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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 December 4, 2018 5 Expires: June 7, 2019 7 SMTP Require TLS Option 8 draft-ietf-uta-smtp-require-tls-06 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 June 7, 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 -05 Draft . . . . . . . . . . . . . . . . 11 80 10.2. Changes since -04 Draft . . . . . . . . . . . . . . . . 11 81 10.3. Changes since -03 Draft . . . . . . . . . . . . . . . . 11 82 10.4. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 83 10.5. Changes since -01 Draft . . . . . . . . . . . . . . . . 12 84 10.6. Changes since -00 Draft . . . . . . . . . . . . . . . . 12 85 10.7. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 86 10.8. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 87 10.9. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 88 10.10. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13 89 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 90 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 91 11.2. Informative References . . . . . . . . . . . . . . . . . 15 92 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 94 1. Introduction 96 The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a 97 means by which an SMTP server and client can establish a Transport 98 Layer Security (TLS) protected session for the transmission of email 99 messages. By default, TLS is used only upon mutual agreement 100 (successful negotiation) of STARTTLS between the client and server; 101 if this is not possible, the message is sent without transport 102 encryption. Furthermore, it is common practice for the client to 103 negotiate TLS even if the SMTP server's certificate is invalid. 105 Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may 106 impose requirements for the use of TLS for email destined for some 107 domains. However, such policies do not allow the sender to specify 108 which messages are more sensitive and require transport-level 109 encryption, and which ones are less sensitive and ought to be relayed 110 even if TLS cannot be negotiated successfully. 112 The default opportunistic nature of SMTP TLS enables several "on the 113 wire" attacks on SMTP security between MTAs. These include passive 114 eavesdropping on connections for which TLS is not used, interference 115 in the SMTP protocol to prevent TLS from being negotiated (presumably 116 accompanied by eavesdropping), and insertion of a man-in-the-middle 117 attacker exploiting the lack of server authentication by the client. 118 Attacks are described in more detail in the Security Considerations 119 section of this document. 121 REQUIRETLS consists of two mechanisms: an SMTP service extension and 122 a message header field. The service extension is used to specify 123 that a given message sent during a particular session MUST be sent 124 over a TLS-protected session with specified security characteristics. 125 It also requires that the SMTP server advertise that it supports 126 REQUIRETLS, in effect promising that it will honor the requirement to 127 enforce TLS transmission and REQUIRETLS support for onward 128 transmission of those messages. 130 The RequireTLS message header field is used to convey a request to 131 ignore recipient-side policy mechanisms such as MTA-STS and DANE, 132 thereby prioritizing delivery over ability to negotiate TLS. Unlike 133 the service extension, the RequireTLS header field allows the message 134 to transit through one or more MTAs that do not support REQUIRETLS. 136 1.1. Requirements Language 138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 140 "OPTIONAL" in this document are to be interpreted as described in BCP 141 14 [RFC2119] [RFC8174] when, and only when, they appear in all 142 capitals, as shown here. 144 2. The REQUIRETLS Service Extension 146 1. The textual name of the extension is "Require TLS". 148 2. The EHLO keyword value associated with this extension is 149 "REQUIRETLS". 151 3. One MAIL FROM option is defined by this extension. 153 4. One new SMTP status code is defined by this extension to convey 154 an error condition resulting from failure of the client to send 155 to a server not also supporting the REQUIRETLS extension. 157 In order to specify REQUIRETLS treatment for a given message, the 158 REQUIRETLS option is specified on the MAIL FROM command when that 159 message is transmitted. This option MUST only be specified in the 160 context of an SMTP session meeting the security requirements that 161 have been specified: 163 o The session itself MUST employ TLS transmission. 165 o If the SMTP server to which the message is being transmitted is 166 identified through an MX record lookup, its name MUST be validated 167 via a DNSSEC signature on the recipient domain's MX record, or the 168 MX hostname MUST be validated by an MTA-STS policy as described in 169 Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033 170 [RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035]. 172 o The certificate presented by the SMTP server MUST either verify 173 successfully in a trust chain leading to a certificate trusted by 174 the SMTP client or it MUST verify successfully using DANE as 175 specified in RFC 7672 [RFC7672]. For trust chains, the choice of 176 trusted (root) certificates is at the discretion of the SMTP 177 client. 179 o Following the negotiation of STARTTLS, the SMTP server MUST 180 advertise in the subsequent EHLO response that it supports 181 REQUIRETLS. 183 3. The RequireTLS Header Field 185 One new message header field, RequireTLS, is defined by this 186 specification. It is used for messages requesting that recipient TLS 187 policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored. 189 The RequireTLS header field has a single REQUIRED parameter: 191 o NO - The SMTP client SHOULD attempt to send the message regardless 192 of its ability to negotiate STARTTLS with the SMTP server, 193 ignoring policy-based mechanisms (including MTA-STS and DANE), if 194 any, asserted by the recipient domain. Nevertheless, the client 195 SHOULD negotiate STARTTLS with the server if available. 197 More than one instance of the RequireTLS header field MUST NOT appear 198 in a given message. 200 4. REQUIRETLS Semantics 202 4.1. REQUIRETLS Receipt Requirements 204 Upon receipt of the REQUIRETLS option on a MAIL FROM command during 205 the receipt of a message, an SMTP server MUST tag that message as 206 needing REQUIRETLS handling. 208 Upon receipt of a message not specifying the REQUIRETLS option on its 209 MAIL FROM command but containing the RequireTLS header field in its 210 message header, an SMTP server implementing this specification MUST 211 tag that message with the option specified in the RequireTLS header 212 field. If the REQUIRETLS MAIL FROM parameter is specified, the 213 RequireTLS header field MUST be ignored but MAY be included in onward 214 relay of the message. 216 The manner in which the above tagging takes place is implementation- 217 dependent. If the message is being locally aliased and redistributed 218 to multiple addresses, all instances of the message MUST be tagged in 219 the same manner. 221 4.2. REQUIRETLS Sender Requirements 223 4.2.1. Sending with TLS Required 225 When sending a message tagged as requiring TLS, the sending (client) 226 MTA MUST: 228 1. Look up the SMTP server to which the message is to be sent as 229 described in [RFC5321] Section 5.1. 231 2. If the server lookup is accomplished via the recipient domain's 232 MX record (the usual case) and is not accompanied by a valid 233 DNSSEC signature, the client MUST also validate the SMTP server 234 name using MTA-STS as described in Section 4.1 of RFC 8461 235 [RFC8461]. 237 3. Open an SMTP session with the peer SMTP server using the EHLO 238 verb. 240 4. Establish a TLS-protected SMTP session with its peer SMTP server 241 and authenticate the server's certificate as specified in 242 [RFC6125] or [RFC7672] as applicable. 244 5. Ensure that the response to the subsequent EHLO following 245 establishment of the TLS protection advertises the REQUIRETLS 246 capability. 248 6. The SMTP client SHOULD also require that meaningfully secure 249 cipher algorithms and key lengths be negotiated with the server. 250 The choices of key lengths and algorithms change over time, so a 251 specific requirement is not presented here. 253 If any of the above steps fail, the client MUST issue a QUIT to the 254 server and repeat steps 2-5 with each host on the recipient domain's 255 list of MX hosts in an attempt to find a mail path that meets the 256 sender's requirements. The client MAY send other, unprotected, 257 messages to that server if it has any prior to issuing the QUIT. If 258 there are no more MX hosts, the client MUST NOT transmit the message 259 to the domain. 261 Following such a failure, the SMTP client MUST send a non-delivery 262 notification to the reverse-path of the failed message as described 263 in section 3.6 of [RFC5321]. The following status codes [RFC5248] 264 SHOULD be used: 266 o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed 268 o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption 269 needed 271 Refer to Section 5 for further requirements regarding non-delivery 272 messages. 274 If all REQUIRETLS requirements have been met, transmit the message, 275 issuing the REQUIRETLS option on the MAIL FROM command with the 276 required option(s), if any. 278 4.2.2. Sending with TLS Optional 280 Messages tagged RequireTLS: NO are handled as follows. When sending 281 such a message, the sending (client) MTA MUST: 283 o Look up the SMTP server to which the message is to be sent as 284 described in [RFC5321] Section 5.1. 286 o Open an SMTP session with the peer SMTP server using the EHLO 287 verb. Attempt to negotiate STARTTLS if possible, and follow any 288 policy published by the recipient domain, but do not fail if this 289 is unsuccessful. 291 Some SMTP servers may be configured to require STARTTLS connections 292 as a matter of policy and not accept messages in the absence of 293 STARTTLS. A non-delivery notification MUST be returned to the sender 294 if message relay fails due to an inability to negotiate STARTTLS when 295 required by the server. 297 Since messages tagged with RequireTLS: NO will sometimes be sent to 298 SMTP servers not supporting REQUIRETLS, that option will not be 299 uniformly observed by all SMTP relay hops. 301 4.3. REQUIRETLS Submission 303 An MUA or other agent making the initial introduction of a message 304 has authority to decide whether to require TLS. When TLS is to be 305 required, it MUST do so by negotiating STARTTLS and REQUIRETLS and 306 include the REQUIRETLS option on the MAIL FROM command, as is done 307 for message relay. 309 When TLS is not to be required, the sender MUST include the 310 RequireTLS header field in the message. SMTP servers implementing 311 this specification MUST interpret this header field as described in 312 Section 4.1. 314 In either case, the decision whether to specify REQUIRETLS MAY be 315 done based on a user interface selection or based on a ruleset or 316 other policy. The manner in which the decision to require TLS is 317 made is implementation-dependent and is beyond the scope of this 318 specification. 320 4.4. Delivery of REQUIRETLS messages 322 Messages are usually retrieved by end users using protocols other 323 than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems. 324 Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD 325 observe the guidelines in [RFC8314]. 327 5. Non-delivery message handling 329 Non-delivery ("bounce") messages usually contain important metadata 330 about the message to which they refer, including the original message 331 header. They therefore MUST be protected in the same manner as the 332 original message. All non-delivery messages resulting from messages 333 with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS 334 error or some other, MUST also specify the REQUIRETLS SMTP option 335 unless redacted as described below. 337 The path from the origination of an error bounce message back to the 338 MAIL FROM address may not share the same REQUIRETLS support as the 339 forward path. Therefore, users requiring TLS are advised to make 340 sure that they are capable of receiving mail using REQUIRETLS as 341 well. Otherwise, such non-delivery messages will be lost. 343 If a REQUIRETLS message is bounced, the server MUST behave as if 344 RET=HDRS was present as described in [RFC3461]. If both RET=FULL and 345 REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be 346 transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS 347 bounce message MUST use an empty MAIL FROM return-path as required by 348 [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS 349 parameter SHOULD NOT cause a bounce message to be discarded even if 350 the next-hop relay does not advertise REQUIRETLS. 352 Senders of messages requiring TLS are advised to consider the 353 possibility that bounce messages will be lost as a result of 354 REQUIRETLS return path failure, and that some information could be 355 leaked if a bounce message is not able to be transmitted with 356 REQUIRETLS. 358 6. Mailing list considerations 360 Mailing lists, upon receipt of a message, originate new messages to 361 list addresses. This is distinct from an aliasing operation that 362 redirects the original message, in some cases to multiple recipients. 363 The requirement to preserve the REQUIRETLS tag therefore does not 364 necessarily extend to mailing lists, although the inclusion of the 365 RequireTLS header field MAY cause messages sent to mailing lists to 366 inherit this characteristic. REQUIRETLS users SHOULD be made aware 367 of this limitation so that they use caution when sending to mailing 368 lists and do not assume that REQUIRETLS applies to messages from the 369 list operator to list members. 371 Mailing list operators MAY apply REQUIRETLS requirements in incoming 372 messages to the resulting messages they originate. If this is done, 373 they SHOULD also apply these requirements to administrative traffic, 374 such as messages to moderators requesting approval of messages. 376 7. IANA Considerations 378 If published as an RFC, this draft requests the addition of the 379 following keyword to the SMTP Service Extensions Registry 380 [MailParams]: 382 Textual name: RequireTLS 383 EHLO keyword value: REQUIRETLS 384 Syntax and parameters: (no parameters) 385 Additional SMTP verbs: none 386 MAIL and RCPT parameters: REQUIRETLS parameter on MAIL 387 Behavior: Use of the REQUIRETLS parameter on the 388 MAIL verb causes that message to require 389 the use of TLS and tagging with 390 REQUIRETLS for all onward relay. 391 Command length increment: 11 characters 393 If published as an RFC, this draft requests the addition of an entry 394 to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 395 Registry [SMTPStatusCodes]: 397 Code: 5.7.YYY 398 Sample Text: REQUIRETLS support required 399 Associated basic status code: 530 400 Description: This indicates that the message was not 401 able to be forwarded because it was 402 received with a REQUIRETLS requirement 403 and none of the SMTP servers to which 404 the message should be forwarded provide 405 this support. 406 Reference: (this document) 407 Submitter: J. Fenton 408 Change controller: IESG 410 If published as an RFC, this draft requests the addition of an entry 411 to the Permanent Message Header Field Names Registry 412 [PermMessageHeaderFields]: 414 Header field name: RequireTLS 415 Applicable protocol: mail 416 Status: standard 417 Author/change controller: IETF 418 Specification document: (this document) 420 This section is to be updated for publication by the RFC Editor. 422 8. Security Considerations 424 The purpose of REQUIRETLS is to improve communications security for 425 email by giving the originator of a message an expectation that it 426 will be transmitted in an encrypted form "over the wire". When used, 427 REQUIRETLS changes the traditional behavior of email transmission, 428 which favors delivery over the ability to send email messages using 429 transport-layer security, to one in which requested security takes 430 precedence over delivery and domain-level policy. 432 The following considerations apply to the REQUIRETLS service 433 extension but not the RequireTLS header field, since messages 434 specifying the header field are less concerned with transport 435 security. 437 8.1. Passive attacks 439 REQUIRETLS is generally effective against passive attackers who are 440 merely trying to eavesdrop on an SMTP exchange between an SMTP client 441 and server. This assumes, of course, the cryptographic integrity of 442 the TLS connection being used. 444 8.2. Active attacks 446 Active attacks against TLS encrypted SMTP connections can take many 447 forms. One such attack is to interfere in the negotiation by 448 changing the STARTTLS command to something illegal such as XXXXXXXX. 449 This causes TLS negotiation to fail and messages to be sent in the 450 clear, where they can be intercepted. REQUIRETLS detects the failure 451 of STARTTLS and declines to send the message rather than send it 452 insecurely. 454 A second form of attack is a man-in-the-middle attack where the 455 attacker terminates the TLS connection rather than the intended SMTP 456 server. This is possible when, as is commonly the case, the SMTP 457 client either does not verify the server's certificate or establishes 458 the connection even when the verification fails. REQUIRETLS requires 459 successful certificate validation before sending the message. 461 Another active attack involves the spoofing of DNS MX records of the 462 recipient domain. An attacker having this capability could 463 potentially cause the message to be redirected to a mail server under 464 the attacker's own control, which would presumably have a valid 465 certificate. REQUIRETLS requires that the recipient domain's MX 466 record lookup be validated either using DNSSEC or via a published 467 MTA-STS policy that specifies the acceptable SMTP server hostname(s) 468 for the recipient domain. 470 8.3. Bad Actor MTAs 472 A bad-actor MTA along the message transmission path could 473 misrepresent its support of REQUIRETLS and/or actively strip 474 REQUIRETLS tags from messages it handles. However, since 475 intermediate MTAs are already trusted with the cleartext of messages 476 they handle, and are not part of the threat model for transport-layer 477 security, they are also not part of the threat model for REQUIRETLS. 479 It should be reemphasized that since SMTP TLS is a transport-layer 480 security protocol, messages sent using REQUIRETLS are not encrypted 481 end-to-end and are visible to MTAs that are part of the message 482 delivery path. Messages containing sensitive information that MTAs 483 should not have access to MUST be sent using end-to-end content 484 encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751]. 486 9. Acknowledgements 488 The author would like to acknowledge many helpful suggestions on the 489 ietf-smtp and uta mailing lists, in particular those of Viktor 490 Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, 491 John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. 493 10. Revision History 495 To be removed by RFC Editor upon publication as an RFC. 497 10.1. Changes since -05 Draft 499 Corrected IANA Permanent Message Header Fields Registry request. 501 10.2. Changes since -04 Draft 503 Require validation of SMTP server hostname via DNSSEC or MTA-STS 504 policy when TLS is required. 506 10.3. Changes since -03 Draft 508 Working Group Last Call changes, including: 510 o Correct reference for SMTP DANE 512 o Clarify that RequireTLS: NO applies to both MTA-STS and DANE 513 policies 515 o Correct newly-defined status codes 517 o Update MTA-STS references to RFC 519 10.4. Changes since -02 Draft 521 o More complete documentation for IANA registration requests. 523 o Changed bounce handling to use RET parameters of RFC 3461, along 524 with slightly more liberal transmission of bounces even if 525 REQUIRETLS can't be negotiated. 527 10.5. Changes since -01 Draft 529 o Converted DEEP references to RFC 8314. 531 o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. 533 o Editorial corrections, notably making the header field name 534 consistent (RequireTLS rather than Require-TLS). 536 10.6. Changes since -00 Draft 538 o Created new header field, Require-TLS, for use by "NO" option. 540 o Removed "NO" option from SMTP service extension. 542 o Recommend DEEP requirements for delivery of messages requiring 543 TLS. 545 o Assorted copy edits 547 10.7. Changes since fenton-03 Draft 549 o Wording improvements from Rolf Sonneveld review 22 July 2017 551 o A few copy edits 553 o Conversion from individual to UTA WG draft 555 10.8. Changes Since -02 Draft 557 o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per 558 suggestion on UTA WG mailing list. 560 o Additional guidance on bounce messages 562 10.9. Changes Since -01 Draft 564 o Specified retries when multiple MX hosts exist for a given domain. 566 o Clarified generation of non-delivery messages 568 o Specified requirements for application of REQUIRETLS to mail 569 forwarders and mailing lists. 571 o Clarified DNSSEC requirements to include MX lookup only. 573 o Corrected terminology regarding message retrieval vs. delivery. 575 o Changed category to standards track. 577 10.10. Changes Since -00 Draft 579 o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM 580 parameter to better associate REQUIRETLS requirements with 581 transmission of individual messages. 583 o Addition of an option to require DNSSEC lookup of the remote mail 584 server, since this affects the common name of the certificate that 585 is presented. 587 o Clarified the wording to more clearly state that TLS sessions must 588 be established and not simply that STARTTLS is negotiated. 590 o Introduced need for minimum encryption standards (key lengths and 591 algorithms) 593 o Substantially rewritten Security Considerations section 595 11. References 597 11.1. Normative References 599 [MailParams] 600 Internet Assigned Numbers Authority (IANA), "IANA Mail 601 Parameters", 2007, 602 . 604 [PermMessageHeaderFields] 605 Internet Assigned Numbers Authority (IANA), "Permanent 606 Message Header Field Names Registry", 2004, 607 . 610 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 611 Requirement Levels", BCP 14, RFC 2119, 612 DOI 10.17487/RFC2119, March 1997, 613 . 615 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 616 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 617 February 2002, . 619 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 620 Extension for Delivery Status Notifications (DSNs)", 621 RFC 3461, DOI 10.17487/RFC3461, January 2003, 622 . 624 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 625 Rose, "DNS Security Introduction and Requirements", 626 RFC 4033, DOI 10.17487/RFC4033, March 2005, 627 . 629 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 630 Rose, "Resource Records for the DNS Security Extensions", 631 RFC 4034, DOI 10.17487/RFC4034, March 2005, 632 . 634 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 635 Rose, "Protocol Modifications for the DNS Security 636 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 637 . 639 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 640 Mail System Status Codes", BCP 138, RFC 5248, 641 DOI 10.17487/RFC5248, June 2008, 642 . 644 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 645 DOI 10.17487/RFC5321, October 2008, 646 . 648 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 649 Verification of Domain-Based Application Service Identity 650 within Internet Public Key Infrastructure Using X.509 651 (PKIX) Certificates in the Context of Transport Layer 652 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 653 2011, . 655 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 656 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 657 May 2017, . 659 [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: 660 Use of Transport Layer Security (TLS) for Email Submission 661 and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, 662 . 664 [RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., 665 and J. Jones, "SMTP MTA Strict Transport Security (MTA- 666 STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018, 667 . 669 [SMTPStatusCodes] 670 Internet Assigned Numbers Authority (IANA), "Simple Mail 671 Transfer Protocol (SMTP) Enhanced Status Codes Registry", 672 2008, . 675 11.2. Informative References 677 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 678 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 679 . 681 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 682 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 683 . 685 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 686 Thayer, "OpenPGP Message Format", RFC 4880, 687 DOI 10.17487/RFC4880, November 2007, 688 . 690 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 691 Mail Extensions (S/MIME) Version 3.2 Message 692 Specification", RFC 5751, DOI 10.17487/RFC5751, January 693 2010, . 695 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 696 Opportunistic DNS-Based Authentication of Named Entities 697 (DANE) Transport Layer Security (TLS)", RFC 7672, 698 DOI 10.17487/RFC7672, October 2015, 699 . 701 Author's Address 703 Jim Fenton 704 Altmode Networks 705 Los Altos, California 94024 706 USA 708 Email: fenton@bluepopcorn.net