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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 September 26, 2018 5 Expires: March 30, 2019 7 SMTP Require TLS Option 8 draft-ietf-uta-smtp-require-tls-04 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, RequireTLS. If the REQUIRETLS option or RequireTLS 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 March 30, 2019. 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 . . . . . . . . . . . . . . 4 62 3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4 63 4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5 64 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5 65 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5 66 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5 67 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6 68 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7 69 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7 70 5. Non-delivery message handling . . . . . . . . . . . . . . . . 7 71 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 72 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 73 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 74 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 9 75 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 76 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 77 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 78 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 79 10.1. Changes since -03 Draft . . . . . . . . . . . . . . . . 11 80 10.2. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 81 10.3. Changes since -01 Draft . . . . . . . . . . . . . . . . 11 82 10.4. Changes since -00 Draft . . . . . . . . . . . . . . . . 11 83 10.5. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 84 10.6. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 85 10.7. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 86 10.8. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12 87 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 88 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 89 11.2. Informative References . . . . . . . . . . . . . . . . . 14 90 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 92 1. Introduction 94 The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a 95 means by which an SMTP server and client can establish a Transport 96 Layer Security (TLS) protected session for the transmission of email 97 messages. By default, TLS is used only upon mutual agreement 98 (successful negotiation) of STARTTLS between the client and server; 99 if this is not possible, the message is sent without transport 100 encryption. Furthermore, it is common practice for the client to 101 negotiate TLS even if the SMTP server's certificate is invalid. 103 Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may 104 impose requirements for the use of TLS for email destined for some 105 domains. However, such policies do not allow the sender to specify 106 which messages are more sensitive and require transport-level 107 encryption, and which ones are less sensitive and ought to be relayed 108 even if TLS cannot be negotiated successfully. 110 The default opportunistic nature of SMTP TLS enables several "on the 111 wire" attacks on SMTP security between MTAs. These include passive 112 eavesdropping on connections for which TLS is not used, interference 113 in the SMTP protocol to prevent TLS from being negotiated (presumably 114 accompanied by eavesdropping), and insertion of a man-in-the-middle 115 attacker exploiting the lack of server authentication by the client. 116 Attacks are described in more detail in the Security Considerations 117 section of this document. 119 REQUIRETLS consists of two mechanisms: an SMTP service extension and 120 a message header field. The service extension is used to specify 121 that a given message sent during a particular session MUST be sent 122 over a TLS-protected session with specified security characteristics. 123 It also requires that the SMTP server advertise that it supports 124 REQUIRETLS, in effect promising that it will honor the requirement to 125 enforce TLS transmission and REQUIRETLS support for onward 126 transmission of those messages. 128 The RequireTLS message header field is used to convey a request to 129 ignore recipient-side policy mechanisms such as MTA-STS and DANE, 130 thereby prioritizing delivery over ability to negotiate TLS. Unlike 131 the service extension, the RequireTLS header field allows the message 132 to transit through one or more MTAs that do not support REQUIRETLS. 134 1.1. Requirements Language 136 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 137 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 138 "OPTIONAL" in this document are to be interpreted as described in BCP 139 14 [RFC2119] [RFC8174] when, and only when, they appear in all 140 capitals, as shown here. 142 2. The REQUIRETLS Service Extension 144 1. The textual name of the extension is "Require TLS". 146 2. The EHLO keyword value associated with this extension is 147 "REQUIRETLS". 149 3. One MAIL FROM option is defined by this extension. 151 4. One new SMTP status code is defined by this extension to convey 152 an error condition resulting from failure of the client to send 153 to a server not also supporting the 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 The certificate presented by the SMTP server MUST either verify 164 successfully in a trust chain leading to a certificate trusted by 165 the SMTP client or it MUST verify succesfully using DANE as 166 specified in RFC 7672 [RFC7672]. For trust chains, the choice of 167 trusted (root) certificates is at the discretion of the SMTP 168 client. 170 o Following the negotiation of STARTTLS, the SMTP server MUST 171 advertise in the subsequent EHLO response that it supports 172 REQUIRETLS. 174 3. The RequireTLS Header Field 176 One new message header field, RequireTLS, is defined by this 177 specification. It is used for messages requesting that recipient TLS 178 policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored. 180 The RequireTLS header field has a single REQUIRED parameter: 182 o NO - The SMTP client SHOULD attempt to send the message regardless 183 of its ability to negotiate STARTTLS with the SMTP server, 184 ignoring policy-based mechanisms (including MTA-STS and DANE), if 185 any, asserted by the recipient domain. Nevertheless, the client 186 SHOULD negotiate STARTTLS with the server if available. 188 More than one instance of the RequireTLS header field MUST NOT appear 189 in a given message. 191 4. REQUIRETLS Semantics 193 4.1. REQUIRETLS Receipt Requirements 195 Upon receipt of the REQUIRETLS option on a MAIL FROM command during 196 the receipt of a message, an SMTP server MUST tag that message as 197 needing REQUIRETLS handling. 199 Upon receipt of a message not specifying the REQUIRETLS option on its 200 MAIL FROM command but containing the RequireTLS header field in its 201 message header, an SMTP server implementing this specification MUST 202 tag that message with the option specified in the RequireTLS header 203 field. If the REQUIRETLS MAIL FROM parameter is specified, the 204 RequireTLS header field MUST be ignored but MAY be included in onward 205 relay of the message. 207 The manner in which the above tagging takes place is implementation- 208 dependent. If the message is being locally aliased and redistributed 209 to multiple addresses, all instances of the message MUST be tagged in 210 the same manner. 212 4.2. REQUIRETLS Sender Requirements 214 4.2.1. Sending with TLS Required 216 When sending a message tagged as requiring TLS, the sending (client) 217 MTA MUST: 219 1. Look up the SMTP server to which the message is to be sent as 220 described in [RFC5321] Section 5.1. 222 2. Open an SMTP session with the peer SMTP server using the EHLO 223 verb. 225 3. Establish a TLS-protected SMTP session with its peer SMTP server 226 and authenticate the server's certificate as specified in 227 [RFC6125] or [RFC7672] as applicable. 229 4. Ensure that the response to the subsequent EHLO following 230 establishment of the TLS protection advertises the REQUIRETLS 231 capability. 233 5. The SMTP client SHOULD also require that meaningfully secure 234 cipher algorithms and key lengths be negotiated with the server. 235 The choices of key lengths and algorithms change over time, so a 236 specific requirement is not presented here. 238 If any of the above steps fail, the client MUST issue a QUIT to the 239 server and repeat steps 2-4 with each host on the recipient domain's 240 list of MX hosts in an attempt to find a mail path that meets the 241 sender's requirements. The client MAY send other, unprotected, 242 messages to that server if it has any prior to issuing the QUIT. If 243 there are no more MX hosts, the client MUST NOT transmit the message 244 to the domain. 246 Following such a failure, the SMTP client MUST send a non-delivery 247 notification to the reverse-path of the failed message as described 248 in section 3.6 of [RFC5321]. The following status codes [RFC5248] 249 SHOULD be used: 251 o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed 253 o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption 254 needed 256 Refer to Section 5 for further requirements regarding non-delivery 257 messages. 259 If all REQUIRETLS requirements have been met, transmit the message, 260 issuing the REQUIRETLS option on the MAIL FROM command with the 261 required option(s), if any. 263 4.2.2. Sending with TLS Optional 265 Messages tagged RequireTLS: NO are handled as follows. When sending 266 such a message, the sending (client) MTA MUST: 268 o Look up the SMTP server to which the message is to be sent as 269 described in [RFC5321] Section 5.1. 271 o Open an SMTP session with the peer SMTP server using the EHLO 272 verb. Attempt to negotiate STARTTLS if possible, and follow any 273 policy published by the recipient domain, but do not fail if this 274 is unsuccessful. 276 Some SMTP servers may be configured to require STARTTLS connections 277 as a matter of policy and not accept messages in the absence of 278 STARTTLS. A non-delivery notification MUST be returned to the sender 279 if message relay fails due to an inability to negotiate STARTTLS when 280 required by the server. 282 Since messages tagged with RequireTLS: NO will sometimes be sent to 283 SMTP servers not supporting REQUIRETLS, that option will not be 284 uniformly observed by all SMTP relay hops. 286 4.3. REQUIRETLS Submission 288 An MUA or other agent making the initial introduction of a message 289 has authority to decide whether to require TLS. When TLS is to be 290 required, it MUST do so by negotiating STARTTLS and REQUIRETLS and 291 include the REQUIRETLS option on the MAIL FROM command, as is done 292 for message relay. 294 When TLS is not to be required, the sender MUST include the 295 RequireTLS header field in the message. SMTP servers implementing 296 this specification MUST interpret this header field as described in 297 Section 4.1. 299 In either case, the decision whether to specify REQUIRETLS MAY be 300 done based on a user interface selection or based on a ruleset or 301 other policy. The manner in which the decision to require TLS is 302 made is implementation-dependent and is beyond the scope of this 303 specification. 305 4.4. Delivery of REQUIRETLS messages 307 Messages are usually retrieved by end users using protocols other 308 than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems. 309 Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD 310 observe the guidelines in [RFC8314]. 312 5. Non-delivery message handling 314 Non-delivery ("bounce") messages usually contain important metadata 315 about the message to which they refer, including the original message 316 header. They therefore MUST be protected in the same manner as the 317 original message. All non-delivery messages resulting from messages 318 with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS 319 error or some other, MUST also specify the REQUIRETLS SMTP option 320 unless redacted as described below. 322 The path from the origination of an error bounce message back to the 323 MAIL FROM address may not share the same REQUIRETLS support as the 324 forward path. Therefore, users requiring TLS are advised to make 325 sure that they are capable of receiving mail using REQUIRETLS as 326 well. Otherwise, such non-delivery messages will be lost. 328 If a REQUIRETLS message is bounced, the server MUST behave as if 329 RET=HDRS was present as described in [RFC3461]. If both RET=FULL and 330 REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be 331 transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS 332 bounce message MUST use an empty MAIL FROM return-path as required by 333 [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS 334 parameter SHOULD NOT cause a bounce message to be discarded even if 335 the next-hop relay does not advertise REQUIRETLS. 337 Senders of messages requiring TLS are advised to consider the 338 possibility that bounce messages will be lost as a result of 339 REQUIRETLS return path failure, and that some information could be 340 leaked if a bounce message is not able to be transmitted with 341 REQUIRETLS. 343 6. Mailing list considerations 345 Mailing lists, upon receipt of a message, originate new messages to 346 list addresses. This is distinct from an aliasing operation that 347 redirects the original message, in some cases to multiple recipients. 348 The requirement to preserve the REQUIRETLS tag therefore does not 349 necessarily extend to mailing lists, although the inclusion of the 350 RequireTLS header field MAY cause messages sent to mailing lists to 351 inherit this characteristic. REQUIRETLS users SHOULD be made aware 352 of this limitation so that they use caution when sending to mailing 353 lists and do not assume that REQUIRETLS applies to messages from the 354 list operator to list members. 356 Mailing list operators MAY apply REQUIRETLS requirements in incoming 357 messages to the resulting messages they originate. If this is done, 358 they SHOULD also apply these requirements to administrative traffic, 359 such as messages to moderators requesting approval of messages. 361 7. IANA Considerations 363 If published as an RFC, this draft requests the addition of the 364 following keyword to the SMTP Service Extensions Registry 365 [MailParams]: 367 Textual name: RequireTLS 368 EHLO keyword value: REQUIRETLS 369 Syntax and parameters: (no parameters) 370 Additional SMTP verbs: none 371 MAIL and RCPT parameters: REQUIRETLS parameter on MAIL 372 Behavior: Use of the REQUIRETLS parameter on the 373 MAIL verb causes that message to require 374 the use of TLS and tagging with 375 REQUIRETLS for all onward relay. 376 Command length increment: 11 characters 378 If published as an RFC, this draft requests the addition of an entry 379 to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 380 Registry [SMTPStatusCodes]: 382 Code: 5.7.YYY 383 Sample Text: REQUIRETLS support required 384 Associated basic status code: 530 385 Description: This indicates that the message was not 386 able to be forwarded because it was 387 received with a REQUIRETLS requirement 388 and none of the SMTP servers to which 389 the message should be forwarded provide 390 this support. 391 Reference: (this document) 392 Submitter: J. Fenton 393 Change controller: IESG 395 If published as an RFC, this draft requests the addition of an entry 396 to the Permanent Message Header Field Names Registry 397 [PermMessageHeaderFields]: 399 Header field name: RequireTLS 400 Applicable protocol: mail 401 Status: provisional 402 Author/change controller: IETF UTA Working Group 403 Specification document: (this document) 405 This section is to be updated for publication by the RFC Editor. 407 8. Security Considerations 409 The purpose of REQUIRETLS is to improve communications security for 410 email by giving the originator of a message an expectation that it 411 will be transmitted in an encrypted form "over the wire". When used, 412 REQUIRETLS changes the traditional behavior of email transmission, 413 which favors delivery over the ability to send email messages using 414 transport-layer security, to one in which requested security takes 415 precedence over delivery and domain-level policy. 417 The following considerations apply to the REQUIRETLS service 418 extension but not the RequireTLS header field, since messages 419 specifying the header field are less concerned with transport 420 security. 422 8.1. Passive attacks 424 REQUIRETLS is generally effective against passive attackers who are 425 merely trying to eavesdrop on an SMTP exchange between an SMTP client 426 and server. This assumes, of course, the cryptographic integrity of 427 the TLS connection being used. 429 8.2. Active attacks 431 Active attacks against TLS encrypted SMTP connections can take many 432 forms. One such attack is to interfere in the negotiation by 433 changing the STARTTLS command to something illegal such as XXXXXXXX. 434 This causes TLS negotiation to fail and messages to be sent in the 435 clear, where they can be intercepted. REQUIRETLS detects the failure 436 of STARTTLS and declines to send the message rather than send it 437 insecurely. 439 A second form of attack is a man-in-the-middle attack where the 440 attacker terminates the TLS connection rather than the intended SMTP 441 server. This is possible when, as is commonly the case, the SMTP 442 client either does not verify the server's certificate or establishes 443 the connection even when the verification fails. REQUIRETLS requires 444 successful certificate validation before sending the message. 446 Another active attack involves the spoofing of DNS MX records of the 447 recipient domain. An attacker having this capability could cause the 448 message to be redirected to a mail server under the attacker's own 449 control, which would presumably have a valid certificate. REQUIRETLS 450 does not address this attack. 452 8.3. Bad Actor MTAs 454 A bad-actor MTA along the message transmission path could 455 misrepresent its support of REQUIRETLS and/or actively strip 456 REQUIRETLS tags from messages it handles. However, since 457 intermediate MTAs are already trusted with the cleartext of messages 458 they handle, and are not part of the threat model for transport-layer 459 security, they are also not part of the threat model for REQUIRETLS. 461 It should be reemphasized that since SMTP TLS is a transport-layer 462 security protocol, messages sent using REQUIRETLS are not encrypted 463 end-to-end and are visible to MTAs that are part of the message 464 delivery path. Messages containing sensitive information that MTAs 465 should not have access to MUST be sent using end-to-end content 466 encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751]. 468 9. Acknowledgements 470 The author would like to acknowledge many helpful suggestions on the 471 ietf-smtp and uta mailing lists, in particular those of Viktor 472 Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, 473 John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. 475 10. Revision History 477 To be removed by RFC Editor upon publication as an RFC. 479 10.1. Changes since -03 Draft 481 Working Group Last Call changes, including: 483 o Correct reference for SMTP DANE 485 o Clarify that RequireTLS: NO applies to both MTA-STS and DANE 486 policies 488 o Correct newly-defined status codes 490 o Update MTA-STS references to RFC 492 10.2. Changes since -02 Draft 494 o More complete documentation for IANA registration requests. 496 o Changed bounce handling to use RET parameters of RFC 3461, along 497 with slightly more liberal transmission of bounces even if 498 REQUIRETLS can't be negotiated. 500 10.3. Changes since -01 Draft 502 o Converted DEEP references to RFC 8314. 504 o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. 506 o Editorial corrections, notably making the header field name 507 consistent (RequireTLS rather than Require-TLS). 509 10.4. Changes since -00 Draft 511 o Created new header field, Require-TLS, for use by "NO" option. 513 o Removed "NO" option from SMTP service extension. 515 o Recommend DEEP requirements for delivery of messages requiring 516 TLS. 518 o Assorted copy edits 520 10.5. Changes since fenton-03 Draft 522 o Wording improvements from Rolf Sonneveld review 22 July 2017 524 o A few copy edits 526 o Conversion from individual to UTA WG draft 528 10.6. Changes Since -02 Draft 530 o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per 531 suggestion on UTA WG mailing list. 533 o Additional guidance on bounce messages 535 10.7. Changes Since -01 Draft 537 o Specified retries when multiple MX hosts exist for a given domain. 539 o Clarified generation of non-delivery messages 541 o Specified requirements for application of REQUIRETLS to mail 542 forwarders and mailing lists. 544 o Clarified DNSSEC requirements to include MX lookup only. 546 o Corrected terminology regarding message retrieval vs. delivery. 548 o Changed category to standards track. 550 10.8. Changes Since -00 Draft 552 o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM 553 parameter to better associate REQUIRETLS requirements with 554 transmission of individual messages. 556 o Addition of an option to require DNSSEC lookup of the remote mail 557 server, since this affects the common name of the certificate that 558 is presented. 560 o Clarified the wording to more clearly state that TLS sessions must 561 be established and not simply that STARTTLS is negotiated. 563 o Introduced need for minimum encryption standards (key lengths and 564 algorithms) 566 o Substantially rewritten Security Considerations section 568 11. References 570 11.1. Normative References 572 [MailParams] 573 Internet Assigned Numbers Authority (IANA), "IANA Mail 574 Parameters", 2007, 575 . 577 [PermMessageHeaderFields] 578 Internet Assigned Numbers Authority (IANA), "Permanent 579 Message Header Field Names Registry", 2004, 580 . 583 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 584 Requirement Levels", BCP 14, RFC 2119, 585 DOI 10.17487/RFC2119, March 1997, 586 . 588 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 589 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 590 February 2002, . 592 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 593 Extension for Delivery Status Notifications (DSNs)", 594 RFC 3461, DOI 10.17487/RFC3461, January 2003, 595 . 597 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 598 Mail System Status Codes", BCP 138, RFC 5248, 599 DOI 10.17487/RFC5248, June 2008, 600 . 602 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 603 DOI 10.17487/RFC5321, October 2008, 604 . 606 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 607 Verification of Domain-Based Application Service Identity 608 within Internet Public Key Infrastructure Using X.509 609 (PKIX) Certificates in the Context of Transport Layer 610 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 611 2011, . 613 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 614 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 615 May 2017, . 617 [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: 618 Use of Transport Layer Security (TLS) for Email Submission 619 and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, 620 . 622 [SMTPStatusCodes] 623 Internet Assigned Numbers Authority (IANA), "Simple Mail 624 Transfer Protocol (SMTP) Enhanced Status Codes Registry", 625 2008, . 628 11.2. Informative References 630 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 631 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 632 . 634 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 635 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 636 . 638 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 639 Thayer, "OpenPGP Message Format", RFC 4880, 640 DOI 10.17487/RFC4880, November 2007, 641 . 643 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 644 Mail Extensions (S/MIME) Version 3.2 Message 645 Specification", RFC 5751, DOI 10.17487/RFC5751, January 646 2010, . 648 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 649 Opportunistic DNS-Based Authentication of Named Entities 650 (DANE) Transport Layer Security (TLS)", RFC 7672, 651 DOI 10.17487/RFC7672, October 2015, 652 . 654 [RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 655 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 656 STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018, 657 . 659 Author's Address 660 Jim Fenton 661 Altmode Networks 662 Los Altos, California 94024 663 USA 665 Email: fenton@bluepopcorn.net