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Internet Engineering Task Force                         J. Fenton                              Altmode Networks Standards Track 2019                        SMTP Require TLS OptionAbstract   The SMTP STARTTLS option, used in negotiating transport-level   encryption of SMTP connections, is not as useful from a security   standpoint as it might be because of its opportunistic nature;   message delivery is, by default, prioritized over security.  This   document describes an SMTP service extension, REQUIRETLS, and   message header field, TLS-Required.  If the REQUIRETLS option or 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 or by requesting that   recipient-side policy mechanisms such as MTA-STS and be ignored when relaying a   message for which security is unimportant.Status of This Memo   This is of the Internet Engineering Task Force   (IETF). of for of   and may be atCopyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (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   6.  Reorigination   7.  IANA Considerations   8.  Security Considerations     8.1.  Passive     8.2.  Active     8.3. MTAs     8.4.  Policy Conflicts   9.  References  Normative References  Informative References   Appendix A.  Examples     A.1.  REQUIRETLS SMTP Option     A.2.  TLS-Required Header Field   Author's Address1.  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 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 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) including the core rules defined in Appendix B of that   document.2.  The REQUIRETLS Service Extension   1.  The textual name of the extension is "Require TLS".   2.  The EHLO keyword value associated with this extension is       "REQUIRETLS".   3.  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 to a server not also the REQUIRETLS       extension.   7.  The REQUIRETLS extension is valid for message relay [RFC5321],       submission [RFC6409], and the Local Mail Transfer Protocol (LMTP)   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 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 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 successfully a trust chain leading to a certificate      trusted by the SMTP or it MUST successfully      using 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 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 Section 3.6.8.   FWS = <as defined in RFC 5322>   CRLF = <as defined in RFC 5322>4.  REQUIRETLS Semantics4.1.  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   onward relay of the message.   The manner in which the above tagging takes place is   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 Requirements4.2.1.  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 as       described in 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 as described in 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 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,   messages to that server if it has any prior to issuing   the QUIT.  If there are no more MX hosts, the client MUST NOT   transmit the message to the domain.   Following such a failure, the SMTP client MUST send a non-delivery   notification to the reverse-path of the failed as described   in 3.6 of [RFC5321].  The following status codes [RFC5248]   SHOULD be used:  REQUIRETLS not supported by server: REQUIRETLS  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 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 as      described in 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 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 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 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 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 mail systems.   Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD   observe the guidelines in [RFC8314].5.  Non-delivery   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 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 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 as required by [RFC5321].  When the MAIL FROM 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 and that some information could be   leaked if a bounce message is not able to be transmitted with   REQUIRETLS.6.  Reorigination   In a number of situations, a mediator [RFC5598] originates a new   message as a result of an incoming message.  These situations   but are not limited 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 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 this the following keyword to the   Service   [MailParams]:      EHLO                 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 this an entry to the Mail Transfer Protocol (SMTP)   Enhanced Status Codes [SMTPStatusCodes]:      Code:      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:      Submitter:                    J.  Fenton      Change            IESG this an entry to the Message   Header Field      Header field name:            TLS-Required      Applicable protocol:          mail      Status:                       standard      Author/change controller:     IETF      Specification document:       RFC8.  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 the or explicitly 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   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   Active attacks against 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 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. 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 messages REQUIRETLS and to of to   with of of of the the of the the to TLS not   STARTTLS is  References  Normative References   [MailParams] Parameters",              <http://www.iana.org/assignments/mail-parameters>. "Permanent Message Header Field   [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] "Simple Mail Transfer Protocol (SMTP) Enhanced              Status Codes Registry",              smtp-enhanced-status-codes>.  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 of the   sender the associated message relayed using TLS.  In the   following example, lines beginning with are transmitted from the   SMTP client to the server, and lines beginning with 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: QUITA.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?    RogerAuthor's Address   Jim Fenton   Altmode Networks   Los Altos, California 94024   Email: fenton@bluepopcorn.net