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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 November 20, 2018 5 Expires: May 24, 2019 7 SMTP Require TLS Option 8 draft-ietf-uta-smtp-require-tls-05 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 May 24, 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 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 -04 Draft . . . . . . . . . . . . . . . . 11 80 10.2. Changes since -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 10.6. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 85 10.7. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 86 10.8. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 87 10.9. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13 88 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 89 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 90 11.2. Informative References . . . . . . . . . . . . . . . . . 15 91 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 93 1. Introduction 95 The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a 96 means by which an SMTP server and client can establish a Transport 97 Layer Security (TLS) protected session for the transmission of email 98 messages. By default, TLS is used only upon mutual agreement 99 (successful negotiation) of STARTTLS between the client and server; 100 if this is not possible, the message is sent without transport 101 encryption. Furthermore, it is common practice for the client to 102 negotiate TLS even if the SMTP server's certificate is invalid. 104 Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may 105 impose requirements for the use of TLS for email destined for some 106 domains. However, such policies do not allow the sender to specify 107 which messages are more sensitive and require transport-level 108 encryption, and which ones are less sensitive and ought to be relayed 109 even if TLS cannot be negotiated successfully. 111 The default opportunistic nature of SMTP TLS enables several "on the 112 wire" attacks on SMTP security between MTAs. These include passive 113 eavesdropping on connections for which TLS is not used, interference 114 in the SMTP protocol to prevent TLS from being negotiated (presumably 115 accompanied by eavesdropping), and insertion of a man-in-the-middle 116 attacker exploiting the lack of server authentication by the client. 117 Attacks are described in more detail in the Security Considerations 118 section of this document. 120 REQUIRETLS consists of two mechanisms: an SMTP service extension and 121 a message header field. The service extension is used to specify 122 that a given message sent during a particular session MUST be sent 123 over a TLS-protected session with specified security characteristics. 124 It also requires that the SMTP server advertise that it supports 125 REQUIRETLS, in effect promising that it will honor the requirement to 126 enforce TLS transmission and REQUIRETLS support for onward 127 transmission of those messages. 129 The RequireTLS message header field is used to convey a request to 130 ignore recipient-side policy mechanisms such as MTA-STS and DANE, 131 thereby prioritizing delivery over ability to negotiate TLS. Unlike 132 the service extension, the RequireTLS header field allows the message 133 to transit through one or more MTAs that do not support REQUIRETLS. 135 1.1. Requirements Language 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 139 "OPTIONAL" in this document are to be interpreted as described in BCP 140 14 [RFC2119] [RFC8174] when, and only when, they appear in all 141 capitals, as shown here. 143 2. The REQUIRETLS Service Extension 145 1. The textual name of the extension is "Require TLS". 147 2. The EHLO keyword value associated with this extension is 148 "REQUIRETLS". 150 3. One MAIL FROM option is defined by this extension. 152 4. One new SMTP status code is defined by this extension to convey 153 an error condition resulting from failure of the client to send 154 to a server not also supporting the 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 If the SMTP server to which the message is being transmitted is 165 identified through an MX record lookup, its name MUST be validated 166 via a DNSSEC signature on the recipient domain's MX record, or the 167 MX hostname MUST be validated by an MTA-STS policy as described in 168 Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033 169 [RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035]. 171 o The certificate presented by the SMTP server MUST either verify 172 successfully in a trust chain leading to a certificate trusted by 173 the SMTP client or it MUST verify succesfully using DANE as 174 specified in RFC 7672 [RFC7672]. For trust chains, the choice of 175 trusted (root) certificates is at the discretion of the SMTP 176 client. 178 o Following the negotiation of STARTTLS, the SMTP server MUST 179 advertise in the subsequent EHLO response that it supports 180 REQUIRETLS. 182 3. The RequireTLS Header Field 184 One new message header field, RequireTLS, is defined by this 185 specification. It is used for messages requesting that recipient TLS 186 policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored. 188 The RequireTLS header field has a single REQUIRED parameter: 190 o NO - The SMTP client SHOULD attempt to send the message regardless 191 of its ability to negotiate STARTTLS with the SMTP server, 192 ignoring policy-based mechanisms (including MTA-STS and DANE), if 193 any, asserted by the recipient domain. Nevertheless, the client 194 SHOULD negotiate STARTTLS with the server if available. 196 More than one instance of the RequireTLS header field MUST NOT appear 197 in a given message. 199 4. REQUIRETLS Semantics 201 4.1. REQUIRETLS Receipt Requirements 203 Upon receipt of the REQUIRETLS option on a MAIL FROM command during 204 the receipt of a message, an SMTP server MUST tag that message as 205 needing REQUIRETLS handling. 207 Upon receipt of a message not specifying the REQUIRETLS option on its 208 MAIL FROM command but containing the RequireTLS header field in its 209 message header, an SMTP server implementing this specification MUST 210 tag that message with the option specified in the RequireTLS header 211 field. If the REQUIRETLS MAIL FROM parameter is specified, the 212 RequireTLS header field MUST be ignored but MAY be included in onward 213 relay of the message. 215 The manner in which the above tagging takes place is implementation- 216 dependent. If the message is being locally aliased and redistributed 217 to multiple addresses, all instances of the message MUST be tagged in 218 the same manner. 220 4.2. REQUIRETLS Sender Requirements 222 4.2.1. Sending with TLS Required 224 When sending a message tagged as requiring TLS, the sending (client) 225 MTA MUST: 227 1. Look up the SMTP server to which the message is to be sent as 228 described in [RFC5321] Section 5.1. 230 2. If the server lookup is accomplished via the recipient domain's 231 MX record (the usual case) and is not accompanied by a valid 232 DNSSEC signature, the client MUST also validate the SMTP server 233 name using MTA-STS as described in Section 4.1 of RFC 8461 234 [RFC8461]. 236 3. Open an SMTP session with the peer SMTP server using the EHLO 237 verb. 239 4. Establish a TLS-protected SMTP session with its peer SMTP server 240 and authenticate the server's certificate as specified in 241 [RFC6125] or [RFC7672] as applicable. 243 5. Ensure that the response to the subsequent EHLO following 244 establishment of the TLS protection advertises the REQUIRETLS 245 capability. 247 6. The SMTP client SHOULD also require that meaningfully secure 248 cipher algorithms and key lengths be negotiated with the server. 249 The choices of key lengths and algorithms change over time, so a 250 specific requirement is not presented here. 252 If any of the above steps fail, the client MUST issue a QUIT to the 253 server and repeat steps 2-5 with each host on the recipient domain's 254 list of MX hosts in an attempt to find a mail path that meets the 255 sender's requirements. The client MAY send other, unprotected, 256 messages to that server if it has any prior to issuing the QUIT. If 257 there are no more MX hosts, the client MUST NOT transmit the message 258 to the domain. 260 Following such a failure, the SMTP client MUST send a non-delivery 261 notification to the reverse-path of the failed message as described 262 in section 3.6 of [RFC5321]. The following status codes [RFC5248] 263 SHOULD be used: 265 o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed 267 o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption 268 needed 270 Refer to Section 5 for further requirements regarding non-delivery 271 messages. 273 If all REQUIRETLS requirements have been met, transmit the message, 274 issuing the REQUIRETLS option on the MAIL FROM command with the 275 required option(s), if any. 277 4.2.2. Sending with TLS Optional 279 Messages tagged RequireTLS: NO are handled as follows. When sending 280 such a message, the sending (client) MTA MUST: 282 o Look up the SMTP server to which the message is to be sent as 283 described in [RFC5321] Section 5.1. 285 o Open an SMTP session with the peer SMTP server using the EHLO 286 verb. Attempt to negotiate STARTTLS if possible, and follow any 287 policy published by the recipient domain, but do not fail if this 288 is unsuccessful. 290 Some SMTP servers may be configured to require STARTTLS connections 291 as a matter of policy and not accept messages in the absence of 292 STARTTLS. A non-delivery notification MUST be returned to the sender 293 if message relay fails due to an inability to negotiate STARTTLS when 294 required by the server. 296 Since messages tagged with RequireTLS: NO will sometimes be sent to 297 SMTP servers not supporting REQUIRETLS, that option will not be 298 uniformly observed by all SMTP relay hops. 300 4.3. REQUIRETLS Submission 302 An MUA or other agent making the initial introduction of a message 303 has authority to decide whether to require TLS. When TLS is to be 304 required, it MUST do so by negotiating STARTTLS and REQUIRETLS and 305 include the REQUIRETLS option on the MAIL FROM command, as is done 306 for message relay. 308 When TLS is not to be required, the sender MUST include the 309 RequireTLS header field in the message. SMTP servers implementing 310 this specification MUST interpret this header field as described in 311 Section 4.1. 313 In either case, the decision whether to specify REQUIRETLS MAY be 314 done based on a user interface selection or based on a ruleset or 315 other policy. The manner in which the decision to require TLS is 316 made is implementation-dependent and is beyond the scope of this 317 specification. 319 4.4. Delivery of REQUIRETLS messages 321 Messages are usually retrieved by end users using protocols other 322 than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems. 323 Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD 324 observe the guidelines in [RFC8314]. 326 5. Non-delivery message handling 328 Non-delivery ("bounce") messages usually contain important metadata 329 about the message to which they refer, including the original message 330 header. They therefore MUST be protected in the same manner as the 331 original message. All non-delivery messages resulting from messages 332 with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS 333 error or some other, MUST also specify the REQUIRETLS SMTP option 334 unless redacted as described below. 336 The path from the origination of an error bounce message back to the 337 MAIL FROM address may not share the same REQUIRETLS support as the 338 forward path. Therefore, users requiring TLS are advised to make 339 sure that they are capable of receiving mail using REQUIRETLS as 340 well. Otherwise, such non-delivery messages will be lost. 342 If a REQUIRETLS message is bounced, the server MUST behave as if 343 RET=HDRS was present as described in [RFC3461]. If both RET=FULL and 344 REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be 345 transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS 346 bounce message MUST use an empty MAIL FROM return-path as required by 347 [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS 348 parameter SHOULD NOT cause a bounce message to be discarded even if 349 the next-hop relay does not advertise REQUIRETLS. 351 Senders of messages requiring TLS are advised to consider the 352 possibility that bounce messages will be lost as a result of 353 REQUIRETLS return path failure, and that some information could be 354 leaked if a bounce message is not able to be transmitted with 355 REQUIRETLS. 357 6. Mailing list considerations 359 Mailing lists, upon receipt of a message, originate new messages to 360 list addresses. This is distinct from an aliasing operation that 361 redirects the original message, in some cases to multiple recipients. 362 The requirement to preserve the REQUIRETLS tag therefore does not 363 necessarily extend to mailing lists, although the inclusion of the 364 RequireTLS header field MAY cause messages sent to mailing lists to 365 inherit this characteristic. REQUIRETLS users SHOULD be made aware 366 of this limitation so that they use caution when sending to mailing 367 lists and do not assume that REQUIRETLS applies to messages from the 368 list operator to list members. 370 Mailing list operators MAY apply REQUIRETLS requirements in incoming 371 messages to the resulting messages they originate. If this is done, 372 they SHOULD also apply these requirements to administrative traffic, 373 such as messages to moderators requesting approval of messages. 375 7. IANA Considerations 377 If published as an RFC, this draft requests the addition of the 378 following keyword to the SMTP Service Extensions Registry 379 [MailParams]: 381 Textual name: RequireTLS 382 EHLO keyword value: REQUIRETLS 383 Syntax and parameters: (no parameters) 384 Additional SMTP verbs: none 385 MAIL and RCPT parameters: REQUIRETLS parameter on MAIL 386 Behavior: Use of the REQUIRETLS parameter on the 387 MAIL verb causes that message to require 388 the use of TLS and tagging with 389 REQUIRETLS for all onward relay. 390 Command length increment: 11 characters 392 If published as an RFC, this draft requests the addition of an entry 393 to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 394 Registry [SMTPStatusCodes]: 396 Code: 5.7.YYY 397 Sample Text: REQUIRETLS support required 398 Associated basic status code: 530 399 Description: This indicates that the message was not 400 able to be forwarded because it was 401 received with a REQUIRETLS requirement 402 and none of the SMTP servers to which 403 the message should be forwarded provide 404 this support. 405 Reference: (this document) 406 Submitter: J. Fenton 407 Change controller: IESG 409 If published as an RFC, this draft requests the addition of an entry 410 to the Permanent Message Header Field Names Registry 411 [PermMessageHeaderFields]: 413 Header field name: RequireTLS 414 Applicable protocol: mail 415 Status: provisional 416 Author/change controller: IETF UTA Working Group 417 Specification document: (this document) 419 This section is to be updated for publication by the RFC Editor. 421 8. Security Considerations 423 The purpose of REQUIRETLS is to improve communications security for 424 email by giving the originator of a message an expectation that it 425 will be transmitted in an encrypted form "over the wire". When used, 426 REQUIRETLS changes the traditional behavior of email transmission, 427 which favors delivery over the ability to send email messages using 428 transport-layer security, to one in which requested security takes 429 precedence over delivery and domain-level policy. 431 The following considerations apply to the REQUIRETLS service 432 extension but not the RequireTLS header field, since messages 433 specifying the header field are less concerned with transport 434 security. 436 8.1. Passive attacks 438 REQUIRETLS is generally effective against passive attackers who are 439 merely trying to eavesdrop on an SMTP exchange between an SMTP client 440 and server. This assumes, of course, the cryptographic integrity of 441 the TLS connection being used. 443 8.2. Active attacks 445 Active attacks against TLS encrypted SMTP connections can take many 446 forms. One such attack is to interfere in the negotiation by 447 changing the STARTTLS command to something illegal such as XXXXXXXX. 448 This causes TLS negotiation to fail and messages to be sent in the 449 clear, where they can be intercepted. REQUIRETLS detects the failure 450 of STARTTLS and declines to send the message rather than send it 451 insecurely. 453 A second form of attack is a man-in-the-middle attack where the 454 attacker terminates the TLS connection rather than the intended SMTP 455 server. This is possible when, as is commonly the case, the SMTP 456 client either does not verify the server's certificate or establishes 457 the connection even when the verification fails. REQUIRETLS requires 458 successful certificate validation before sending the message. 460 Another active attack involves the spoofing of DNS MX records of the 461 recipient domain. An attacker having this capability could 462 potentially cause the message to be redirected to a mail server under 463 the attacker's own control, which would presumably have a valid 464 certificate. REQUIRETLS requires that the recipient domain's MX 465 record lookup be validated either using DNSSEC or via a published 466 MTA-STS policy that specifies the acceptable SMTP server hostname(s) 467 for the recipient domain. 469 8.3. Bad Actor MTAs 471 A bad-actor MTA along the message transmission path could 472 misrepresent its support of REQUIRETLS and/or actively strip 473 REQUIRETLS tags from messages it handles. However, since 474 intermediate MTAs are already trusted with the cleartext of messages 475 they handle, and are not part of the threat model for transport-layer 476 security, they are also not part of the threat model for REQUIRETLS. 478 It should be reemphasized that since SMTP TLS is a transport-layer 479 security protocol, messages sent using REQUIRETLS are not encrypted 480 end-to-end and are visible to MTAs that are part of the message 481 delivery path. Messages containing sensitive information that MTAs 482 should not have access to MUST be sent using end-to-end content 483 encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751]. 485 9. Acknowledgements 487 The author would like to acknowledge many helpful suggestions on the 488 ietf-smtp and uta mailing lists, in particular those of Viktor 489 Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, 490 John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. 492 10. Revision History 494 To be removed by RFC Editor upon publication as an RFC. 496 10.1. Changes since -04 Draft 498 Require validation of SMTP server hostname via DNSSEC or MTA-STS 499 policy when TLS is required. 501 10.2. Changes since -03 Draft 503 Working Group Last Call changes, including: 505 o Correct reference for SMTP DANE 507 o Clarify that RequireTLS: NO applies to both MTA-STS and DANE 508 policies 510 o Correct newly-defined status codes 512 o Update MTA-STS references to RFC 514 10.3. Changes since -02 Draft 516 o More complete documentation for IANA registration requests. 518 o Changed bounce handling to use RET parameters of RFC 3461, along 519 with slightly more liberal transmission of bounces even if 520 REQUIRETLS can't be negotiated. 522 10.4. Changes since -01 Draft 524 o Converted DEEP references to RFC 8314. 526 o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. 528 o Editorial corrections, notably making the header field name 529 consistent (RequireTLS rather than Require-TLS). 531 10.5. Changes since -00 Draft 533 o Created new header field, Require-TLS, for use by "NO" option. 535 o Removed "NO" option from SMTP service extension. 537 o Recommend DEEP requirements for delivery of messages requiring 538 TLS. 540 o Assorted copy edits 542 10.6. Changes since fenton-03 Draft 544 o Wording improvements from Rolf Sonneveld review 22 July 2017 546 o A few copy edits 548 o Conversion from individual to UTA WG draft 550 10.7. Changes Since -02 Draft 552 o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per 553 suggestion on UTA WG mailing list. 555 o Additional guidance on bounce messages 557 10.8. Changes Since -01 Draft 559 o Specified retries when multiple MX hosts exist for a given domain. 561 o Clarified generation of non-delivery messages 563 o Specified requirements for application of REQUIRETLS to mail 564 forwarders and mailing lists. 566 o Clarified DNSSEC requirements to include MX lookup only. 568 o Corrected terminology regarding message retrieval vs. delivery. 570 o Changed category to standards track. 572 10.9. Changes Since -00 Draft 574 o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM 575 parameter to better associate REQUIRETLS requirements with 576 transmission of individual messages. 578 o Addition of an option to require DNSSEC lookup of the remote mail 579 server, since this affects the common name of the certificate that 580 is presented. 582 o Clarified the wording to more clearly state that TLS sessions must 583 be established and not simply that STARTTLS is negotiated. 585 o Introduced need for minimum encryption standards (key lengths and 586 algorithms) 588 o Substantially rewritten Security Considerations section 590 11. References 592 11.1. Normative References 594 [MailParams] 595 Internet Assigned Numbers Authority (IANA), "IANA Mail 596 Parameters", 2007, 597 . 599 [PermMessageHeaderFields] 600 Internet Assigned Numbers Authority (IANA), "Permanent 601 Message Header Field Names Registry", 2004, 602 . 605 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 606 Requirement Levels", BCP 14, RFC 2119, 607 DOI 10.17487/RFC2119, March 1997, 608 . 610 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 611 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 612 February 2002, . 614 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 615 Extension for Delivery Status Notifications (DSNs)", 616 RFC 3461, DOI 10.17487/RFC3461, January 2003, 617 . 619 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 620 Rose, "DNS Security Introduction and Requirements", 621 RFC 4033, DOI 10.17487/RFC4033, March 2005, 622 . 624 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 625 Rose, "Resource Records for the DNS Security Extensions", 626 RFC 4034, DOI 10.17487/RFC4034, March 2005, 627 . 629 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 630 Rose, "Protocol Modifications for the DNS Security 631 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 632 . 634 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 635 Mail System Status Codes", BCP 138, RFC 5248, 636 DOI 10.17487/RFC5248, June 2008, 637 . 639 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 640 DOI 10.17487/RFC5321, October 2008, 641 . 643 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 644 Verification of Domain-Based Application Service Identity 645 within Internet Public Key Infrastructure Using X.509 646 (PKIX) Certificates in the Context of Transport Layer 647 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 648 2011, . 650 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 651 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 652 May 2017, . 654 [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: 655 Use of Transport Layer Security (TLS) for Email Submission 656 and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, 657 . 659 [RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 660 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 661 STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018, 662 . 664 [SMTPStatusCodes] 665 Internet Assigned Numbers Authority (IANA), "Simple Mail 666 Transfer Protocol (SMTP) Enhanced Status Codes Registry", 667 2008, . 670 11.2. Informative References 672 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 673 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 674 . 676 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 677 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 678 . 680 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 681 Thayer, "OpenPGP Message Format", RFC 4880, 682 DOI 10.17487/RFC4880, November 2007, 683 . 685 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 686 Mail Extensions (S/MIME) Version 3.2 Message 687 Specification", RFC 5751, DOI 10.17487/RFC5751, January 688 2010, . 690 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 691 Opportunistic DNS-Based Authentication of Named Entities 692 (DANE) Transport Layer Security (TLS)", RFC 7672, 693 DOI 10.17487/RFC7672, October 2015, 694 . 696 Author's Address 698 Jim Fenton 699 Altmode Networks 700 Los Altos, California 94024 701 USA 703 Email: fenton@bluepopcorn.net