blame: rfcs/rfc8689.txt

Internet Engineering Task Force (IETF) J. Fenton

Request for Comments: 8689 Altmode Networks

Category: Standards Track November 2019

ISSN: 2070-1721

SMTP Require TLS Option

Abstract

The SMTP STARTTLS option, used in negotiating transport-level

encryption of SMTP connections, is not as useful from a security

standpoint as it might be because of its opportunistic nature;

message delivery is, by default, prioritized over security. This

document describes an SMTP service extension, REQUIRETLS, and a

message header field, TLS-Required. If the REQUIRETLS option or TLS-

Required message header field is used when sending a message, it

asserts a request on the part of the message sender to override the

default negotiation of TLS, either by requiring that TLS be

negotiated when the message is relayed or by requesting that

recipient-side policy mechanisms such as MTA-STS and DNS-Based

Authentication of Named Entities (DANE) be ignored when relaying a

message for which security is unimportant.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force

(IETF). It represents the consensus of the IETF community. It has

received public review and has been approved for publication by the

Internet Engineering Steering Group (IESG). Further information on

Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,

and how to provide feedback on it may be obtained at

https://www.rfc-editor.org/info/rfc8689.

This document is subject to BCP 78 and the IETF Trust's Legal

Provisions Relating to IETF Documents

(https://trustee.ietf.org/license-info) in effect on the date of

publication of this document. Please review these documents

carefully, as they describe your rights and restrictions with respect

to this document. Code Components extracted from this document must

include Simplified BSD License text as described in Section 4.e of

the Trust Legal Provisions and are provided without warranty as

described in the Simplified BSD License.

Table of Contents

1. Introduction

1.1. Requirements Language

2. The REQUIRETLS Service Extension

3. The TLS-Required Header Field

4. REQUIRETLS Semantics

4.1. REQUIRETLS Receipt Requirements

4.2. REQUIRETLS Sender Requirements

4.2.1. Sending with TLS Required

4.2.2. Sending with TLS Optional

4.3. REQUIRETLS Submission

4.4. Delivery of REQUIRETLS messages

5. Non-delivery Message Handling

6. Reorigination Considerations

7. IANA Considerations

8. Security Considerations

8.1. Passive Attacks

8.2. Active Attacks

8.3. Bad-Actor MTAs

8.4. Policy Conflicts

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Examples

A.1. REQUIRETLS SMTP Option

A.2. TLS-Required Header Field

Acknowledgements

Author's Address

1. Introduction

The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a

means by which an SMTP server and client can establish a Transport

Layer Security (TLS) protected session for the transmission of email

messages. By default, TLS is used only upon mutual agreement

(successful negotiation) of STARTTLS between the client and server;

if this is not possible, the message is sent without transport

encryption. Furthermore, it is common practice for the client to

negotiate TLS even if the SMTP server's certificate is invalid.

Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may

impose requirements for the use of TLS for email destined for some

domains. However, such policies do not allow the sender to specify

which messages are more sensitive and require transport-level

encryption and which ones are less sensitive and ought to be relayed

even if TLS cannot be negotiated successfully.

The default opportunistic nature of SMTP TLS enables several on-the-

wire attacks on SMTP security between MTAs. These include passive

eavesdropping on connections for which TLS is not used, interference

in the SMTP protocol to prevent TLS from being negotiated (presumably

accompanied by eavesdropping), and insertion of a man-in-the-middle

attacker exploiting the lack of server authentication by the client.

Attacks are described in more detail in the Security Considerations

section of this document.

REQUIRETLS consists of two mechanisms: an SMTP service extension and

a message header field. The service extension is used to specify

that a given message sent during a particular session MUST be sent

over a TLS-protected session with specified security characteristics.

It also requires that the SMTP server advertise that it supports

REQUIRETLS, in effect promising that it will honor the requirement to

enforce TLS transmission and REQUIRETLS support for onward

transmission of those messages.

The TLS-Required message header field is used to convey a request to

ignore recipient-side policy mechanisms such as MTA-STS and DANE,

thereby prioritizing delivery over ability to negotiate TLS. Unlike

the service extension, the TLS-Required header field allows the

message to transit through one or more MTAs that do not support

REQUIRETLS.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and

"OPTIONAL" in this document are to be interpreted as described in

BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all

capitals, as shown here.

The formal syntax uses the Augmented Backus-Naur Form (ABNF)

[RFC5234], including the core rules defined in Appendix B of that

document.

2. The REQUIRETLS Service Extension

The REQUIRETLS SMTP service extension has the following

characteristics:

1. The textual name of the extension is "Require TLS".

2. The EHLO keyword value associated with this extension is

"REQUIRETLS".

3. No additional SMTP verbs are defined by this extension.

4. One optional parameter ("REQUIRETLS") is added to the MAIL FROM

command by this extension. No value is associated with this

parameter.

5. The maximum length of a MAIL FROM command line is increased by 11

octets by the possible addition of a space and the REQUIRETLS

keyword.

6. One new SMTP status code is defined by this extension to convey

an error condition resulting from failure of the client to send

data to a server that does not also support the REQUIRETLS

extension.

7. The REQUIRETLS extension is valid for message relay [RFC5321],

submission [RFC6409], and the Local Mail Transfer Protocol (LMTP)

[RFC2033].

8. The ABNF syntax for the MAIL FROM parameter is as follows:

requiretls-param = "REQUIRETLS"

; where requiretls-param is an instance of an

; esmtp-param used in Mail-parameters in

; RFC 5321, Section 4.1.2. There is no esmtp-value

; associated with requiretls-param.

In order to specify REQUIRETLS treatment for a given message, the

REQUIRETLS option is specified in 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

REQUIRETLS:

* The session itself MUST employ TLS transmission.

* If the SMTP server to which the message is being transmitted is

identified through an MX record lookup, its name MUST be validated

via a DNSSEC signature on the recipient domain's MX record, or the

MX hostname MUST be validated by an MTA-STS policy as described in

Section 4.1 of [RFC8461]. DNSSEC is defined in [RFC4033],

[RFC4034], and [RFC4035].

* The certificate presented by the SMTP server either MUST be

verified successfully by a trust chain leading to a certificate

trusted by the SMTP client, or it MUST be verified successfully

using DANE, as specified in [RFC7672]. For trust chains, the

choice of trusted (root) certificates is at the discretion of the

SMTP client.

* Following the negotiation of STARTTLS, the SMTP server MUST

advertise in the subsequent EHLO response that it supports

REQUIRETLS.

3. The TLS-Required Header Field

One new message header field [RFC5322], TLS-Required, is defined by

this specification. It is used for messages for which the originator

requests that the recipient TLS policy (including MTA-STS [RFC8461]

and DANE [RFC7672]) be ignored. This might be done, for example, to

report a misconfigured mail server, such as an expired TLS

certificate.

The TLS-Required header field has a single REQUIRED parameter:

* No - The SMTP client SHOULD attempt to send the message regardless

of its ability to negotiate STARTTLS with the SMTP server,

ignoring policy-based mechanisms (including MTA-STS and DANE), if

any, asserted by the recipient domain. Nevertheless, the client

SHOULD negotiate STARTTLS with the server if available.

More than one instance of the TLS-Required header field MUST NOT

appear in a given message.

The ABNF syntax for the TLS-Required header field is as follows:

requiretls-field = "TLS-Required:" [FWS] "No" CRLF

; where requiretls-field in an instance of an

; optional-field defined in RFC 5322, Section 3.6.8.

FWS = <as defined in RFC 5322>

CRLF = <as defined in RFC 5234>

4. REQUIRETLS Semantics

4.1. REQUIRETLS Receipt Requirements

Upon receipt of the REQUIRETLS option on a MAIL FROM command during

the receipt of a message, an SMTP server MUST tag that message as

needing REQUIRETLS handling.

Upon receipt of a message not specifying the REQUIRETLS option on its

MAIL FROM command but containing the TLS-Required header field in its

message header, an SMTP server implementing this specification MUST

tag that message with the option specified in the TLS-Required header

field. If the REQUIRETLS MAIL FROM parameter is specified, the TLS-

Required header field MUST be ignored but MAY be included in the

onward relay of the message.

The manner in which the above tagging takes place is implementation

dependent. If the message is being locally aliased and redistributed

to multiple addresses, all instances of the message MUST be tagged in

the same manner.

4.2. REQUIRETLS Sender Requirements

4.2.1. Sending with TLS Required

When sending a message tagged as requiring TLS for which the MAIL

FROM return-path is not empty (an empty MAIL FROM return-path

indicating a bounce message), the sending (client) MTA MUST:

1. Look up the SMTP server to which the message is to be sent, as

described in [RFC5321], Section 5.1.

2. If the server lookup is accomplished via the recipient domain's

MX record (the usual case) and is not accompanied by a valid

DNSSEC signature, the client MUST also validate the SMTP server

name using MTA-STS, as described in [RFC8461], Section 4.1.

3. Open an SMTP session with the peer SMTP server using the EHLO

verb.

4. Establish a TLS-protected SMTP session with its peer SMTP server

and authenticate the server's certificate as specified in

[RFC6125] or [RFC7672], as applicable. The hostname from the 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 or CN-

ID of the certificate presented by the server.

5. Ensure that the response to the subsequent EHLO following

establishment of the TLS protection advertises the REQUIRETLS

capability.

The SMTP client SHOULD follow the recommendations in [RFC7525] or its

successor with respect to negotiation of the TLS session.

If any of the above steps fail, the client MUST issue a QUIT 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 mail path that meets the

sender's requirements. The client MAY send other, unprotected

messages to that server if it has any such messages prior to issuing

the QUIT. If there are no more MX hosts, the client MUST NOT

transmit the message to the domain.

Following such a failure, the SMTP client MUST send a non-delivery

notification to the reverse-path of the failed message, as described

in Section 3.6 of [RFC5321]. The following status codes [RFC5248]

SHOULD be used:

* REQUIRETLS not supported by server: 5.7.30 REQUIRETLS support

required

* Unable to establish TLS-protected SMTP session: 5.7.10 Encryption

needed

Refer to Section 5 for further requirements regarding non-delivery

messages.

If all REQUIRETLS requirements have been met, transmit the message,

issuing the REQUIRETLS option on the MAIL FROM command with the

required option(s), if any.

4.2.2. Sending with TLS Optional

Messages tagged "TLS-Required: No" are handled as follows. When

sending such a message, the sending (client) MTA MUST:

* Look up the SMTP server to which the message is to be sent, as

described in [RFC5321], Section 5.1.

* Open an SMTP session with the peer SMTP server using the EHLO

verb. Attempt to negotiate STARTTLS if possible, and follow any

policy published by the recipient domain, but do not fail if this

is unsuccessful.

Some SMTP servers may be configured to require STARTTLS connections

as a matter of policy and not accept messages in the absence of

STARTTLS. A non-delivery notification MUST be returned to the sender

if message relay fails due to an inability to negotiate STARTTLS when

required by the server.

Since messages tagged with "TLS-Required: No" will sometimes be sent

to SMTP servers not supporting REQUIRETLS, that option will not be

uniformly observed by all SMTP relay hops.

4.3. REQUIRETLS Submission

A Mail User Agent (MUA) or other agent making the initial

introduction of a message has the option to decide whether to require

TLS. If TLS is to be required, it MUST do so by negotiating STARTTLS

and REQUIRETLS and including the REQUIRETLS option on the MAIL FROM

command, as is done for message relay.

When TLS is not to be required, the sender MUST include the TLS-

Required header field in the message. SMTP servers implementing this

specification MUST interpret this header field as described in

Section 4.1.

In either case, the decision whether to specify REQUIRETLS MAY be

done based on a user interface selection or based on a ruleset or

other policy. The manner in which the decision to require TLS is

made is implementation dependent and is beyond the scope of this

specification.

4.4. Delivery of REQUIRETLS messages

Messages are usually retrieved by end users using protocols other

than SMTP such as IMAP [RFC3501], POP [RFC1939], or Web mail systems.

Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD

observe the guidelines in [RFC8314].

5. Non-delivery Message Handling

Non-delivery ("bounce") messages usually contain important metadata

about the message to which they refer, including the original message

header. They therefore MUST be protected in the same manner as the

original message. All non-delivery messages resulting from messages

with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS

error or some other issue, MUST also specify the REQUIRETLS SMTP

option unless redacted as described below.

The path from the origination of an error bounce message back to the

MAIL FROM address may not share the same REQUIRETLS support as the

forward path. Therefore, users requiring TLS are advised to make

sure that they are capable of receiving mail using REQUIRETLS as

well. Otherwise, such non-delivery messages will be lost.

If a REQUIRETLS message is bounced, the server MUST behave as if

RET=HDRS was present, as described in [RFC3461]. If both RET=FULL

and REQUIRETLS are present, the RET=FULL MUST be disregarded. The

SMTP client for a REQUIRETLS bounce message uses an empty MAIL FROM

return-path, as required by [RFC5321]. When the MAIL FROM return-

path is empty, the REQUIRETLS parameter SHOULD NOT cause a bounce

message to be discarded even if the next-hop relay does not advertise

REQUIRETLS.

Senders of messages requiring TLS are advised to consider the

possibility that bounce messages will be lost as a result of

REQUIRETLS return path failure and that some information could be

leaked if a bounce message is not able to be transmitted with

REQUIRETLS.

6. Reorigination Considerations

In a number of situations, a mediator [RFC5598] originates a new

message as a result of an incoming message. These situations include

but are not limited to mailing lists (including administrative

traffic such as message approval requests), Sieve [RFC5228],

"vacation" responders, and other filters to which incoming messages

may be piped. These newly originated messages may essentially 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

message or a delivery notification, they will be different but might

contain parts of the original message or other information for which

the original message sender wants to influence the requirement to use

TLS transmission.

Mediators that reoriginate messages should apply REQUIRETLS

requirements in incoming messages (both requiring TLS transmission

and requesting that TLS not be required) to the reoriginated messages

to the extent feasible. A limitation to this might be that for a

message requiring TLS, redistribution to multiple addresses while

retaining the TLS requirement could result in the message not being

delivered to some of the intended recipients.

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

not be aware of the REQUIRETLS requirement on a delivered message.

Recipients that expect sensitive traffic should avoid the use of

user-side mediators. Alternatively, if operationally feasible (such

as when forwarding to a specific, known address), they should apply

REQUIRETLS to all reoriginated messages that do not contain the "TLS-

Required: No" header field.

7. IANA Considerations

Per this document, IANA has added the following keyword to the "SMTP

Service Extensions" subregistry of the "Mail Parameters" registry

[MailParams]:

EHLO Keyword: REQUIRETLS

Description: Require TLS

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

Per this document, IANA has added an entry to the "Enumerated Status

Codes" subregistry of the "Simple Mail Transfer Protocol (SMTP)

Enhanced Status Codes Registry" [SMTPStatusCodes]:

Code: X.7.30

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: RFC 8689

Submitter: J. Fenton

Change Controller: IESG

Per this document, IANA has added an entry to the "Permanent Message

Header Field Names" subregistry of the "Message Headers" registry

[MessageHeaders] as follows:

Header field name: TLS-Required

Applicable protocol: mail

Status: standard

Author/change controller: IETF

Specification document: RFC 8689

8. Security Considerations

The purpose of REQUIRETLS is to give the originator of a message

control over the security of email they send, either by conveying an

expectation that it will be transmitted in an encrypted form over the

wire or explicitly indicating that transport encryption is not

required if it cannot be successfully negotiated.

The following considerations apply to the REQUIRETLS service

extension but not the TLS-Required header field, since messages

specifying the header field are less concerned with transport

security.

8.1. Passive Attacks

REQUIRETLS is generally effective against passive attackers who are

merely trying to eavesdrop on an SMTP exchange between an SMTP client

and server. This assumes, of course, the cryptographic integrity of

the TLS connection being used.

8.2. Active Attacks

Active attacks against TLS-encrypted SMTP connections can take many

forms. One such attack is to interfere in the negotiation by

changing the STARTTLS command to something illegal such as XXXXXXXX.

This causes TLS negotiation to fail and messages to be sent in the

clear, where they can be intercepted. REQUIRETLS detects the failure

of STARTTLS and declines to send the message rather than send it

insecurely.

A second form of attack is a man-in-the-middle attack where the

attacker terminates the TLS connection rather than the intended SMTP

server. This is possible when, as is commonly the case, the SMTP

client either does not verify the server's certificate or establishes

the connection even when the verification fails. REQUIRETLS requires

successful certificate validation before sending the message.

Another active attack involves the spoofing of DNS MX records of the

recipient domain. An attacker with this capability could potentially

cause the message to be redirected to a mail server under the

attacker's own control, which would presumably have a valid

certificate. REQUIRETLS requires that the recipient domain's MX

record lookup be validated either using DNSSEC or via a published

MTA-STS policy that specifies the acceptable SMTP server hostname(s)

for the recipient domain.

8.3. Bad-Actor MTAs

A bad-actor MTA along the message transmission path could

misrepresent its support of REQUIRETLS and/or actively strip

REQUIRETLS tags from messages it handles. However, since

intermediate MTAs are already trusted with the cleartext of messages

they handle, and are not part of the threat model for transport-layer

security, they are also not part of the threat model for REQUIRETLS.

It should be reemphasized that since SMTP TLS is a transport-layer

security protocol, messages sent using REQUIRETLS are not encrypted

end-to-end and are visible to MTAs that are part of the message

delivery path. Messages containing sensitive information that MTAs

should not have access to MUST be sent using end-to-end content

encryption such as OpenPGP [RFC4880] or S/MIME [RFC8551].

8.4. Policy Conflicts

In some cases, the use of the TLS-Required header field may conflict

with a recipient domain policy expressed through the DANE [RFC7672]

or MTA-STS [RFC8461] protocols. Although these protocols encourage

the use of TLS transport by advertising the availability of TLS, the

use of the "TLS-Required: No" header field represents an 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 ability to require TLS by not accepting messages when

STARTTLS has not been negotiated; otherwise, "TLS-Required: No" is

effectively directing the client MTA to behave as if it does not

support DANE or MTA-STS.

9. References

9.1. Normative References

[MailParams]

IANA, "Mail Parameters",

<http://www.iana.org/assignments/mail-parameters>.

[MessageHeaders]

IANA, "Permanent Message Header Field Names",

<https://www.iana.org/assignments/message-headers>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC 2119,

DOI 10.17487/RFC2119, March 1997,

<https://www.rfc-editor.org/info/rfc2119>.

[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over

Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,

February 2002, <https://www.rfc-editor.org/info/rfc3207>.

[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service

Extension for Delivery Status Notifications (DSNs)",

RFC 3461, DOI 10.17487/RFC3461, January 2003,

<https://www.rfc-editor.org/info/rfc3461>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.

Rose, "DNS Security Introduction and Requirements",

RFC 4033, DOI 10.17487/RFC4033, March 2005,

<https://www.rfc-editor.org/info/rfc4033>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.

Rose, "Resource Records for the DNS Security Extensions",

RFC 4034, DOI 10.17487/RFC4034, March 2005,

<https://www.rfc-editor.org/info/rfc4034>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.

Rose, "Protocol Modifications for the DNS Security

Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

<https://www.rfc-editor.org/info/rfc4035>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax

Specifications: ABNF", STD 68, RFC 5234,

DOI 10.17487/RFC5234, January 2008,

<https://www.rfc-editor.org/info/rfc5234>.

[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced

Mail System Status Codes", BCP 138, RFC 5248,

DOI 10.17487/RFC5248, June 2008,

<https://www.rfc-editor.org/info/rfc5248>.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,

DOI 10.17487/RFC5321, October 2008,

<https://www.rfc-editor.org/info/rfc5321>.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,

DOI 10.17487/RFC5322, October 2008,

<https://www.rfc-editor.org/info/rfc5322>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and

Verification of Domain-Based Application Service Identity

within Internet Public Key Infrastructure Using X.509

(PKIX) Certificates in the Context of Transport Layer

Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March

2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,

"Recommendations for Secure Use of Transport Layer

Security (TLS) and Datagram Transport Layer Security

(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May

2015, <https://www.rfc-editor.org/info/rfc7525>.

[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via

Opportunistic DNS-Based Authentication of Named Entities

(DANE) Transport Layer Security (TLS)", RFC 7672,

DOI 10.17487/RFC7672, October 2015,

<https://www.rfc-editor.org/info/rfc7672>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC

2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,

May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:

Use of Transport Layer Security (TLS) for Email Submission

and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,

<https://www.rfc-editor.org/info/rfc8314>.

[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,

and J. Jones, "SMTP MTA Strict Transport Security (MTA-

STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,

<https://www.rfc-editor.org/info/rfc8461>.

[SMTPStatusCodes]

IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced

Status Codes Registry", <https://www.iana.org/assignments/

smtp-enhanced-status-codes>.

9.2. Informative References

[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",

STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,

<https://www.rfc-editor.org/info/rfc1939>.

[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,

DOI 10.17487/RFC2033, October 1996,

<https://www.rfc-editor.org/info/rfc2033>.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION

4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,

<https://www.rfc-editor.org/info/rfc3501>.

[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.

Thayer, "OpenPGP Message Format", RFC 4880,

DOI 10.17487/RFC4880, November 2007,

<https://www.rfc-editor.org/info/rfc4880>.

[RFC5228] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email

Filtering Language", RFC 5228, DOI 10.17487/RFC5228,

January 2008, <https://www.rfc-editor.org/info/rfc5228>.

[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,

DOI 10.17487/RFC5598, July 2009,

<https://www.rfc-editor.org/info/rfc5598>.

[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",

STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,

<https://www.rfc-editor.org/info/rfc6409>.

[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/

Multipurpose Internet Mail Extensions (S/MIME) Version 4.0

Message Specification", RFC 8551, DOI 10.17487/RFC8551,

April 2019, <https://www.rfc-editor.org/info/rfc8551>.

Appendix A. Examples

This section is informative.

A.1. REQUIRETLS SMTP Option

The TLS-Required SMTP option is used to express the intention of 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.

S: 220 mail.example.net ESMTP

C: EHLO mail.example.org

S: 250-mail.example.net Hello example.org [192.0.2.1]

S: 250-SIZE 52428800

S: 250-8BITMIME

S: 250-PIPELINING

S: 250-STARTTLS

S: 250 HELP

C: STARTTLS

S: TLS go ahead

(at this point TLS negotiation takes place. The remainder of this

session occurs within TLS.)

S: 220 mail.example.net ESMTP

C: EHLO mail.example.org

S: 250-mail.example.net Hello example.org [192.0.2.1]

S: 250-SIZE 52428800

S: 250-8BITMIME

S: 250-PIPELINING

S: 250-REQUIRETLS

S: 250 HELP

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

S: 250 OK

C: RCPT TO:<editor@example.net>

S: 250 Accepted

C: DATA

S: 354 Enter message, ending with "." on a line by itself

(message follows)

C: .

S: 250 OK

C: QUIT

A.2. TLS-Required Header Field

The TLS-Required header field is used when the sender requests that

the mail system not heed a default policy of the recipient domain

requiring TLS. It might be used, for example, to allow problems with

the recipient domain's TLS certificate to be reported:

From: Roger Reporter <roger@example.org>

To: Andy Admin <admin@example.com>

Subject: Certificate problem?

TLS-Required: No

Date: Fri, 18 Jan 2019 10:26:55 -0800

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

Andy, there seems to be a problem with the TLS certificate

on your mail server. Are you aware of this?

Roger

Acknowledgements

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.

Author's Address

Jim Fenton

Altmode Networks

Los Altos, California 94024

United States of America

Email: fenton@bluepopcorn.net

Kevin Schoon 7 months ago