	as described in <xref target="RFC5321"></xref> Section
	5.1.</t>


	<t>If the server lookup is accomplished via the recipient	</t>
		<ol spacing="normal" type="1">
		<li>Look up the SMTP server to which the message is to be sent,
		as described in <xref target="RFC5321" sectionFormat="comma"
		section="5.1" />.</li>
		<li>If the server lookup is accomplished via the recipient
	domain's MX record (the usual case) and is not accompanied by a valid	domain's MX record (the usual case) and is not accompanied by a valid

	DNSSEC signature, the client MUST also validate the SMTP	DNSSEC signature, the client <bcp14>MUST</bcp14> also validate the SMTP
	server name using MTA-STS as described in	server name using MTA-STS, as described in
	<xref target="RFC8461">RFC 8461</xref> Section 4.1.</t>	<xref target="RFC8461" sectionFormat="comma" section="4.1"/>.</li>
		<li>Open an SMTP session with the peer SMTP server using the
	<t>Open an SMTP session with the peer SMTP server using the	EHLO verb.</li>
	EHLO verb.</t>	<li>Establish a TLS-protected SMTP session with its peer SMTP

	<t>Establish a TLS-protected SMTP session with its peer SMTP
	server and authenticate the server's certificate as specified	server and authenticate the server's certificate as specified

	in <xref target="RFC6125"></xref> or <xref	in <xref target="RFC6125" format="default"/> or <xref target="RFC7672" f
	target="RFC7672"></xref> as applicable. The hostname from the	ormat="default"/>, as applicable. The hostname from the
	MX record lookup (or the domain name in the absence of an MX	MX record lookup (or the domain name in the absence of an MX

	record where an A record is used directly) MUST match the DNS-ID	record where an A record is used directly) <bcp14>MUST</bcp14> match the DNS-
	or CN-ID of the certificate presented by the server.</t>	ID
		or CN-ID of the certificate presented by the server.</li>
	<t>Ensure that the response to the subsequent EHLO following	<li>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.</t>	capability.</li>
		</ol>
	</list></t>	<t>The SMTP client <bcp14>SHOULD</bcp14> follow the recommendations in
		<xref target="RFC7525" format="default"/> or its successor with respect to
	<t>The SMTP client SHOULD follow the recommendations in <xref
	target="RFC7525"></xref> or its successor with respect to
	negotiation of the TLS session.</t>	negotiation of the TLS session.</t>


	<t>If any of the above steps fail, the client MUST issue a QUIT	<t>If any of the above steps fail, the client <bcp14>MUST</bcp14> issu e a QUIT
	to the server and repeat steps 2-5 with each host on the	to the server and repeat steps 2-5 with each host on the
	recipient domain's list of MX hosts in an attempt to find a	recipient domain's list of MX hosts in an attempt to find a

	mail path that meets the sender's requirements. The client MAY	mail path that meets the sender's requirements. The client <bcp14>MAY</bc
	send other, unprotected, messages to that server if it has any	p14>
	prior to issuing the QUIT. If there are no more MX hosts, the	send other, unprotected messages to that server if it has any
	client MUST NOT transmit the message to the domain. </t>	such messages prior to issuing the QUIT. If there are no more MX hosts, t
		he
	<t>Following such a failure, the SMTP client MUST send a	client <bcp14>MUST NOT</bcp14> transmit the message to the domain. </t>
		<t>Following such a failure, the SMTP client <bcp14>MUST</bcp14> send
		a
	non-delivery notification to the reverse-path of the failed	non-delivery notification to the reverse-path of the failed

	message as described in section 3.6 of <xref target="RFC5321" />. The	message, as described in <xref target="RFC5321"
	following <xref target="RFC5248">status codes</xref> SHOULD be used:	sectionFormat="of" section="3.6"/>. The
		following <xref target="RFC5248" format="default">status codes</xref> <bc
	<list style="symbols">	p14>SHOULD</bcp14> be used:
	<t>REQUIRETLS not supported by server: 5.7.YYY REQUIRETLS	</t>
	needed</t>
	<t>Unable to establish TLS-protected SMTP session: 5.7.10
	Encryption needed</t>
	</list></t>


	<t>Refer to <xref target="errors" /> for further requirements regarding	<ul spacing="normal">
		<li>REQUIRETLS not supported by server: 5.7.30 REQUIRETLS
		support required</li>
		<li>Unable to establish TLS-protected SMTP session: 5.7.10
		Encryption needed</li>
		</ul>
		<t>Refer to <xref target="errors" format="default"/> for further requi
		rements regarding
	non-delivery messages.</t>	non-delivery messages.</t>

		<t>If all REQUIRETLS requirements have been met, transmit the
	<t>If all REQUIRETLS requirements have been met, transmit the
	message, issuing the REQUIRETLS option on the MAIL FROM command	message, issuing the REQUIRETLS option on the MAIL FROM command
	with the required option(s), if any.</t>	with the required option(s), if any.</t>

		</section>
	</section>	<section anchor="maytls" numbered="true" toc="default">
	<section anchor="maytls" title="Sending with TLS Optional">	<name>Sending with TLS Optional</name>
	<t>Messages tagged TLS-Required: No are handled as	<t>Messages tagged "TLS-Required: No" are handled as
	follows. When sending such a message, the sending (client)	follows. When sending such a message, the sending (client)

	MTA MUST:	MTA <bcp14>MUST</bcp14>:
	<list style="symbols">	</t>
	<t>Look up the SMTP server to which the message is to be	<ul spacing="normal">
	sent as described in <xref target="RFC5321"></xref> Section	<li>Look up the SMTP server to which the message is to be
	5.1.</t>	sent, as described in <xref target="RFC5321" sectionFormat="comma" sec
		tion="5.1"/>.</li>
	<t>Open an SMTP session with the peer SMTP server using the	<li>Open an SMTP session with the peer SMTP server using the
	EHLO verb. Attempt to negotiate STARTTLS if possible, and	EHLO verb. Attempt to negotiate STARTTLS if possible, and
	follow any policy published by the recipient domain, but do	follow any policy published by the recipient domain, but do

	not fail if this is unsuccessful.</t>	not fail if this is unsuccessful.</li>
		</ul>
	</list>	<t>Some SMTP servers may be configured to require STARTTLS
	</t>

	<t>Some SMTP servers may be configured to require STARTTLS
	connections as a matter of policy and not accept messages in	connections as a matter of policy and not accept messages in

	the absence of STARTTLS. A non-delivery notification MUST be	the absence of STARTTLS. A non-delivery notification <bcp14>MUST</bcp14> be
	returned to the sender if message relay fails due to an	returned to the sender if message relay fails due to an
	inability to negotiate STARTTLS when required by the	inability to negotiate STARTTLS when required by the
	server.</t>	server.</t>

		<t>Since messages tagged with "TLS-Required: No" will sometimes
	<t>Since messages tagged with TLS-Required: No will sometimes
	be sent to SMTP servers not supporting REQUIRETLS, that	be sent to SMTP servers not supporting REQUIRETLS, that
	option will not be uniformly observed by all SMTP relay	option will not be uniformly observed by all SMTP relay
	hops.</t>	hops.</t>

		</section>
		</section>
		<section anchor="submission" numbered="true" toc="default">
		<name>REQUIRETLS Submission</name>


	</section>	<t>A Mail User Agent (MUA) or other agent making the initial introductio
		n of a
	</section>

	<section anchor="submission" title="REQUIRETLS Submission">
	<t>An MUA or other agent making the initial introduction of a
	message has the option to decide whether to require TLS. If	message has the option to decide whether to require TLS. If

	TLS is to be required, it MUST do so by negotiating STARTTLS	TLS is to be required, it <bcp14>MUST</bcp14> do so by negotiating STARTT
	and REQUIRETLS and include the REQUIRETLS option on the MAIL	LS
		and REQUIRETLS and including the REQUIRETLS option on the MAIL
	FROM command, as is done for message relay.</t>	FROM command, as is done for message relay.</t>

		<t>When TLS is not to be required, the sender <bcp14>MUST</bcp14> includ
	<t>When TLS is not to be required, the sender MUST include the	e the
	TLS-Required header field in the message. SMTP servers	TLS-Required header field in the message. SMTP servers

	implementing this specification MUST interpret this header	implementing this specification <bcp14>MUST</bcp14> interpret this header
	field as described in <xref target="receipt" />.</t>	field as described in <xref target="receipt" format="default"/>.</t>
		<t>In either case, the decision whether to specify REQUIRETLS
	<t>In either case, the decision whether to specify REQUIRETLS	<bcp14>MAY</bcp14> be done based on a user interface selection or based o
	MAY be done based on a user interface selection or based on a	n a
	ruleset or other policy. The manner in which the decision to	ruleset or other policy. The manner in which the decision to

	require TLS is made is implementation-dependent and is beyond	require TLS is made is implementation dependent and is beyond
	the scope of this specification.</t>	the scope of this specification.</t>

	</section>	</section>
		<section anchor="delivery" numbered="true" toc="default">
	<section anchor="delivery" title="Delivery of REQUIRETLS messages">	<name>Delivery of REQUIRETLS messages</name>
	<t>Messages are usually retrieved by end users using protocols	<t>Messages are usually retrieved by end users using protocols
	other than SMTP such as <xref target="RFC3501">IMAP</xref>,	other than SMTP such as <xref target="RFC3501" format="default">IMAP</xre
	<xref target="RFC1939">POP</xref>, or web mail systems. Mail	f>,
	delivery agents supporting the REQUIRETLS SMTP option SHOULD	<xref target="RFC1939" format="default">POP</xref>, or Web mail systems.
	observe the guidelines in <xref target="RFC8314" />.</t>	Mail
	</section>	delivery agents supporting the REQUIRETLS SMTP option <bcp14>SHOULD</bcp1
	</section>	4>
		observe the guidelines in <xref target="RFC8314" format="default"/>.</t>
	<section anchor="errors" title="Non-delivery message handling">	</section>
		</section>
		<section anchor="errors" numbered="true" toc="default">
		<name>Non-delivery Message Handling</name>


	<t>Non-delivery ("bounce") messages usually contain important	<t>Non-delivery ("bounce") messages usually contain important
	metadata about the message to which they refer, including the	metadata about the message to which they refer, including the

	original message header. They therefore MUST be protected in	original message header. They therefore <bcp14>MUST</bcp14> be protected
	the same manner as the original message. All non-delivery	in
	messages resulting from messages with the REQUIRETLS SMTP	the same manner as the original message.
	option, whether resulting from a REQUIRETLS error or some	All non-delivery messages resulting from messages with the REQUIRETLS SMTP
	other, MUST also specify the REQUIRETLS SMTP option unless	option, whether resulting from a REQUIRETLS error or some other issue, <bcp14
	redacted as described below.</t>	>MUST</bcp14>
		also specify the REQUIRETLS SMTP option unless redacted as described below.
	<t>The path from the origination of an error bounce message	</t>
		<t>The path from the origination of an error bounce message
	back to the MAIL FROM address may not share the same REQUIRETLS	back to the MAIL FROM address may not share the same REQUIRETLS
	support as the forward path. Therefore, users requiring TLS are	support as the forward path. Therefore, users requiring TLS are
	advised to make sure that they are capable of receiving mail	advised to make sure that they are capable of receiving mail
	using REQUIRETLS as well. Otherwise, such non-delivery messages	using REQUIRETLS as well. Otherwise, such non-delivery messages
	will be lost.</t>	will be lost.</t>

		<t>If a REQUIRETLS message is bounced, the server <bcp14>MUST</bcp14> beha
	<t>If a REQUIRETLS message is bounced, the server MUST behave	ve
	as if RET=HDRS was present as described in <xref	as if RET=HDRS was present, as described in <xref target="RFC3461" format
	target="RFC3461"></xref>. If both RET=FULL and REQUIRETLS are	="default"/>. If both RET=FULL and REQUIRETLS are
	present, the RET=FULL MUST be disregarded. The SMTP client for a	present, the RET=FULL <bcp14>MUST</bcp14> be disregarded. The SMTP client
		for a
	REQUIRETLS bounce message uses an empty MAIL FROM	REQUIRETLS bounce message uses an empty MAIL FROM

	return-path as required by <xref target="RFC5321"></xref>. When	return-path, as required by <xref target="RFC5321" format="default"/>. Wh en
	the MAIL FROM return-path is empty, the REQUIRETLS parameter	the MAIL FROM return-path is empty, the REQUIRETLS parameter

	SHOULD NOT cause a bounce message to be discarded even if the	<bcp14>SHOULD NOT</bcp14> cause a bounce message to be discarded even if the
	next-hop relay does not advertise REQUIRETLS.</t>	next-hop relay does not advertise REQUIRETLS.</t>

		<t>Senders of messages requiring TLS are advised to consider
	<t>Senders of messages requiring TLS are advised to consider
	the possibility that bounce messages will be lost as a result	the possibility that bounce messages will be lost as a result

	of REQUIRETLS return path failure, and that some information	of REQUIRETLS return path failure and that some information
	could be leaked if a bounce message is not able to be	could be leaked if a bounce message is not able to be
	transmitted with REQUIRETLS.</t>	transmitted with REQUIRETLS.</t>

	</section>	</section>
		<section anchor="reorigination" numbered="true" toc="default">
	<section anchor="reorigination" title="Reorigination considerations">	<name>Reorigination Considerations</name>
		<t>In a number of situations, a <xref target="RFC5598" format="default">me
	<t>In a number of situations, a <xref target="RFC5598">mediator</xref>	diator</xref>
	originates a new message as a result of an incoming message. These	originates a new message as a result of an incoming message. These

	situations include, but are not limited to, mailing lists (including	situations include but are not limited to mailing lists (including
	administrative traffic such as message approval requests),	administrative traffic such as message approval requests),

	<xref target="RFC5228">Sieve</xref>, "vacation" responders, and other	<xref target="RFC5228" format="default">Sieve</xref>, "vacation" responder s, and other
	filters to which incoming	filters to which incoming
	messages may be piped. These newly originated messages may essentially	messages may be piped. These newly originated messages may essentially
	be copies of the incoming message, such as with a forwarding service	be copies of the incoming message, such as with a forwarding service
	or a mailing list expander. In other cases, such as with a vacation	or a mailing list expander. In other cases, such as with a vacation
	message or a delivery notification, they will be different but might	message or a delivery notification, they will be different but might
	contain parts of the original message or other information for which	contain parts of the original message or other information for which
	the original message sender wants to influence the requirement to use	the original message sender wants to influence the requirement to use
	TLS transmission.</t>	TLS transmission.</t>


	<t>Mediators that reoriginate messages should apply REQUIRETLS requirement s	<t>Mediators that reoriginate messages should apply REQUIRETLS requirement s
	in incoming messages (both requiring TLS transmission and requesting	in incoming messages (both requiring TLS transmission and requesting
	that TLS not be required) to the reoriginated messages to the extent	that TLS not be required) to the reoriginated messages to the extent
	feasible. A limitation to this might be that for a message requiring	feasible. A limitation to this might be that for a message requiring
	TLS, redistribution to multiple addresses while retaining the TLS	TLS, redistribution to multiple addresses while retaining the TLS
	requirement could result in the message not being delivered to some of	requirement could result in the message not being delivered to some of
	the intended recipients.</t>	the intended recipients.</t>


	<t>User-side mediators (such as use of Sieve rules on a user agent)	<t>User-side mediators (such as use of Sieve rules on a user agent)

	typically do not have access to the SMTP details, and therefore may	typically do not have access to the SMTP details and therefore may
	not be aware of the REQUIRETLS requirement on a delivered message.	not be aware of the REQUIRETLS requirement on a delivered message.
	Recipients that expect sensitive traffic should avoid the use of	Recipients that expect sensitive traffic should avoid the use of
	user-side mediators. Alternatively, if operationally feasible (such as whe n	user-side mediators. Alternatively, if operationally feasible (such as whe n
	forwarding to a specific, known address), they should apply REQUIRETLS	forwarding to a specific, known address), they should apply REQUIRETLS
	to all reoriginated messages that do not contain the "TLS-Required: No" he ader	to all reoriginated messages that do not contain the "TLS-Required: No" he ader
	field.</t>	field.</t>

		</section>


	</section>	<section anchor="IANA" numbered="true" toc="default">
		<name>IANA Considerations</name>
	<!-- Possibly a 'Contributors' section ... -->	<t>Per this document, IANA has added
		the following keyword to the "SMTP Service Extensions" subregistry of the
	<section anchor="IANA" title="IANA Considerations">	<xref target="MailParams" format="default">"Mail Parameters" registry</xre
	<t>If published as an RFC, this draft requests the addition of	f>:</t>
	the following keyword to the <xref target="MailParams">SMTP
	Service Extensions Registry</xref>:</t>

	<figure align="left"><artwork align="left">
	Textual name: Require TLS
	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
	</artwork></figure>

	<t>If published as an RFC, this draft requests the addition of an
	entry to the <xref
	target="SMTPStatusCodes">Simple Mail Transfer Protocol (SMTP) Enhanced Stat
	us
	Codes Registry</xref>:</t>

	<figure align="left"><artwork align="left">
	Code: 5.7.YYY
	Sample Text: REQUIRETLS support required
	Associated basic status code: 550
	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
	</artwork></figure>

	<t>If published as an RFC, this draft requests the addition of
	an entry to the <xref
	target="PermMessageHeaderFields">Permanent Message Header Field
	Names Registry</xref>:</t>

	<figure align="left"><artwork align="left">
	Header field name: TLS-Required
	Applicable protocol: mail
	Status: standard
	Author/change controller: IETF
	Specification document: (this document)
	</artwork></figure>

	<t>This section is to be updated for publication by
	the RFC Editor.</t>
	</section>


	<section anchor="Security" title="Security Considerations">	<ul empty="true"><li>
	<t>The purpose of REQUIRETLS is to give the originator of a	<dl newline="false" spacing="compact" indent="30">
		<dt>EHLO Keyword:</dt><dd>REQUIRETLS</dd>
		<dt>Description:</dt><dd>Require TLS</dd>
		<dt>Syntax and parameters:</dt><dd>(no parameters)</dd>
		<dt>Additional SMTP verbs:</dt><dd>none</dd>
		<dt>MAIL and RCPT parameters:</dt><dd>REQUIRETLS parameter on MAIL</dd>
		<dt>Behavior:</dt><dd>Use of the REQUIRETLS parameter on the MAIL verb causes
		that message to require the use of TLS and tagging with REQUIRETLS for all onwar
		d relay.</dd>
		<dt>Command length increment:</dt><dd>11 characters</dd>
		</dl>
		</li>
		</ul>
		<t>Per this document, IANA has added an
		entry to the "Enumerated Status Codes" subregistry of the <xref target="SMT
		PStatusCodes" format="default">"Simple Mail Transfer Protocol (SMTP) Enhanced St
		atus
		Codes Registry"</xref>:</t>
		<ul empty="true"><li>
		<dl newline="false" spacing="compact" indent="30">
		<dt>Code:</dt><dd>X.7.30</dd>
		<dt>Sample Text:</dt><dd>REQUIRETLS support required</dd>
		<dt>Associated basic status code:</dt><dd>550</dd>
		<dt>Description:</dt><dd>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.<
		/dd>
		<dt>Reference:</dt><dd>RFC 8689</dd>
		<dt>Submitter:</dt><dd>J. Fenton</dd>
		<dt>Change Controller:</dt><dd>IESG</dd>
		</dl>
		</li>
		</ul>
		<t>Per this document, IANA has added
		an entry to the "Permanent Message Header Field
		Names" subregistry of the <xref target="MessageHeaders"
		format="default">"Message Headers" registry</xref> as follows:</t>
		<ul empty="true"><li>
		<dl newline="false" spacing="compact" indent="30">
		<dt>Header field name:</dt><dd>TLS-Required</dd>
		<dt>Applicable protocol:</dt><dd>mail</dd>
		<dt>Status:</dt><dd>standard</dd>
		<dt>Author/change controller:</dt><dd>IETF</dd>
		<dt>Specification document:</dt><dd>RFC 8689</dd>
		</dl>
		</li>
		</ul>
		</section>
		<section anchor="Security" numbered="true" toc="default">
		<name>Security Considerations</name>
		<t>The purpose of REQUIRETLS is to give the originator of a
	message control over the security of email they send, either by	message control over the security of email they send, either by
	conveying an expectation that it will be transmitted in an	conveying an expectation that it will be transmitted in an

	encrypted form "over the wire" or explicitly that transport	encrypted form over the wire or explicitly indicating that transport
	encryption is not required if it cannot be successfully	encryption is not required if it cannot be successfully
	negotiated.</t>	negotiated.</t>

		<t>The following considerations apply to the REQUIRETLS service
	<t>The following considerations apply to the REQUIRETLS service
	extension but not the TLS-Required header field, since messages	extension but not the TLS-Required header field, since messages
	specifying the header field are less concerned with transport	specifying the header field are less concerned with transport
	security.</t>	security.</t>

		<section anchor="Passive" numbered="true" toc="default">
	<section anchor="Passive" title="Passive attacks">	<name>Passive Attacks</name>
	<t>REQUIRETLS is generally effective against passive	<t>REQUIRETLS is generally effective against passive
	attackers who are merely trying to eavesdrop on an SMTP	attackers who are merely trying to eavesdrop on an SMTP
	exchange between an SMTP client and server. This assumes, of	exchange between an SMTP client and server. This assumes, of
	course, the cryptographic integrity of the TLS connection	course, the cryptographic integrity of the TLS connection
	being used.</t>	being used.</t>

	</section>	</section>
		<section anchor="Active" numbered="true" toc="default">
	<section anchor="Active" title="Active attacks">	<name>Active Attacks</name>
		<t>Active attacks against TLS-encrypted SMTP connections can
	<t>Active attacks against TLS encrypted SMTP connections can
	take many forms. One such attack is to interfere in the	take many forms. One such attack is to interfere in the
	negotiation by changing the STARTTLS command to something	negotiation by changing the STARTTLS command to something
	illegal such as XXXXXXXX. This causes TLS negotiation to fail	illegal such as XXXXXXXX. This causes TLS negotiation to fail
	and messages to be sent in the clear, where they can be	and messages to be sent in the clear, where they can be
	intercepted. REQUIRETLS detects the failure of STARTTLS and	intercepted. REQUIRETLS detects the failure of STARTTLS and
	declines to send the message rather than send it	declines to send the message rather than send it
	insecurely.</t>	insecurely.</t>

		<t>A second form of attack is a man-in-the-middle attack
	<t>A second form of attack is a man-in-the-middle attack
	where the attacker terminates the TLS connection rather than	where the attacker terminates the TLS connection rather than
	the intended SMTP server. This is possible when, as is	the intended SMTP server. This is possible when, as is
	commonly the case, the SMTP client either does not verify the	commonly the case, the SMTP client either does not verify the
	server's certificate or establishes the connection even when	server's certificate or establishes the connection even when
	the verification fails. REQUIRETLS requires successful	the verification fails. REQUIRETLS requires successful
	certificate validation before sending the message.</t>	certificate validation before sending the message.</t>

		<t>Another active attack involves the spoofing of DNS MX
	<t>Another active attack involves the spoofing of DNS MX	records of the recipient domain. An attacker with this
	records of the recipient domain. An attacker having this
	capability could potentially cause the message to be redirected to a mai l	capability could potentially cause the message to be redirected to a mai l
	server under the attacker's own control, which would	server under the attacker's own control, which would
	presumably have a valid certificate. REQUIRETLS requires that	presumably have a valid certificate. REQUIRETLS requires that
	the recipient domain's MX record lookup be validated either	the recipient domain's MX record lookup be validated either
	using DNSSEC or via a published MTA-STS policy that specifies	using DNSSEC or via a published MTA-STS policy that specifies
	the acceptable SMTP server hostname(s) for the recipient domain.</t>	the acceptable SMTP server hostname(s) for the recipient domain.</t>

		</section>
	</section>	<section anchor="badactor" numbered="true" toc="default">
		<name>Bad-Actor MTAs</name>
	<section anchor="badactor" title="Bad Actor MTAs">	<t>A bad-actor MTA along the message transmission path could

	<t>A bad-actor MTA along the message transmission path could
	misrepresent its support of REQUIRETLS and/or actively strip	misrepresent its support of REQUIRETLS and/or actively strip
	REQUIRETLS tags from messages it handles. However, since	REQUIRETLS tags from messages it handles. However, since
	intermediate MTAs are already trusted with the cleartext of	intermediate MTAs are already trusted with the cleartext of
	messages they handle, and are not part of the threat model	messages they handle, and are not part of the threat model
	for transport-layer security, they are also not part of the	for transport-layer security, they are also not part of the
	threat model for REQUIRETLS.</t>	threat model for REQUIRETLS.</t>

		<t>It should be reemphasized that since SMTP TLS is a
	<t>It should be reemphasized that since SMTP TLS is a
	transport-layer security protocol, messages sent using	transport-layer security protocol, messages sent using
	REQUIRETLS are not encrypted end-to-end and are visible to	REQUIRETLS are not encrypted end-to-end and are visible to
	MTAs that are part of the message delivery path. Messages	MTAs that are part of the message delivery path. Messages
	containing sensitive information that MTAs should not have	containing sensitive information that MTAs should not have

	access to MUST be sent using end-to-end content encryption	access to <bcp14>MUST</bcp14> be sent using end-to-end content encryptio
	such as <xref target="RFC4880">OpenPGP</xref> or <xref	n
	target="RFC8551">S/MIME</xref>.</t>	such as <xref target="RFC4880" format="default">OpenPGP</xref> or <xref
	</section>	target="RFC8551" format="default">S/MIME</xref>.</t>
		</section>
	<section anchor="conflicts" title="Policy Conflicts">	<section anchor="conflicts" numbered="true" toc="default">
		<name>Policy Conflicts</name>
	<t>In some cases, the use of the TLS-Required header field may conflict	<t>In some cases, the use of the TLS-Required header field may conflict

	with a recipient domain policy expressed through the <xref	with a recipient domain policy expressed through the <xref target="RFC7672" form
	target="RFC7672">DANE</xref> or <xref target="RFC8461">MTA-STS</xref> protocols.	at="default">DANE</xref> or <xref target="RFC8461" format="default">MTA-STS</xre
		f> protocols.
	Although these protocols encourage the use of TLS transport by advertising	Although these protocols encourage the use of TLS transport by advertising

	availability of TLS, the use of "TLS-Required: No" header field represents an	the availability of TLS, the use of the "TLS-Required: No" header field represen ts an
	explicit decision on the part of the sender not to require the use of TLS, such	explicit decision on the part of the sender not to require the use of TLS, such
	as to overcome a configuration error. The recipient domain has the ultimate	as to overcome a configuration error. The recipient domain has the ultimate
	ability to require TLS by not accepting messages when STARTTLS has not been	ability to require TLS by not accepting messages when STARTTLS has not been
	negotiated; otherwise, "TLS-Required: No" is effectively directing the client	negotiated; otherwise, "TLS-Required: No" is effectively directing the client

	MTA to behave as if it does not support DANE nor MTA-STS.</t>	MTA to behave as if it does not support DANE or MTA-STS.</t>
	</section>	</section>
	</section>	</section>

		</middle>
		<back>
		<references>
		<name>References</name>
		<references>
		<name>Normative References</name>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.2119.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.3207.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.4033.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.4034.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.4035.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.3461.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.5234.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.5248.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.5321.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.5322.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.6125.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.7525.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.7672.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.8174.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.8314.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.8461.xml"/>


	<section anchor="acknowledgements" title="Acknowledgements">	<reference anchor="MailParams" target="http://www.iana.org/assignments/m
	<t>The author would like to acknowledge many helpful suggestions	ail-parameters">
	on the ietf-smtp and uta mailing lists, in particular those of	<front>
	Viktor Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel	<title>Mail Parameters</title>
	Hathcock, John Klensin, Barry Leiba, John Levine, Rolf Sonneveld,	<author>
	and Per Thorsheim.</t>	<organization>IANA</organization>
		</author>
	</section>	</front>
		</reference>
	<section anchor="history" title="Revision History">

	<t>To be removed by RFC Editor upon publication as an RFC.</t>

	<section title="Changes since -08 Draft">
	<t>Additional changes in response to IESG review:</t>
	<t><list style="symbols">
	<t>Unify wording describing TLS-Required in Appendix A.2.</t>
	<t>Add specifics on verification of mail server hostnames with
	certificates.</t>
	<t>Wording tweak in 4.3 to emphasize optional nature of REQUIRETLS.</t>
	<t>Update S/MIME reference from RFC 5751 to 8551</t>
	</list></t></section>

	<section title="Changes since -07 Draft">
	<t>Changes in response to IESG review and IETF Last Call comments:</t>
	<t><list style="symbols">
	<t>Change associated status code for 5.7.YYY from 530 to
	550.</t>
	<t>Correct textual name of extension in IANA Considerations
	for consistency with the rest of the document.</t>
	<t>Remove special handling of bounce messages in Section
	4.1.</t>
	<t>Change name of header field from RequireTLS to
	TLS-Required and make capitalization of parameter
	consistent.</t>
	<t>Remove mention of transforming RET=FULL to RET=HDRS on
	relay in Section 5.</t>
	<t>Replace Section 6 dealing with mailing lists with a more
	general section on reorigination by mediators.</t>
	<t>Add security considerations section on policy conflicts.</t>

	</list></t>
	</section>

	<section title="Changes since -06 Draft">
	<t>Various changes in response to AD review:</t>

	<t><list style="symbols">
	<t>Reference RFC 7525 for TLS negotiation
	recommendations.</t>
	<t>Make reference to requested 5.7.YYY error code
	consistent.</t>
	<t>Clarify applicability to LMTP and submission.</t>
	<t>Provide ABNF for syntax of SMTP option and header field
	and examples in Appendix A.</t>
	<t>Correct use of normative language in Section 5.</t>
	<t>Clarify case where REQUIRETLS option is used on bounce
	messages.</t>
	<t>Improve Security Requirements wording to be inclusive of
	both SMTP option and header field.</t>
	</list></t>
	</section>

	<section title="Changes since -05 Draft">
	<t>Corrected IANA Permanent Message Header Fields Registry
	request.</t>
	</section>

	<section title="Changes since -04 Draft">
	<t>Require validation of SMTP server hostname via DNSSEC or
	MTA-STS policy when TLS is required.</t>
	</section>

	<section title="Changes since -03 Draft">
	<t>Working Group Last Call changes, including:</t>
	<t><list style="symbols">
	<t>Correct reference for SMTP DANE</t>
	<t>Clarify that RequireTLS: NO applies to both MTA-STS and
	DANE policies</t>
	<t>Correct newly-defined status codes</t>
	<t>Update MTA-STS references to RFC</t>
	</list></t>
	</section>

	<section title="Changes since -02 Draft">
	<t><list style="symbols">
	<t>More complete documentation for IANA registration requests.</t>
	<t>Changed bounce handling to use RET parameters of <xref
	target="RFC3461"></xref>, along with slightly more liberal transmission of
	bounces even if REQUIRETLS can't be negotiated.</t>
	</list></t>
	</section>

	<section title="Changes since -01 Draft">
	<t><list style="symbols">
	<t>Converted DEEP references to RFC 8314.</t>
	<t>Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.</t>
	<t>Editorial corrections, notably making the header field
	name consistent (RequireTLS rather than Require-TLS).</t>
	</list></t>
	</section>

	<section title="Changes since -00 Draft">
	<t><list style="symbols">
	<t>Created new header field, Require-TLS, for use by "NO"
	option.</t>
	<t>Removed "NO" option from SMTP service extension.</t>
	<t>Recommend DEEP requirements for delivery of messages
	requiring TLS.</t>
	<t>Assorted copy edits</t>
	</list></t>
	</section>

	<section title="Changes since fenton-03 Draft">
	<t><list style="symbols">
	<t>Wording improvements from Rolf Sonneveld review 22 July
	2017</t>
	<t>A few copy edits</t>
	<t>Conversion from individual to UTA WG draft</t>
	</list></t>
	</section>

	<section title="Changes Since -02 Draft">
	<t><list style="symbols">
	<t>Incorporation of "MAY TLS" functionality as
	REQUIRETLS=NO per suggestion on UTA WG mailing list.</t>
	<t>Additional guidance on bounce messages</t>
	</list></t>
	</section>

	<section title="Changes Since -01 Draft">
	<t><list style="symbols">
	<t>Specified retries when multiple MX hosts exist for a
	given domain.</t>
	<t>Clarified generation of non-delivery messages</t>
	<t>Specified requirements for application of REQUIRETLS to mail forwar
	ders
	and mailing lists.</t>
	<t>Clarified DNSSEC requirements to include MX lookup only.</t>
	<t>Corrected terminology regarding message retrieval
	vs. delivery.</t>
	<t>Changed category to standards track.</t>

	</list></t>
	</section>
	<section title="Changes Since -00 Draft">

	<t><list style="symbols">
	<t>Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
	parameter to better associate REQUIRETLS requirements with
	transmission of individual messages.</t>

	<t>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.</t>

	<t>Clarified the wording to more clearly state that TLS
	sessions must be established and not simply that STARTTLS is
	negotiated.</t>

	<t>Introduced need for minimum encryption standards (key
	lengths and algorithms)</t>

	<t>Substantially rewritten Security Considerations section</t>

	</list></t>

	</section>

	</section>

	</middle>

	<!-- ***BACK MATTER *** -->

	<back>
	<!-- References split into informative and normative -->

	<!-- There are 2 ways to insert reference entries from the citation libraries
	:
	1. define an ENTITY at the top, and use "ampersand character"RFC2629; here (
	as shown)
	2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml
	"?> here
	(for I-Ds: include="reference.I-D.narten-iana-considerations-rfc2434bis.x
	ml")

	Both are cited textually in the same manner: by using xref elements.
	If you use the PI option, xml2rfc will, by default, try to find included fil
	es in the same
	directory as the including file. You can also define the XML_LIBRARY environ
	ment variable
	with a value containing a set of directories to search. These can be either
	in the local
	filing system or remote ones accessed by http (http://domain/dir/... ).-->

	<references title="Normative References">
	<!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2
	119.xml"?-->
	&RFC2119;
	&RFC3207;
	&RFC4033;
	&RFC4034;
	&RFC4035;
	&RFC3461;
	&RFC5234;
	&RFC5248;
	&RFC5321;
	&RFC5322;
	&RFC6125;
	&RFC7525;
	&RFC7672;
	&RFC8174;
	&RFC8314;
	&RFC8461;

	<!-- [rfced] [MailParams] This URL is correct -->

	<reference anchor="MailParams"
	target="http://www.iana.org/assignments/mail-parameters">
	<front>
	<title>IANA Mail Parameters</title>

	<author>
	<organization>Internet Assigned Numbers Authority (IANA)</organizatio
	n>
	</author>

	<date year="2007" />
	</front>
	</reference>

	<!-- [rfced] [SMTPStatusCodes] This URL is correct -->


	<reference anchor="SMTPStatusCodes"	<reference anchor="SMTPStatusCodes" target="https://www.iana.org/assignm
	target="http://www.iana.org/assignments/smtp-enhanced-status-cod	ents/smtp-enhanced-status-codes">
	es">	<front>
	<front>	<title>Simple Mail Transfer Protocol (SMTP) Enhanced Status
	<title>Simple Mail Transfer Protocol (SMTP) Enhanced Status
	Codes Registry</title>	Codes Registry</title>

		<author>
		<organization>IANA</organization>
		</author>
		</front>
		</reference>


	<author>	<reference anchor="MessageHeaders" target="https://www.iana.org/assignme
	<organization>Internet Assigned Numbers Authority (IANA)</organizatio	nts/message-headers">
	n>	<front>
	</author>	<title>Permanent Message Header Field Names</title>
		<author>
	<date year="2008" />	<organization>IANA</organization>
	</front>	</author>
	</reference>	</front>
		</reference>
	<!-- [rfced] [PermMessageHeaderFields] This URL is correct -->	</references>
		<references>
	<reference anchor="PermMessageHeaderFields"	<name>Informative References</name>
	target="https://www.iana.org/assignments/message-headers/message	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
	-headers.xhtml#perm-headers">	ence.RFC.1939.xml"/>
	<front>	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
	<title>Permanent Message Header Field Names Registry</title>	ence.RFC.2033.xml"/>
		<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
	<author>	ence.RFC.3501.xml"/>
	<organization>Internet Assigned Numbers Authority (IANA)</organizatio	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
	n>	ence.RFC.4880.xml"/>
	</author>	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.5228.xml"/>
	<date year="2004" />	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
	</front>	ence.RFC.5598.xml"/>
	</reference>	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.6409.xml"/>
	</references>	<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
		ence.RFC.8551.xml"/>
	<references title="Informative References">	</references>
	<!-- Here we use entities that we defined at the beginning. -->	</references>

	&RFC1939;
	&RFC2033;
	&RFC3501;
	&RFC4880;
	&RFC5228;
	&RFC5598;
	&RFC6409;
	&RFC8551;

	</references>


	<section title="Examples">	<section numbered="true" toc="default">
	<t>This section is informative.</t>	<name>Examples</name>
	<section title="REQUIRETLS SMTP Option">	<t>This section is informative.</t>
	<t>The TLS-Required SMTP option is used to express the intent of	<section numbered="true" toc="default">
	the sender that the associated message be relayed using TLS. In	<name>REQUIRETLS SMTP Option</name>
	the following example, lines beginning with C: are transmitted	<t>The TLS-Required SMTP option is used to express the intention of
	from the SMTP client to the server, and lines beginning with S:	the sender to have the associated message relayed using TLS. In
		the following example, lines beginning with "C:" are transmitted
		from the SMTP client to the server, and lines beginning with "S:"
	are transmitted in the opposite direction.</t>	are transmitted in the opposite direction.</t>

	<figure>	<artwork name="" type="" align="left" alt=""><![CDATA[
	<artwork>
	S: 220 mail.example.net ESMTP	S: 220 mail.example.net ESMTP
	C: EHLO mail.example.org	C: EHLO mail.example.org
	S: 250-mail.example.net Hello example.org [192.0.2.1]	S: 250-mail.example.net Hello example.org [192.0.2.1]
	S: 250-SIZE 52428800	S: 250-SIZE 52428800
	S: 250-8BITMIME	S: 250-8BITMIME
	S: 250-PIPELINING	S: 250-PIPELINING
	S: 250-STARTTLS	S: 250-STARTTLS
	S: 250 HELP	S: 250 HELP
	C: STARTTLS	C: STARTTLS
	S: TLS go ahead	S: TLS go ahead

		]]></artwork>
		<t>(at this point TLS negotiation takes place. The remainder of this
		session occurs within TLS.)</t>


	(at this point TLS negotiation takes place. The remainder of this	<artwork name="" type="" align="left" alt=""><![CDATA[
	session occurs within TLS.)

	S: 220 mail.example.net ESMTP	S: 220 mail.example.net ESMTP
	C: EHLO mail.example.org	C: EHLO mail.example.org
	S: 250-mail.example.net Hello example.org [192.0.2.1]	S: 250-mail.example.net Hello example.org [192.0.2.1]
	S: 250-SIZE 52428800	S: 250-SIZE 52428800
	S: 250-8BITMIME	S: 250-8BITMIME
	S: 250-PIPELINING	S: 250-PIPELINING
	S: 250-REQUIRETLS	S: 250-REQUIRETLS
	S: 250 HELP	S: 250 HELP

	C: MAIL FROM:<roger@example.org> REQUIRETLS	C: MAIL FROM:<roger@example.org> REQUIRETLS
	S: 250 OK	S: 250 OK

	C: RCPT TO:<editor@example.net>	C: RCPT TO:<editor@example.net>
	S: 250 Accepted	S: 250 Accepted
	C: DATA	C: DATA
	S: 354 Enter message, ending with "." on a line by itself	S: 354 Enter message, ending with "." on a line by itself

		]]></artwork>


	(message follows)	<t>(message follows)</t>
		<artwork name="" type="" align="left" alt=""><![CDATA[
	C: .	C: .
	S: 250 OK	S: 250 OK
	C: QUIT	C: QUIT

	</artwork>	]]></artwork>
	</figure>	</section>
	</section>	<section numbered="true" toc="default">
	<section title="TLS-Required Header Field">	<name>TLS-Required Header Field</name>
	<t>The TLS-Required header field is used when the sender requests	<t>The TLS-Required header field is used when the sender requests
	that the mail system not heed a default policy of the recipient	that the mail system not heed a default policy of the recipient
	domain requiring TLS. It might be used, for example, to allow	domain requiring TLS. It might be used, for example, to allow
	problems with the recipient domain's TLS certificate to be	problems with the recipient domain's TLS certificate to be
	reported:</t>	reported:</t>

	<figure>	<artwork name="" type="" align="left" alt=""><![CDATA[
	<artwork>	From: Roger Reporter <roger@example.org>
	From: Roger Reporter <roger@example.org>	To: Andy Admin <admin@example.com>
	To: Andy Admin <admin@example.com>
	Subject: Certificate problem?	Subject: Certificate problem?
	TLS-Required: No	TLS-Required: No
	Date: Fri, 18 Jan 2019 10:26:55 -0800	Date: Fri, 18 Jan 2019 10:26:55 -0800

	Message-ID: <5c421a6f79c0e_d153ff8286d45c468473@mail.example.org>	Message-ID: <5c421a6f79c0e_d153ff8286d45c468473@mail.example.org>

	Andy, there seems to be a problem with the TLS certificate	Andy, there seems to be a problem with the TLS certificate
	on your mail server. Are you aware of this?	on your mail server. Are you aware of this?


	Roger	Roger ]]></artwork>
	</artwork>	</section>
	</figure>	</section>
	</section>


	</section>	<section anchor="acknowledgements" numbered="false" toc="default">
	</back>	<name>Acknowledgements</name>
		<t>
		The author would like to acknowledge many helpful suggestions on the
		ietf-smtp and uta mailing lists, in particular those of Viktor Dukhovni,
		Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin, Barry Leiba, John
		Levine, Chris Newman, Rolf Sonneveld, and Per Thorsheim.
		</t>
		</section>
		</back>
	</rfc>	</rfc>

End of changes. 103 change blocks.
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