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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force J. Fenton 3 Internet-Draft Altmode Networks 4 Intended status: Standards Track January 16, 2018 5 Expires: July 20, 2018 7 SMTP Require TLS Option 8 draft-ietf-uta-smtp-require-tls-01 10 Abstract 12 The SMTP STARTTLS option, used in negotiating transport-level 13 encryption of SMTP connections, is not as useful from a security 14 standpoint as it might be because of its opportunistic nature; 15 message delivery is, by default, prioritized over security. This 16 document describes an SMTP service extension, REQUIRETLS, and message 17 header field, Require-TLS. If the REQUIRETLS option or Require-TLS 18 message header field is used when sending a message, it asserts a 19 request on the part of the message sender to override the default 20 negotiation of TLS, either by requiring that TLS be negotiated when 21 the message is relayed, or by requesting that recipient-side policy 22 mechanisms such as MTA-STS and DANE be ignored when relaying a 23 message for which security is unimportant. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at https://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on July 20, 2018. 42 Copyright Notice 44 Copyright (c) 2018 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (https://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 60 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 61 2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 3 62 3. The Require-TLS Header Field . . . . . . . . . . . . . . . . 5 63 4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5 64 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5 65 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 6 66 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 6 67 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 7 68 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7 69 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 8 70 5. Non-delivery message handling . . . . . . . . . . . . . . . . 8 71 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 72 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 73 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 74 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10 75 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 76 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 77 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 78 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 79 10.1. Changes since -00 Draft . . . . . . . . . . . . . . . . 11 80 10.2. Changes since fenton-03 Draft . . . . . . . . . . . . . 11 81 10.3. Changes Since -02 Draft . . . . . . . . . . . . . . . . 11 82 10.4. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 83 10.5. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12 84 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 85 11.1. Normative References . . . . . . . . . . . . . . . . . . 12 86 11.2. Informative References . . . . . . . . . . . . . . . . . 14 87 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 89 1. Introduction 91 The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a 92 means by which an SMTP server and client can establish a Transport 93 Layer Security (TLS) protected session for the transmission of email 94 messages. By default, TLS is used only upon mutual agreement 95 (successful negotiation) of STARTTLS between the client and server; 96 if this is not possible, the message is sent without transport 97 encryption. Furthermore, it is common practice for the client to 98 negotiate TLS even if the SMTP server's certificate is invalid. 100 Policy mechanisms such as DANE [RFC7672] and MTA-STS 101 [I-D.ietf-uta-mta-sts] may impose requirements for the use of TLS for 102 email destined for some domains. However, such policies do not allow 103 the sender to specify which messages are more sensitive and require 104 transport-level encryption, and which ones are less sensitive and 105 ought to be relayed even if TLS cannot be negotiated successfully. 107 The default opportunistic nature of SMTP TLS enables several "on the 108 wire" attacks on SMTP security between MTAs. These include passive 109 eavesdropping on connections for which TLS is not used, interference 110 in the SMTP protocol to prevent TLS from being negotiated (presumably 111 accompanied by eavesdropping), and insertion of a man-in-the-middle 112 attacker exploiting the lack of server authentication by the client. 113 Attacks are described in more detail in the Security Considerations 114 section of this document. 116 REQUIRETLS consists of two mechanisms: an SMTP service extension and 117 a message header field. The service extension is used to specify 118 that a given message sent during a particular session MUST be sent 119 over a TLS-protected session with specified security characteristics. 120 It also requires that the SMTP server advertise that it supports 121 REQUIRETLS, in effect promising that it will honor the requirement to 122 enforce TLS transmission and REQUIRETLS support for onward 123 transmission of those messages. 125 The Require-TLS message header field is used to convey a request to 126 ignore recipient-side policy mechanisms such as MTA-STS and DANE, 127 thereby prioritizing delivery over ability to negotiate TLS. Unlike 128 the service extension, the Require-TLS header field allows the 129 message to transit through one or more MTAs that do not support 130 REQUIRETLS. 132 1.1. Requirements Language 134 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 135 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 136 "OPTIONAL" in this document are to be interpreted as described in BCP 137 14 [RFC2119] [RFC8174] when, and only when, they appear in all 138 capitals, as shown here. 140 2. The REQUIRETLS Service Extension 142 1. The textual name of the extension is "Require TLS". 144 2. The EHLO keyword value associated with this extension is 145 "REQUIRETLS". 147 3. One MAIL FROM option is defined by this extension. 149 4. Two new SMTP status codes are defined by this extension to convey 150 error conditions resulting from failure of the client to 151 negotiate a TLS connection with the required security and as a 152 result of an attempt to send to a server not also supporting the 153 REQUIRETLS extension. 155 In order to specify REQUIRETLS treatment for a given message, the 156 REQUIRETLS option is specified on the MAIL FROM command when that 157 message is transmitted. This option MUST only be specified in the 158 context of an SMTP session meeting the security requirements that 159 have been specified: 161 o The session itself MUST employ TLS transmission. 163 o Any server authentication requirements included as an option to 164 the REQUIRETLS option (see below) MUST have been satisfied in 165 establishing the current session. 167 o Following the negotiation of STARTTLS, the SMTP server MUST 168 advertise in the subsequent EHLO response that it supports 169 REQUIRETLS. 171 An optional parameter to the REQUIRETLS MAIL FROM option specifies 172 the requirements for server authentication that MUST be used for any 173 onward transmission of the following message. The parameter takes 174 the form of either a single value or comma-separated list, separated 175 from the REQUIRETLS option by a single "=" (equals-sign) character. 176 If present, the parameter MUST take one or more of the following 177 values: 179 o CHAIN - The certificate presented by the SMTP server MUST verify 180 successfully in a trust chain leading to a certificate trusted by 181 the SMTP client. The choice of trusted (root) certificates by the 182 client is at their own discretion. 184 o DANE - The certificate presented by the SMTP server MUST verify 185 succesfully using DANE as specified in RFC 7672 [RFC7672]. 187 o DNSSEC - The server MUST confirm that any MX record or CNAME 188 lookup used to locate the SMTP server must be DNSSEC [RFC4035] 189 signed and valid. 191 The CHAIN and DANE parameters are additive; if both are specified, 192 either method of certificate validation is acceptable. If neither 193 CHAIN nor DANE is specified, the certificate presented by the SMTP 194 server is not required to be verified. 196 3. The Require-TLS Header Field 198 One new message header field, Require-TLS, is defined by this 199 specification. It is used for messages requesting that recipient TLS 200 policy (MTA-STS [I-D.ietf-uta-mta-sts] or DANE [RFC7672]) be ignored. 202 The Require-TLS header field has a single required parameter: 204 o NO - The SMTP client SHOULD attempt to send the message regardless 205 of its ability to negotiate STARTTLS with the SMTP server, 206 ignoring policy-based mechanisms, if any, asserted by the 207 recipient domain. Nevertheless, the client MAY negotiate STARTTLS 208 with the server if available. 210 More than one instance of the Require-TLS header field MUST NOT 211 appear in a given message. 213 4. REQUIRETLS Semantics 215 4.1. REQUIRETLS Receipt Requirements 217 Upon receipt of the REQUIRETLS option on a MAIL FROM command during 218 the receipt of a message, an SMTP server MUST tag that message as 219 needing REQUIRETLS handling with the option(s) specified in the 220 REQUIRETLS parameter. 222 Upon receipt of a message not specifying the REQUIRETLS option on its 223 MAIL FROM command but containing the Require-TLS header field in its 224 message header, an SMTP server implementing this specification MUST 225 tag that message with the option specified in the Require-TLS header 226 field. If the REQUIRETLS MAIL FROM parameter is specified, the 227 Require-TLS header field MUST be ignored but MAY be included in 228 onward relay of the message. 230 The manner in which the above tagging takes place is implementation- 231 dependent. If the message is being locally aliased and redistributed 232 to multiple addresses, all instances of the message MUST be tagged in 233 the same manner. 235 4.2. REQUIRETLS Sender Requirements 237 4.2.1. Sending with TLS Required 239 When sending a message tagged as requiring TLS, the sending (client) 240 MTA MUST: 242 1. Look up the SMTP server to which the message is to be sent as 243 described in [RFC5321] Section 5.1. If the DNSSEC option is 244 included in the message tag, the MX record lookups in this 245 process MUST use DNSSEC verification and the response(s) MUST be 246 DNSSEC-signed in order to ensure the integrity of the resource 247 identifier [RFC6125] used to authenticate the SMTP server. 249 2. Open an SMTP session with the peer SMTP server using the EHLO 250 verb. The server MUST advertise the REQUIRETLS capability. 252 3. Establish a TLS-protected SMTP session with its peer SMTP server 253 and authenticate the server's certificate with the specified 254 authentication method as specified in [RFC6125] or [RFC6698] as 255 applicable. 257 4. The SMTP client SHOULD also require that meaningfully secure 258 cipher algorithms and key lengths be negotiated with the server. 259 The choices of key lengths and algorithms change over time, so a 260 specific requirement is not presented here. 262 If any of the above steps fail, the client MUST issue a QUIT to the 263 server and repeat steps 2-4 with each host on the recipient domain's 264 list of MX hosts in an attempt to find a mail path that meets the 265 sender's requirements. The client MAY send other, unprotected, 266 messages to that server if it has any prior to issuing the QUIT. If 267 there are no more MX hosts or if the MX record lookup is not DNSSEC- 268 protected and DNSSEC verification is required, the client MUST NOT 269 transmit the message to the domain. 271 Following such a failure, the SMTP client MUST send a non-delivery 272 notification to the reverse-path of the failed message as described 273 in section 3.6 of [RFC5321]. The following status codes [RFC5248] 274 SHOULD be used: 276 o DNSSEC lookup failure: 5.x.x DNSSEC lookup required 278 o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed 280 o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption 281 needed 283 Refer to Section 5 for further requirements regarding non-delivery 284 messages. 286 If all REQUIRETLS requirements have been met, transmit the message, 287 issuing the REQUIRETLS option on the MAIL FROM command with the 288 required option(s), if any. 290 4.2.2. Sending with TLS Optional 292 Messages tagged RequireTLS: NO are handled as follows. When sending 293 such a message, the sending (client) MTA MUST: 295 o Look up the SMTP server to which the message is to be sent as 296 described in [RFC5321] Section 5.1. 298 o Open an SMTP session with the peer SMTP server using the EHLO 299 verb. Attempt to negotiate STARTTLS if possible, and follow any 300 policy published by the recipient domain, but do not fail if this 301 is unsuccessful. 303 Some SMTP servers may be configured to require STARTTLS connections 304 as a matter of policy and not accept messages in the absence of 305 STARTTLS. This MUST be expected, and a non-delivery notification 306 returned to the sender. 308 Since messages tagged with RequireTLS: NO will sometimes be sent to 309 SMTP servers not supporting REQUIRETLS, that option will not be 310 uniformly observed by all SMTP relay hops. 312 4.3. REQUIRETLS Submission 314 An MUA or other agent making the initial introduction of a message 315 has authority to decide whether to require TLS, and if so, using what 316 authentication method(s). When TLS is to be required, it MUST do so 317 by negotiating STARTTLS and REQUIRETLS and include the REQUIRETLS 318 option on the MAIL FROM command, as is done for message relay. 320 When TLS is not to be required, the sender MUST include the Require- 321 TLS header field in the message. SMTP servers implementing this 322 specification will interpret this header field as described in 323 Section 4.1. 325 In either case, the decision whether to specify REQUIRETLS, and with 326 what option(s), MAY be done based on a user interface selection or 327 based on a ruleset or other policy. The manner in which the decision 328 to require TLS is made is implementation-dependent and is beyond the 329 scope of this specification. 331 4.4. Delivery of REQUIRETLS messages 333 Messages are usually retrieved by end users using protocols other 334 than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems. 335 Mail delivery agents supporting REQUIRETLS SHOULD observe the 336 guidelines in [I-D.ietf-uta-email-deep]. 338 5. Non-delivery message handling 340 Non-delivery ("bounce") messages usually contain important metadata 341 about the message to which they refer, including the original message 342 header. They therefore MUST be protected in the same manner as the 343 original message. All non-delivery messages, whether resulting from 344 a REQUIRETLS error or some other, MUST employ REQUIRETLS using the 345 same authentication method(s) as the message that caused the error to 346 occur. 348 The path from the origination of an error bounce message back to the 349 MAIL FROM address may not share the same REQUIRETLS support as the 350 forward path. Therefore, users requiring TLS are advised to make 351 sure that they are capable of receiving mail using REQUIRETLS at the 352 same authentication method(s) as messages they send. Otherwise, such 353 non-delivery messages will be lost. 355 If unable to send a bounce message due to a REQUIRETLS failure (the 356 return path not supporting the TLS requirements in the original 357 message), the MTA sending the bounce message MAY send a redacted non- 358 delivery message to the postmaster of the domain identified in the 359 envelope-From address identifying the message only by Message-ID and 360 indicating the type of failure. The original From, Return-path, To, 361 Sender, Cc, and related header fields MUST NOT be included in this 362 message. 364 Senders of messages requiring TLS are advised to consider the 365 increased likelihood that bounce messages will be lost as a result of 366 REQUIRETLS return path failure. 368 6. Mailing list considerations 370 Mailing lists, upon receipt of a message, originate new messages to 371 list addresses, as distinct from an aliasing operation that redirects 372 the original message, in some cases to multiple recipients. The 373 requirement to preserve the REQUIRETLS tag and options therefore does 374 not necessarily extend to mailing lists, although the inclusion of 375 the Require-TLS header field MAY cause messages sent to mailing lists 376 to inherit this characteristic. REQUIRETLS users SHOULD be made 377 aware of this limitation so that they use caution when sending to 378 mailing lists and do not assume that REQUIRETLS applies to messages 379 from the list operator to list members. 381 Mailing list operators MAY apply REQUIRETLS requirements in incoming 382 messages to the resulting messages they originate. If this is done, 383 they SHOULD also apply these requirements to administrative traffic, 384 such as messages to moderators requesting approval of messages. 386 7. IANA Considerations 388 If published as an RFC, this draft requests the addition of the 389 keyword REQUIRETLS to the SMTP Service Extensions Registry 390 [MailParams]. 392 If published as an RFC, this draft also requests the creation of a 393 registry, REQUIRETLS Security Requirements, to be initially populated 394 with the CHAIN, DANE, DNSSEC, and NO keywords. 396 If published as an RFC, this draft requests the addition of an entry 397 to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 398 Registry [SMTPStatusCodes] in the 5.7.YYY range to indicate lack of 399 REQUIRETLS support by an SMTP server to which a message is being 400 routed. 402 If published as an RFC, this draft requests the addition of the 403 header field name Require-TLS to the Permanent Message Header Field 404 Names Registry [PermMessageHeaderFields]. 406 This section is to be removed during conversion into an RFC by the 407 RFC Editor. 409 8. Security Considerations 411 The purpose of REQUIRETLS is to improve communications security for 412 email by giving the originator of a message an expectation that it 413 will be transmitted in an encrypted form "over the wire". When used, 414 REQUIRETLS changes the traditional behavior of email transmission, 415 which favors delivery over the ability to send email messages using 416 transport-layer security, to one in which requested security takes 417 precedence over delivery and domain-level policy. 419 The following considerations apply to the REQUIRETLS service 420 extension but not the RequireTLS header field, since messages 421 specifying the header field are less concerned with transport 422 security. 424 8.1. Passive attacks 426 REQUIRETLS is generally effective against passive attackers who are 427 merely trying to eavesdrop on an SMTP exchange between an SMTP client 428 and server. This assumes, of course, the cryptographic integrity of 429 the TLS connection being used. 431 8.2. Active attacks 433 Active attacks against TLS encrypted SMTP connections can take many 434 forms. One such attack is to interfere in the negotiation by 435 changing the STARTTLS command to something illegal such as XXXXXXXX. 436 This causes TLS negotiation to fail and messages to be sent in the 437 clear, where they can be intercepted. REQUIRETLS detects the failure 438 of STARTTLS and declines to send the message rather than send it 439 insecurely. 441 A second form of attack is a man-in-the-middle attack where the 442 attacker terminates the TLS connection rather than the intended SMTP 443 server. This is possible when, as is commonly the case, the SMTP 444 client either does not verify the server's certificate or establishes 445 the connection even when the verification fails. The REQUIRETLS 446 CHAIN and DANE options allow the message sender to specify that 447 successful certificate validation, using either or both of two 448 different methods, is required before sending the message. 450 Another active attack involves the spoofing of DNS MX records of the 451 recipient domain. An attacker having this capability could cause the 452 message to be redirected to a mail server under the attacker's own 453 control, which would presumably have a valid certificate. The 454 REQUIRETLS DNSSEC option allows the message sender to require that 455 valid DNSSEC [RFC4033] signatures be obtained when locating the 456 recipient's mail server, in order to address that attack. 458 In addition to support of the DNSSEC option, domains receiving email 459 SHOULD deploy DNSSEC and SMTP clients SHOULD deploy DNSSEC 460 verification. 462 8.3. Bad Actor MTAs 464 A bad-actor MTA along the message transmission path could 465 misrepresent its support of REQUIRETLS and/or actively strip 466 REQUIRETLS tags from messages it handles. However, since 467 intermediate MTAs are already trusted with the cleartext of messages 468 they handle, and are not part of the threat model for transport-layer 469 security, they are also not part of the threat model for REQUIRETLS. 471 It should be reemphasized that since SMTP TLS is a transport-layer 472 security protocol, messages sent using REQUIRETLS are not encrypted 473 end-to-end and are visible to MTAs that are part of the message 474 delivery path. Messages containing sensitive information that MTAs 475 should not have access to MUST be sent using end-to-end content 476 encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751]. 478 9. Acknowledgements 480 The author would like to acknowledge many helpful suggestions on the 481 ietf-smtp and uta mailing lists, in particular those of Viktor 482 Dukhovni, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin, 483 John Levine, Rolf Sonneveld, and Per Thorsheim. 485 10. Revision History 487 To be removed by RFC Editor upon publication as an RFC. 489 10.1. Changes since -00 Draft 491 o Created new header field, Require-TLS, for use by "NO" option. 493 o Removed "NO" option from SMTP service extension. 495 o Recommend DEEP requirements for delivery of messages requiring 496 TLS. 498 o Assorted copy edits 500 10.2. Changes since fenton-03 Draft 502 o Wording improvements from Rolf Sonneveld review 22 July 2017 504 o A few copy edits 506 o Conversion from individual to UTA WG draft 508 10.3. Changes Since -02 Draft 510 o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per 511 suggestion on UTA WG mailing list. 513 o Additional guidance on bounce messages 515 10.4. Changes Since -01 Draft 517 o Specified retries when multiple MX hosts exist for a given domain. 519 o Clarified generation of non-delivery messages 521 o Specified requirements for application of REQUIRETLS to mail 522 forwarders and mailing lists. 524 o Clarified DNSSEC requirements to include MX lookup only. 526 o Corrected terminology regarding message retrieval vs. delivery. 528 o Changed category to standards track. 530 10.5. Changes Since -00 Draft 532 o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM 533 parameter to better associate REQUIRETLS requirements with 534 transmission of individual messages. 536 o Addition of an option to require DNSSEC lookup of the remote mail 537 server, since this affects the common name of the certificate that 538 is presented. 540 o Clarified the wording to more clearly state that TLS sessions must 541 be established and not simply that STARTTLS is negotiated. 543 o Introduced need for minimum encryption standards (key lengths and 544 algorithms) 546 o Substantially rewritten Security Considerations section 548 11. References 550 11.1. Normative References 552 [I-D.ietf-uta-email-deep] 553 Moore, K. and C. Newman, "Cleartext Considered Obsolete: 554 Use of TLS for Email Submission and Access", draft-ietf- 555 uta-email-deep-12 (work in progress), December 2017. 557 [MailParams] 558 Internet Assigned Numbers Authority (IANA), "IANA Mail 559 Parameters", 2007, 560 . 562 [PermMessageHeaderFields] 563 Internet Assigned Numbers Authority (IANA), "Permanent 564 Message Header Field Names Registry", 2004, 565 . 568 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 569 Requirement Levels", BCP 14, RFC 2119, 570 DOI 10.17487/RFC2119, March 1997, 571 . 573 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 574 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 575 February 2002, . 577 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 578 Rose, "Protocol Modifications for the DNS Security 579 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 580 . 582 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 583 Mail System Status Codes", BCP 138, RFC 5248, 584 DOI 10.17487/RFC5248, June 2008, 585 . 587 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 588 DOI 10.17487/RFC5321, October 2008, 589 . 591 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 592 Verification of Domain-Based Application Service Identity 593 within Internet Public Key Infrastructure Using X.509 594 (PKIX) Certificates in the Context of Transport Layer 595 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 596 2011, . 598 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 599 of Named Entities (DANE) Transport Layer Security (TLS) 600 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 601 2012, . 603 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 604 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 605 May 2017, . 607 [SMTPStatusCodes] 608 Internet Assigned Numbers Authority (IANA), "Simple Mail 609 Transfer Protocol (SMTP) Enhanced Status Codes Registry", 610 2008, . 613 11.2. Informative References 615 [I-D.ietf-uta-mta-sts] 616 Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 617 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 618 STS)", draft-ietf-uta-mta-sts-14 (work in progress), 619 January 2018. 621 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 622 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 623 . 625 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 626 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 627 . 629 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 630 Rose, "DNS Security Introduction and Requirements", 631 RFC 4033, DOI 10.17487/RFC4033, March 2005, 632 . 634 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 635 Thayer, "OpenPGP Message Format", RFC 4880, 636 DOI 10.17487/RFC4880, November 2007, 637 . 639 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 640 Mail Extensions (S/MIME) Version 3.2 Message 641 Specification", RFC 5751, DOI 10.17487/RFC5751, January 642 2010, . 644 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 645 Opportunistic DNS-Based Authentication of Named Entities 646 (DANE) Transport Layer Security (TLS)", RFC 7672, 647 DOI 10.17487/RFC7672, October 2015, 648 . 650 Author's Address 651 Jim Fenton 652 Altmode Networks 653 Los Altos, California 94024 654 USA 656 Email: fenton@bluepopcorn.net