idnits 2.17.1 draft-ietf-uta-smtp-require-tls-03.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (June 22, 2018) is 2136 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'MailParams' -- Possible downref: Non-RFC (?) normative reference: ref. 'PermMessageHeaderFields' ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) -- Possible downref: Non-RFC (?) normative reference: ref. 'SMTPStatusCodes' -- Obsolete informational reference (is this intentional?): RFC 3501 (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 5751 (Obsoleted by RFC 8551) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 6 comments (--). 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 June 22, 2018 5 Expires: December 24, 2018 7 SMTP Require TLS Option 8 draft-ietf-uta-smtp-require-tls-03 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 December 24, 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 . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . 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 -02 Draft . . . . . . . . . . . . . . . . 11 80 10.2. Changes since -01 Draft . . . . . . . . . . . . . . . . 11 81 10.3. Changes since -00 Draft . . . . . . . . . . . . . . . . 11 82 10.4. Changes since fenton-03 Draft . . . . . . . . . . . . . 11 83 10.5. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 84 10.6. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 85 10.7. Changes Since -00 Draft . . . . . . . . . . . . . . . . 12 86 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 87 11.1. Normative References . . . . . . . . . . . . . . . . . . 12 88 11.2. Informative References . . . . . . . . . . . . . . . . . 14 89 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 91 1. Introduction 93 The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a 94 means by which an SMTP server and client can establish a Transport 95 Layer Security (TLS) protected session for the transmission of email 96 messages. By default, TLS is used only upon mutual agreement 97 (successful negotiation) of STARTTLS between the client and server; 98 if this is not possible, the message is sent without transport 99 encryption. Furthermore, it is common practice for the client to 100 negotiate TLS even if the SMTP server's certificate is invalid. 102 Policy mechanisms such as DANE [RFC7672] and MTA-STS 103 [I-D.ietf-uta-mta-sts] may impose requirements for the use of TLS for 104 email destined for some domains. However, such policies do not allow 105 the sender to specify which messages are more sensitive and require 106 transport-level encryption, and which ones are less sensitive and 107 ought to be relayed even if TLS cannot be negotiated successfully. 109 The default opportunistic nature of SMTP TLS enables several "on the 110 wire" attacks on SMTP security between MTAs. These include passive 111 eavesdropping on connections for which TLS is not used, interference 112 in the SMTP protocol to prevent TLS from being negotiated (presumably 113 accompanied by eavesdropping), and insertion of a man-in-the-middle 114 attacker exploiting the lack of server authentication by the client. 115 Attacks are described in more detail in the Security Considerations 116 section of this document. 118 REQUIRETLS consists of two mechanisms: an SMTP service extension and 119 a message header field. The service extension is used to specify 120 that a given message sent during a particular session MUST be sent 121 over a TLS-protected session with specified security characteristics. 122 It also requires that the SMTP server advertise that it supports 123 REQUIRETLS, in effect promising that it will honor the requirement to 124 enforce TLS transmission and REQUIRETLS support for onward 125 transmission of those messages. 127 The RequireTLS message header field is used to convey a request to 128 ignore recipient-side policy mechanisms such as MTA-STS and DANE, 129 thereby prioritizing delivery over ability to negotiate TLS. Unlike 130 the service extension, the RequireTLS header field allows the message 131 to transit through one or more MTAs that do not support REQUIRETLS. 133 1.1. Requirements Language 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 137 "OPTIONAL" in this document are to be interpreted as described in BCP 138 14 [RFC2119] [RFC8174] when, and only when, they appear in all 139 capitals, as shown here. 141 2. The REQUIRETLS Service Extension 143 1. The textual name of the extension is "Require TLS". 145 2. The EHLO keyword value associated with this extension is 146 "REQUIRETLS". 148 3. One MAIL FROM option is defined by this extension. 150 4. Two new SMTP status codes are defined by this extension to convey 151 error conditions resulting from failure of the client to 152 negotiate a TLS connection with the required security and as a 153 result of an attempt to send to a server not also supporting the 154 REQUIRETLS extension. 156 In order to specify REQUIRETLS treatment for a given message, the 157 REQUIRETLS option is specified on the MAIL FROM command when that 158 message is transmitted. This option MUST only be specified in the 159 context of an SMTP session meeting the security requirements that 160 have been specified: 162 o The session itself MUST employ TLS transmission. 164 o The certificate presented by the SMTP server MUST either verify 165 successfully in a trust chain leading to a certificate trusted by 166 the SMTP client or it MUST verify succesfully using DANE as 167 specified in RFC 7672 [RFC7672]. For trust chains, the choice of 168 trusted (root) certificates is at the discretion of the SMTP 169 client. 171 o Following the negotiation of STARTTLS, the SMTP server MUST 172 advertise in the subsequent EHLO response that it supports 173 REQUIRETLS. 175 3. The RequireTLS Header Field 177 One new message header field, RequireTLS, is defined by this 178 specification. It is used for messages requesting that recipient TLS 179 policy (MTA-STS [I-D.ietf-uta-mta-sts] or DANE [RFC7672]) be ignored. 181 The RequireTLS header field has a single REQUIRED parameter: 183 o NO - The SMTP client SHOULD attempt to send the message regardless 184 of its ability to negotiate STARTTLS with the SMTP server, 185 ignoring policy-based mechanisms, if any, asserted by the 186 recipient domain. Nevertheless, the client SHOULD negotiate 187 STARTTLS with the server if available. 189 More than one instance of the RequireTLS header field MUST NOT appear 190 in a given message. 192 4. REQUIRETLS Semantics 194 4.1. REQUIRETLS Receipt Requirements 196 Upon receipt of the REQUIRETLS option on a MAIL FROM command during 197 the receipt of a message, an SMTP server MUST tag that message as 198 needing REQUIRETLS handling. 200 Upon receipt of a message not specifying the REQUIRETLS option on its 201 MAIL FROM command but containing the RequireTLS header field in its 202 message header, an SMTP server implementing this specification MUST 203 tag that message with the option specified in the RequireTLS header 204 field. If the REQUIRETLS MAIL FROM parameter is specified, the 205 RequireTLS header field MUST be ignored but MAY be included in onward 206 relay of the message. 208 The manner in which the above tagging takes place is implementation- 209 dependent. If the message is being locally aliased and redistributed 210 to multiple addresses, all instances of the message MUST be tagged in 211 the same manner. 213 4.2. REQUIRETLS Sender Requirements 215 4.2.1. Sending with TLS Required 217 When sending a message tagged as requiring TLS, the sending (client) 218 MTA MUST: 220 1. Look up the SMTP server to which the message is to be sent as 221 described in [RFC5321] Section 5.1. 223 2. Open an SMTP session with the peer SMTP server using the EHLO 224 verb. 226 3. Establish a TLS-protected SMTP session with its peer SMTP server 227 and authenticate the server's certificate as specified in 228 [RFC6125] or [RFC6698] as applicable. 230 4. Ensure that the response to the subsequent EHLO following 231 establishment of the TLS protection advertises the REQUIRETLS 232 capability. 234 5. The SMTP client SHOULD also require that meaningfully secure 235 cipher algorithms and key lengths be negotiated with the server. 237 The choices of key lengths and algorithms change over time, so a 238 specific requirement is not presented here. 240 If any of the above steps fail, the client MUST issue a QUIT to the 241 server and repeat steps 2-4 with each host on the recipient domain's 242 list of MX hosts in an attempt to find a mail path that meets the 243 sender's requirements. The client MAY send other, unprotected, 244 messages to that server if it has any prior to issuing the QUIT. If 245 there are no more MX hosts, the client MUST NOT transmit the message 246 to the domain. 248 Following such a failure, the SMTP client MUST send a non-delivery 249 notification to the reverse-path of the failed message as described 250 in section 3.6 of [RFC5321]. The following status codes [RFC5248] 251 SHOULD be used: 253 o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed 255 o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption 256 needed 258 Refer to Section 5 for further requirements regarding non-delivery 259 messages. 261 If all REQUIRETLS requirements have been met, transmit the message, 262 issuing the REQUIRETLS option on the MAIL FROM command with the 263 required option(s), if any. 265 4.2.2. Sending with TLS Optional 267 Messages tagged RequireTLS: NO are handled as follows. When sending 268 such a message, the sending (client) MTA MUST: 270 o Look up the SMTP server to which the message is to be sent as 271 described in [RFC5321] Section 5.1. 273 o Open an SMTP session with the peer SMTP server using the EHLO 274 verb. Attempt to negotiate STARTTLS if possible, and follow any 275 policy published by the recipient domain, but do not fail if this 276 is unsuccessful. 278 Some SMTP servers may be configured to require STARTTLS connections 279 as a matter of policy and not accept messages in the absence of 280 STARTTLS. A non-delivery notification MUST be returned to the sender 281 if message relay fails due to an inability to negotiate STARTTLS when 282 required by the server. 284 Since messages tagged with RequireTLS: NO will sometimes be sent to 285 SMTP servers not supporting REQUIRETLS, that option will not be 286 uniformly observed by all SMTP relay hops. 288 4.3. REQUIRETLS Submission 290 An MUA or other agent making the initial introduction of a message 291 has authority to decide whether to require TLS. When TLS is to be 292 required, it MUST do so by negotiating STARTTLS and REQUIRETLS and 293 include the REQUIRETLS option on the MAIL FROM command, as is done 294 for message relay. 296 When TLS is not to be required, the sender MUST include the 297 RequireTLS header field in the message. SMTP servers implementing 298 this specification MUST interpret this header field as described in 299 Section 4.1. 301 In either case, the decision whether to specify REQUIRETLS MAY be 302 done based on a user interface selection or based on a ruleset or 303 other policy. The manner in which the decision to require TLS is 304 made is implementation-dependent and is beyond the scope of this 305 specification. 307 4.4. Delivery of REQUIRETLS messages 309 Messages are usually retrieved by end users using protocols other 310 than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems. 311 Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD 312 observe the guidelines in [RFC8314]. 314 5. Non-delivery message handling 316 Non-delivery ("bounce") messages usually contain important metadata 317 about the message to which they refer, including the original message 318 header. They therefore MUST be protected in the same manner as the 319 original message. All non-delivery messages resulting from messages 320 with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS 321 error or some other, MUST also specify the REQUIRETLS SMTP option 322 unless redacted as described below. 324 The path from the origination of an error bounce message back to the 325 MAIL FROM address may not share the same REQUIRETLS support as the 326 forward path. Therefore, users requiring TLS are advised to make 327 sure that they are capable of receiving mail using REQUIRETLS as 328 well. Otherwise, such non-delivery messages will be lost. 330 If a REQUIRETLS message is bounced, the server MUST behave as if 331 RET=HDRS was present as described in [RFC3461]. If both RET=FULL and 332 REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be 333 transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS 334 bounce message MUST use an empty MAIL FROM return-path as required by 335 [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS 336 parameter SHOULD NOT cause a bounce message to be discarded even if 337 the next-hop relay does not advertise REQUIRETLS. 339 Senders of messages requiring TLS are advised to consider the 340 possibility that bounce messages will be lost as a result of 341 REQUIRETLS return path failure, and that some information could be 342 leaked if a bounce message is not able to be transmitted with 343 REQUIRETLS. 345 6. Mailing list considerations 347 Mailing lists, upon receipt of a message, originate new messages to 348 list addresses. This is distinct from an aliasing operation that 349 redirects the original message, in some cases to multiple recipients. 350 The requirement to preserve the REQUIRETLS tag therefore does not 351 necessarily extend to mailing lists, although the inclusion of the 352 RequireTLS header field MAY cause messages sent to mailing lists to 353 inherit this characteristic. REQUIRETLS users SHOULD be made aware 354 of this limitation so that they use caution when sending to mailing 355 lists and do not assume that REQUIRETLS applies to messages from the 356 list operator to list members. 358 Mailing list operators MAY apply REQUIRETLS requirements in incoming 359 messages to the resulting messages they originate. If this is done, 360 they SHOULD also apply these requirements to administrative traffic, 361 such as messages to moderators requesting approval of messages. 363 7. IANA Considerations 365 If published as an RFC, this draft requests the addition of the 366 following keyword to the SMTP Service Extensions Registry 367 [MailParams]: 369 Textual name: RequireTLS 370 EHLO keyword value: REQUIRETLS 371 Syntax and parameters: (no parameters) 372 Additional SMTP verbs: none 373 MAIL and RCPT parameters: REQUIRETLS parameter on MAIL 374 Behavior: Use of the REQUIRETLS parameter on the 375 MAIL verb causes that message to require 376 the use of TLS and tagging with REQUIRETLS 377 for all onward relay. 378 Command length increment: 11 characters 379 If published as an RFC, this draft requests the addition of an entry 380 to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 381 Registry [SMTPStatusCodes]: 383 Code: 5.7.YYY 384 Sample Text: REQUIRETLS support required 385 Associated basic status code: 530 386 Description: This indicates that the message was not 387 able to be forwarded because it was 388 received with a REQUIRETLS requirement 389 and none of the SMTP servers to which 390 the message should be forwarded provide 391 this support. 392 Reference: (this document) 393 Submitter: J. Fenton 394 Change controller: IESG 396 If published as an RFC, this draft requests the addition of an entry 397 to the Permanent Message Header Field Names Registry 398 [PermMessageHeaderFields]: 400 Header field name: RequireTLS 401 Applicable protocol: mail 402 Status: provisional 403 Author/change controller: IETF UTA Working Group 404 Specification document: (this document) 406 This section is to be updated for publication by the RFC Editor. 408 8. Security Considerations 410 The purpose of REQUIRETLS is to improve communications security for 411 email by giving the originator of a message an expectation that it 412 will be transmitted in an encrypted form "over the wire". When used, 413 REQUIRETLS changes the traditional behavior of email transmission, 414 which favors delivery over the ability to send email messages using 415 transport-layer security, to one in which requested security takes 416 precedence over delivery and domain-level policy. 418 The following considerations apply to the REQUIRETLS service 419 extension but not the RequireTLS header field, since messages 420 specifying the header field are less concerned with transport 421 security. 423 8.1. Passive attacks 425 REQUIRETLS is generally effective against passive attackers who are 426 merely trying to eavesdrop on an SMTP exchange between an SMTP client 427 and server. This assumes, of course, the cryptographic integrity of 428 the TLS connection being used. 430 8.2. Active attacks 432 Active attacks against TLS encrypted SMTP connections can take many 433 forms. One such attack is to interfere in the negotiation by 434 changing the STARTTLS command to something illegal such as XXXXXXXX. 435 This causes TLS negotiation to fail and messages to be sent in the 436 clear, where they can be intercepted. REQUIRETLS detects the failure 437 of STARTTLS and declines to send the message rather than send it 438 insecurely. 440 A second form of attack is a man-in-the-middle attack where the 441 attacker terminates the TLS connection rather than the intended SMTP 442 server. This is possible when, as is commonly the case, the SMTP 443 client either does not verify the server's certificate or establishes 444 the connection even when the verification fails. REQUIRETLS requires 445 successful certificate validation before sending the message. 447 Another active attack involves the spoofing of DNS MX records of the 448 recipient domain. An attacker having this capability could cause the 449 message to be redirected to a mail server under the attacker's own 450 control, which would presumably have a valid certificate. REQUIRETLS 451 does not address this attack. 453 8.3. Bad Actor MTAs 455 A bad-actor MTA along the message transmission path could 456 misrepresent its support of REQUIRETLS and/or actively strip 457 REQUIRETLS tags from messages it handles. However, since 458 intermediate MTAs are already trusted with the cleartext of messages 459 they handle, and are not part of the threat model for transport-layer 460 security, they are also not part of the threat model for REQUIRETLS. 462 It should be reemphasized that since SMTP TLS is a transport-layer 463 security protocol, messages sent using REQUIRETLS are not encrypted 464 end-to-end and are visible to MTAs that are part of the message 465 delivery path. Messages containing sensitive information that MTAs 466 should not have access to MUST be sent using end-to-end content 467 encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751]. 469 9. Acknowledgements 471 The author would like to acknowledge many helpful suggestions on the 472 ietf-smtp and uta mailing lists, in particular those of Viktor 473 Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, 474 John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. 476 10. Revision History 478 To be removed by RFC Editor upon publication as an RFC. 480 10.1. Changes since -02 Draft 482 o More complete documentation for IANA registration requests. 484 o Changed bounce handling to use RET parameters of RFC 3461, along 485 with slightly more liberal transmission of bounces even if 486 REQUIRETLS can't be negotiated. 488 10.2. Changes since -01 Draft 490 o Converted DEEP references to RFC 8314. 492 o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. 494 o Editorial corrections, notably making the header field name 495 consistent (RequireTLS rather than Require-TLS). 497 10.3. Changes since -00 Draft 499 o Created new header field, Require-TLS, for use by "NO" option. 501 o Removed "NO" option from SMTP service extension. 503 o Recommend DEEP requirements for delivery of messages requiring 504 TLS. 506 o Assorted copy edits 508 10.4. Changes since fenton-03 Draft 510 o Wording improvements from Rolf Sonneveld review 22 July 2017 512 o A few copy edits 514 o Conversion from individual to UTA WG draft 516 10.5. Changes Since -02 Draft 518 o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per 519 suggestion on UTA WG mailing list. 521 o Additional guidance on bounce messages 523 10.6. Changes Since -01 Draft 525 o Specified retries when multiple MX hosts exist for a given domain. 527 o Clarified generation of non-delivery messages 529 o Specified requirements for application of REQUIRETLS to mail 530 forwarders and mailing lists. 532 o Clarified DNSSEC requirements to include MX lookup only. 534 o Corrected terminology regarding message retrieval vs. delivery. 536 o Changed category to standards track. 538 10.7. Changes Since -00 Draft 540 o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM 541 parameter to better associate REQUIRETLS requirements with 542 transmission of individual messages. 544 o Addition of an option to require DNSSEC lookup of the remote mail 545 server, since this affects the common name of the certificate that 546 is presented. 548 o Clarified the wording to more clearly state that TLS sessions must 549 be established and not simply that STARTTLS is negotiated. 551 o Introduced need for minimum encryption standards (key lengths and 552 algorithms) 554 o Substantially rewritten Security Considerations section 556 11. References 558 11.1. Normative References 560 [MailParams] 561 Internet Assigned Numbers Authority (IANA), "IANA Mail 562 Parameters", 2007, 563 . 565 [PermMessageHeaderFields] 566 Internet Assigned Numbers Authority (IANA), "Permanent 567 Message Header Field Names Registry", 2004, 568 . 571 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 572 Requirement Levels", BCP 14, RFC 2119, 573 DOI 10.17487/RFC2119, March 1997, 574 . 576 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 577 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 578 February 2002, . 580 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 581 Extension for Delivery Status Notifications (DSNs)", 582 RFC 3461, DOI 10.17487/RFC3461, January 2003, 583 . 585 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 586 Mail System Status Codes", BCP 138, RFC 5248, 587 DOI 10.17487/RFC5248, June 2008, 588 . 590 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 591 DOI 10.17487/RFC5321, October 2008, 592 . 594 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 595 Verification of Domain-Based Application Service Identity 596 within Internet Public Key Infrastructure Using X.509 597 (PKIX) Certificates in the Context of Transport Layer 598 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 599 2011, . 601 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 602 of Named Entities (DANE) Transport Layer Security (TLS) 603 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 604 2012, . 606 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 607 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 608 May 2017, . 610 [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: 611 Use of Transport Layer Security (TLS) for Email Submission 612 and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, 613 . 615 [SMTPStatusCodes] 616 Internet Assigned Numbers Authority (IANA), "Simple Mail 617 Transfer Protocol (SMTP) Enhanced Status Codes Registry", 618 2008, . 621 11.2. Informative References 623 [I-D.ietf-uta-mta-sts] 624 Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 625 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 626 STS)", draft-ietf-uta-mta-sts-21 (work in progress), June 627 2018. 629 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 630 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 631 . 633 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 634 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 635 . 637 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 638 Thayer, "OpenPGP Message Format", RFC 4880, 639 DOI 10.17487/RFC4880, November 2007, 640 . 642 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 643 Mail Extensions (S/MIME) Version 3.2 Message 644 Specification", RFC 5751, DOI 10.17487/RFC5751, January 645 2010, . 647 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 648 Opportunistic DNS-Based Authentication of Named Entities 649 (DANE) Transport Layer Security (TLS)", RFC 7672, 650 DOI 10.17487/RFC7672, October 2015, 651 . 653 Author's Address 654 Jim Fenton 655 Altmode Networks 656 Los Altos, California 94024 657 USA 659 Email: fenton@bluepopcorn.net