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Checking references for intended status: Best Current Practice ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Downref: Normative reference to an Historic RFC: RFC 5617 (ref. 'ADSP') ** Obsolete normative reference: RFC 5451 (ref. 'AUTH-RESULTS') (Obsoleted by RFC 7001) == Outdated reference: draft-ietf-dkim-rfc4871bis has been published as RFC 6376 -- Possible downref: Normative reference to a draft: ref. 'DKIM' ** Downref: Normative reference to an Informational RFC: RFC 5598 (ref. 'EMAIL-ARCH') -- Obsolete informational reference (is this intentional?): RFC 5070 (ref. 'IODEF') (Obsoleted by RFC 7970) Summary: 3 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DKIM Working Group M. Kucherawy 3 Internet-Draft Cloudmark 4 Intended status: BCP May 10, 2011 5 Expires: November 11, 2011 7 DKIM And Mailing Lists 8 draft-ietf-dkim-mailinglists-10 10 Abstract 12 DomainKeys Identified Mail (DKIM) allows an administrative mail 13 domain (ADMD) to assume some responsibility for a message. Based on 14 deployment experience with DKIM, this Best Current Practices document 15 provides guidance for the use of DKIM with scenarios that include 16 Mailing List Managers (MLMs). 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on November 11, 2011. 35 Copyright Notice 37 Copyright (c) 2011 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4 54 1.2. MLMs In Infrastructure . . . . . . . . . . . . . . . . . . 4 55 1.3. Feedback Loops And Other Bi-Lateral Agreements . . . . . . 5 56 1.4. Document Scope and Goals . . . . . . . . . . . . . . . . . 5 57 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7 58 2.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 7 59 2.2. Messaging Terms . . . . . . . . . . . . . . . . . . . . . 7 60 2.3. DKIM-Specific References . . . . . . . . . . . . . . . . . 7 61 2.4. 'DKIM-Friendly' . . . . . . . . . . . . . . . . . . . . . 7 62 2.5. Message Streams . . . . . . . . . . . . . . . . . . . . . 7 63 3. Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . . 9 64 3.1. Roles and Realities . . . . . . . . . . . . . . . . . . . 9 65 3.2. Types Of Mailing Lists . . . . . . . . . . . . . . . . . . 10 66 3.3. Current MLM Effects On Signatures . . . . . . . . . . . . 11 67 4. Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 14 68 4.1. Author-Related Signing . . . . . . . . . . . . . . . . . . 14 69 4.2. Verification Outcomes at Receivers . . . . . . . . . . . . 15 70 4.3. Handling Choices at Receivers . . . . . . . . . . . . . . 15 71 4.4. Wrapping A Non-Participating MLM . . . . . . . . . . . . . 15 72 5. Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 16 73 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 16 74 5.2. DKIM Author Domain Signing Practices . . . . . . . . . . . 16 75 5.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 17 76 5.4. Exceptions To ADSP Recommendations . . . . . . . . . . . . 18 77 5.5. Author-Related Signing . . . . . . . . . . . . . . . . . . 18 78 5.6. Verification Outcomes at MLMs . . . . . . . . . . . . . . 18 79 5.7. Signature Removal Issues . . . . . . . . . . . . . . . . . 19 80 5.8. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 21 81 5.9. Verification Outcomes at Final Receiving Sites . . . . . . 22 82 5.10. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 22 83 5.11. Handling Choices at Receivers . . . . . . . . . . . . . . 23 84 6. DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 25 85 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 86 8. Security Considerations . . . . . . . . . . . . . . . . . . . 27 87 8.1. Security Considerations from DKIM and ADSP . . . . . . . . 27 88 8.2. Authentication Results When Relaying . . . . . . . . . . . 27 89 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 90 9.1. Normative References . . . . . . . . . . . . . . . . . . . 28 91 9.2. Informative References . . . . . . . . . . . . . . . . . . 28 92 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 30 93 Appendix B. Example Scenarios . . . . . . . . . . . . . . . . . . 31 94 B.1. MLMs and ADSP . . . . . . . . . . . . . . . . . . . . . . 31 95 B.2. MLMs and FBLs . . . . . . . . . . . . . . . . . . . . . . 31 96 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32 98 1. Introduction 100 DomainKeys Identified Mail [DKIM] allows an Administrative Mail 101 Domain to take some responsibility for a [MAIL] message. Such 102 responsibility can be taken by an author's organization, an 103 operational relay (Mail Transfer Agent, or MTA) or one of their 104 agents. Assertion of responsibility is made through a cryptographic 105 signature. Message transit from author to recipient is through 106 relays that typically make no substantive change to the message 107 content and thus preserve the validity of the DKIM signature. 109 In contrast to relays, there are intermediaries, such as mailing list 110 managers (MLMs), that actively take delivery of messages, re-format 111 them, and re-post them, often invalidating DKIM signatures. The goal 112 for this document is to explore the use of DKIM for scenarios that 113 include intermediaries, and recommend Best Current Practices based on 114 acquired experience. Questions that will be discussed include: 116 o Under what circumstances is it advisable for an author, or its 117 organization, to apply DKIM to mail sent to mailing lists? 119 o What are the tradeoffs regarding having an MLM verify and use DKIM 120 identifiers? 122 o What are the tradeoffs regarding having an MLM remove existing 123 DKIM signatures prior to re-posting the message? 125 o What are the tradeoffs regarding having an MLM add its own DKIM 126 signature? 128 These and others are open questions for which there may be no 129 definitive answers. However, based on experience since the 130 publication of the original version of [DKIM] and its gradual 131 deployment, there are some views that are useful to consider and some 132 recommended procedures. 134 In general there are, in relation to DKIM, two categories of MLMs: 135 participating and non-participating. As each type has its own issues 136 regarding DKIM-signed messages that are either handled or produced by 137 them (or both), the types are discussed in separate sections. 139 The best general recommendation for dealing with MLMs is that the MLM 140 or an MTA in the MLM's domain apply its own DKIM signature to each 141 message it forwards, and for assessors on the receiving end to 142 consider the MLM's domain signature in making their assessments. 143 With the understanding that that is not always possible or practical, 144 and the consideration that it might not always be sufficient, this 145 document provides additional guidance. 147 1.1. Background 149 DKIM signatures permit an agent of the email architecture (see 150 [EMAIL-ARCH]) to make a claim of responsibility for a message by 151 affixing a validated domain-level identifier to the message as it 152 passes through a relay. Although not the only possibility, this is 153 most commonly done as a message passes through a boundary Mail 154 Transport Agent (MTA) as it departs an Administrative Mail Domain 155 (ADMD) across the open Internet. 157 A DKIM signature will fail to verify if a portion of the message 158 covered by one of its hashes is altered. An MLM commonly alters 159 messages to provide information specific to the mailing list for 160 which it is providing service. Common modifications are enumerated 161 and described in Section 3.3. However, note that MLMs vary widely in 162 behaviour as well as often allowing subscribers to select individual 163 behaviours. Further, the MTA might make changes that are independent 164 of those applied by the MLM. 166 The DKIM signing specification deliberately rejects the notion of 167 tying the signing domain (the "d=" tag in a DKIM signature) to any 168 other identifier within a message; any ADMD that handles a message 169 could sign it, regardless of its origin or author domain. In 170 particular, DKIM does not define any meaning to the occurrence of a 171 match between the content of a "d=" tag and the value of, for 172 example, a domain name in the RFC5322.From field, nor is there any 173 obvious degraded value to a signature where they do not match. Since 174 any DKIM signature is merely an assertion of "some" responsibility by 175 an ADMD, a DKIM signature added by an MLM has no more, nor less, 176 meaning than a signature with any other "d=" value. 178 1.2. MLMs In Infrastructure 180 An MLM is an autonomous agent that takes delivery of a message and 181 can re-post it as a new message, or construct a digest of it along 182 with other messages to the members of the list (see [EMAIL-ARCH], 183 Section 5.3). However, the fact that the RFC5322.From field of such 184 a message (in the non-digest case) is typically the same as that of 185 the original message, and that recipients perceive the message as 186 "from" the original author rather than the MLM, creates confusion 187 about responsibility and autonomy for the re-posted message. This 188 has important implications for use of DKIM. 190 Section 3.3 describes some of the things MLMs commonly do that 191 produce broken signatures, thus reducing the perceived value of DKIM. 193 Further, while there are published standards that are specific to MLM 194 behaviour (e.g. [MAIL], [LIST-ID] and [LIST-URLS]), their adoption 195 has been spotty at best. Hence, efforts to specify the use of DKIM 196 in the context of MLMs needs to be incremental and value-based. 198 Some MLM behaviours are well-established and their effects on DKIM 199 signature validity can be argued as frustrating wider DKIM adoption. 200 Still, those behaviors are not standards violations. Hence, the best 201 approach for a BCP effort is to specify practices for all parties 202 involved, defining the minimum changes possible to MLMs themselves. 204 A DKIM signature on a message is an expression of some responsibility 205 for the message taken by the signing domain. An open issue that is 206 addressed by this document is the ways a signature might be used by a 207 recipient's evaluation module, after the message has gone through a 208 mailing list and might or might not have been rendered invalid. The 209 document also considers how invalidation might have happened. 211 Note that where in this document there is discussion of an MLM 212 conducting validation of DKIM signatures or ADSP policies, the actual 213 implementation could be one where the validation is done by the MTA 214 or an agent attached to it, and the results of that work are relayed 215 by a trusted channel not specified here. See [AUTH-RESULTS] for a 216 discussion of this. This document does not favour any particular 217 arrangement of these agents over another, but merely talks about the 218 MLM itself doing the work as a matter of simplicity. 220 1.3. Feedback Loops And Other Bi-Lateral Agreements 222 A Feedback Loop (FBL) is a bi-lateral agreement between two parties 223 to exchange reports of abuse. Typically, a sender registers with a 224 receiving site to receive abuse reports from that site for mail 225 coming from the sender. 227 An FBL reporting address (i.e., an address to which FBL reports are 228 sent) is part of this bi-lateral registration. Some FBLs require 229 DKIM use by the registrant. 231 See Section 6 for additional discussion. 233 FBLs tend to use the [ARF] or the [IODEF] formats. 235 1.4. Document Scope and Goals 237 This document provides discussion on the above issues, to improve the 238 handling of possible interactions between DKIM and MLMs. In general, 239 the preference is to impose changes to behaviour at the signer and 240 verifier rather than at the MLM. 242 Wherever possible, the document's discussion of MLMs is conceptually 243 decoupled from MTAs despite the very tight integration that is 244 sometimes observed in implementation. This is done to emphasize the 245 functional independence of MLM services and responsibilities from 246 those of an MTA. 248 Parts of this document explore possible changes to common practice by 249 signers, verifiers and MLMs. The suggested enhancements are largely 250 predictive in nature, taking into account the current email 251 infrastructure, the facilities DKIM can provide as it gains wider 252 deployment, and working group consensus. There is no substantial 253 implementation history upon which these suggestions are based, and 254 the efficacy, performance and security characteristics of them have 255 not yet been fully explored. 257 2. Definitions 259 2.1. Key Words 261 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 262 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 263 "OPTIONAL" in this document are to be interpreted as described in 264 [KEYWORDS]. 266 2.2. Messaging Terms 268 See [EMAIL-ARCH] for a general description of the current messaging 269 architecture, and for definitions of various terms used in this 270 document. 272 2.3. DKIM-Specific References 274 Readers are encouraged to become familiar with [DKIM] and [ADSP], 275 which are core specification documents, as well as [DKIM-OVERVIEW] 276 and [DKIM-DEPLOYMENT], which are DKIM's primary tutorial documents. 278 2.4. 'DKIM-Friendly' 280 The term "DKIM-Friendly" is used to describe an email intermediary 281 that, when handling a message, makes no changes to that message which 282 cause valid [DKIM] signatures present on the message on input to fail 283 to verify on output. 285 Various features of MTAs and MLMs seen as helpful to users often have 286 side effects that do render DKIM signatures unverifiable. These 287 would not qualify for this label. 289 2.5. Message Streams 291 A "message stream" identifies a group of messages originating from 292 within an ADMD that are distinct in intent, origin and/or use, and 293 partitions them somehow (i.e., via changing the value in the "d=" tag 294 value in the context of DKIM) so as to keep them associated to users 295 yet distinct in terms of their evaluation and handling by verifiers 296 or receivers. 298 A good example might be user mail generated by a company's employees, 299 versus operational or transactional mail that comes from automated 300 sources, versus marketing or sales campaigns. Each of these could 301 have different security policies imposed against them, or there might 302 be a desire to insulate one from the other (e.g., a marketing 303 campaign that gets reported by many spam filters could cause the 304 marketing stream's reputation to degrade without automatically 305 punishing the transactional or user streams). 307 3. Mailing Lists and DKIM 309 It is important to make some distinctions among different styles of 310 intermediaries, their typical implementations, and the effects they 311 have in a DKIM-aware environment. 313 3.1. Roles and Realities 315 Across DKIM activities, there are several key roles in the transit of 316 a message. Most of these are defined in [EMAIL-ARCH], but are 317 reviewed here for quick reference. 319 author: The agent that provided the content of the message being 320 sent through the system. The author delivers that content to the 321 originator in order to begin a message's journey to its intended 322 final recipients. The author can be a human using an MUA (Mail 323 User Agent) or a common system utility such as "cron", etc. 325 originator: The agent that accepts a message from the author, 326 ensures it conforms to the relevant standards such as [MAIL], and 327 then sends it toward its destination(s). This is often referred 328 to as the Mail Submission Agent (MSA). 330 signer: Any agent that affixes one or more DKIM signature(s) to a 331 message on its way toward its ultimate destination. There is 332 typically a signer running at the MTA that sits between the 333 author's ADMD and the general Internet. The originator and/or 334 author might also be a signer. 336 verifier: Any agent that conducts DKIM signature analysis. One is 337 typically running at the MTA that sits between the public Internet 338 and the receiver's ADMD. Note that any agent that handles a 339 signed message can conduct verification; this document only 340 considers that action and its outcomes either at an MLM or at the 341 receiver. Filtering decisions could be made by this agent based 342 on verification results. 344 receiver: The agent that is the final transit relay for the message 345 and performs final delivery to the recipient(s) of the message. 346 Filtering decisions based on results made by the verifier could be 347 applied by the receiver. The verifier and the receiver could be 348 the same agent. 350 In the case of simple user-to-user mail, these roles are fairly 351 straightforward. However, when one is sending mail to a list, which 352 then gets relayed to all of that list's subscribers, the roles are 353 often less clear to the general user as particular agents may hold 354 multiple important but separable roles. The above definitions are 355 intended to enable more precise discussion of the mechanisms 356 involved. 358 3.2. Types Of Mailing Lists 360 There are four common MLM implementation modes: 362 aliasing: An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one 363 that makes no changes to the message itself as it redistributes; 364 any modifications are constrained to changes to the [SMTP] 365 envelope recipient list (RCPT commands) only. There are no 366 changes to the message header or body at all, except for the 367 addition of [MAIL] trace header fields. The output of such an MLM 368 is considered to be a continuation of the author's original 369 message transit. An example of such an MLM is an address that 370 expands directly in the MTA, such as a list of local system 371 administrators used for relaying operational or other internal- 372 only messages. See also Section 3.9.2 of [SMTP]. 374 resending: A resending MLM (see Sections 5.2 and 5.3 of 375 [EMAIL-ARCH]) is one that may make changes to a message. The 376 output of such an MLM is considered to be a new message; delivery 377 of the original has been completed prior to distribution of the 378 re-posted message. Such messages are often re-formatted, such as 379 with list-specific header fields or other properties, to 380 facilitate discussion among list subscribers. 382 authoring: An authoring MLM is one that creates the content being 383 sent as well as initiating its transport, rather than basing it on 384 one or more messages received earlier. This is not a "mediator" 385 in terms of [EMAIL-ARCH] since it originates the message, but 386 after creation, its message processing and posting behavior 387 otherwise do match the MLM paradigm. Typically replies are not 388 generated, or if they are, they go to a specific recipient and not 389 back to the list's full set of recipients. Examples include 390 newsletters and bulk marketing mail. 392 digesting: A special case of the resending MLM is one that sends a 393 single message comprising an aggregation of recent MLM 394 submissions, which might be a message of [MIME] type "multipart/ 395 digest" (see [MIME-TYPES]). This is obviously a new message but 396 it may contain a sequence of original messages that may themselves 397 have been DKIM-signed. 399 In the remainder of this document we distinguish two relevant steps, 400 corresponding to the following SMTP transactions: 402 MLM Input: Originating user is author; originating ADMD is 403 originator and signer; MLM's ADMD is verifier; MLM's input 404 function is receiver. 406 MLM Output: MLM (sending its reconstructed copy of the originating 407 user's message) is author; MLM's ADMD is originator and signer; 408 the ADMD of each subscriber of the list is a verifier; each 409 subscriber is a receiver. 411 Much of this document focuses on the resending class of MLM as it has 412 the most direct conflict operationally with DKIM. 414 The dissection of the overall MLM operation into these two distinct 415 phases allows the DKIM-specific issues with respect to MLMs to be 416 isolated and handled in a logical way. The main issue is that the 417 repackaging and reposting of a message by an MLM is actually the 418 construction of a completely new message, and as such the MLM is 419 introducing new content into the email ecosystem, consuming the 420 author's copy of the message and creating its own. When considered 421 in this way, the dual role of the MLM and its ADMD becomes clear. 423 Some issues about these activities are discussed in Section 3.6.4 of 424 [MAIL] and in Section 3.4.1 of [EMAIL-ARCH]. 426 3.3. Current MLM Effects On Signatures 428 As described above, an aliasing MLM does not affect any existing 429 signature, and an authoring MLM is always creating new content and 430 thus there is never an existing signature. However, the changes a 431 resending MLM typically make affect the RFC5322.Subject header field, 432 addition of some list-specific header fields, and/or modification of 433 the message body. The effects of each of these on DKIM verification 434 are discussed below. 436 Subject tags: A popular feature of MLMs is the "tagging" of an 437 RFC5322.Subject field by prefixing the field's contents with the 438 name of the list, such as "[example]" for a list called "example". 439 Altering the RFC5322.Subject field on new submissions by adding a 440 list-specific prefix or suffix will invalidate the signer's 441 signature if that header field was included in the hash when 442 creating that signature. Section 5.5 of [DKIM] lists 443 RFC5322.Subject as one that should be covered as it contains 444 important user-visible text, so this is expected to be an issue 445 for any list that makes such changes. 447 List-specific header fields: Some lists will add header fields 448 specific to list administrative functions such as those defined in 449 [LIST-ID] and [LIST-URLS], or the "Resent-" fields defined in 450 [MAIL]. It is unlikely that a typical MUA would include such 451 fields in an original message, and DKIM is resilient to the 452 addition of header fields in general (see notes about the "h=" tag 453 in Section 3.5 of [DKIM]). Therefore not seen as a concern. 455 Other header fields: Some lists will add or replace header fields 456 such as "Reply-To" or "Sender" in order to establish that the 457 message is being sent in the context of the mailing list, so that 458 the list is identified ("Sender") and any user replies go to the 459 list ("Reply-To"). If these fields were included in the original 460 message, it is possible that one or more of them may have been 461 included in the signature hash, and those signatures will thus be 462 broken. 464 Minor body changes: Some lists prepend or append a few lines to each 465 message to remind subscribers of an administrative URL for 466 subscription issues, or of list policy, etc. Changes to the body 467 will alter the body hash computed at the DKIM verifier, so these 468 will render any existing signatures that cover those portions of 469 the message body unverifiable. [DKIM] includes the capability to 470 limit the length of the body covered by its body hash so that 471 appended text will not interfere with signature validation, but 472 this has security implications. 474 Major body changes: There are some MLMs that make more substantial 475 changes to message bodies when preparing them for re-distribution, 476 such as adding, deleting, reordering, or reformatting [MIME] 477 parts, "flattening" HTML messages into plain text, or inserting 478 headers or footers within HTML messages. Most or all of these 479 changes will invalidate a DKIM signature. 481 MIME part removal: Some MLMs that are MIME-aware will remove large 482 MIME parts from submissions and replace them with URLs to reduce 483 the size of the distributed form of the message and to prevent 484 inadvertent automated malware delivery. Except in some cases 485 where a body length limit is applied in generation of the DKIM 486 signature, the signature will be broken. 488 There reportedly still exist some mailing lists in operation that are 489 actually run manually by a human list manager, whose workings in 490 preparing a message for distribution could include the above or even 491 some other changes. 493 In general, absent a general movement by MLM developers and operators 494 toward more DKIM-friendly practices, an MLM subscriber cannot expect 495 signatures applied before the message was processed by the MLM to be 496 valid on delivery to a receiver. Such an evolution is not expected 497 in the short term due to general development and deployment inertia. 498 Moreover, even if an MLM currently passes messages unmodified such 499 that author signatures validate, it is possible that a configuration 500 change or software upgrade to that MLM will cause that no longer to 501 be true. 503 4. Non-Participating MLMs 505 This section contains a discussion of issues regarding sending DKIM- 506 signed mail to or through an MLM that is not DKIM-aware. 507 Specifically, the header fields introduced by [DKIM] and 508 [AUTH-RESULTS] carry no special meaning to such an MLM. 510 4.1. Author-Related Signing 512 In an idealized world, if an author knows that the MLM to which a 513 message is being sent is a non-participating resending MLM, the 514 author SHOULD be cautious when deciding whether or not to send a 515 signed message to the list. The MLM could make a change that would 516 invalidate the author's signature but not remove it prior to re- 517 distribution. Hence, list recipients would receive a message 518 purportedly from the author but bearing a DKIM signature that would 519 not verify. Some mail filtering software incorrectly penalizes a 520 message containing a DKIM signature that fails verification. This 521 may have detrimental effects outside of the author's control. 522 (Additional discussion of this is below.) This problem can be 523 compounded if there are receivers that apply signing policies (e.g., 524 [ADSP]) and the author publishes any kind of strict policy, i.e., a 525 policy that requests that receivers reject or otherwise deal severely 526 with non-compliant messages. 528 For domains that do publish strict ADSP policies, the originating 529 site SHOULD use a separate message stream (see Section 2.5), such as 530 a signing and author subdomain, for the "personal" mail -- a 531 subdomain that is different from domain(s) used for other mail 532 streams. This allows each to develop an independent reputation, and 533 more stringent policies (including ADSP) can be applied to the mail 534 stream(s) that do not go through mailing lists or perhaps do not get 535 signed at all. 537 However, all of this presupposes a level of infrastructure 538 understanding that is not expected to be common. Thus, it will be 539 incumbent upon site administrators to consider how support of users 540 wishing to participate in mailing lists might be accomplished as DKIM 541 achieves wider adoption. 543 In general, the more strict practices and policies are likely to be 544 successful only for the mail streams subject to the most end-to-end 545 control by the originating organization. That typically excludes 546 mail going through MLMs. Therefore, site administrators wishing to 547 employ ADSP with a "discardable" setting SHOULD separate the 548 controlled mail stream warranting this handling from other mail 549 streams that are less controlled, such as personal mail that transits 550 MLMs. (See also in Section 5.7 below.) 552 4.2. Verification Outcomes at Receivers 554 There is no reliable way to determine that a piece of mail arrived 555 via a non-participating MLM. Sites whose users subscribe to non- 556 participating MLMs SHOULD ensure that such user mail streams are not 557 subject to strict DKIM-related handling policies. 559 4.3. Handling Choices at Receivers 561 In order to exempt some mail from the expectation of signature 562 verification, as discussed in Section 4.1, receiving ADMDs would need 563 to register non-participating lists and confirm that mail transited 564 them. However, such an approach requires excessive effort and even 565 then is likely to be unreliable. Hence, it is not a scalable 566 solution. 568 Any treatment of a verification failure as having special meaning is 569 a violation of the basic DKIM signing specification. The only valid, 570 standardized basis for going beyond that specification is with 571 specific ADSP direction. 573 Use of restrictive domain policies such as [ADSP] "discardable" 574 presents an additional challenge. In that case, when a message is 575 unsigned or the signature can no longer be verified, discarding of 576 the message is requested. There is no exception in the policy for a 577 message that may have been altered by an MLM, nor is there a reliable 578 way to identify such mail. Therefore, participants SHOULD honour the 579 policy and disallow the message. 581 4.4. Wrapping A Non-Participating MLM 583 One approach for adding DKIM support to an otherwise non- 584 participating MLM is to "wrap" the MLM, or in essence place it 585 between other DKIM-aware components (such as MTAs) that provide some 586 DKIM services. For example, the ADMD operating a non-participating 587 MLM could have its DKIM verifier act on messages from list 588 subscribers, enforcing some of the features and recommendations of 589 Section 5 on behalf of the MLM, and the MTA or MSA receiving the MLM 590 Output could also add a DKIM signature for the MLM's domain. 592 5. Participating MLMs 594 This section contains a discussion of issues regarding DKIM-signed 595 mail that transits an MLM which is DKIM-aware. 597 5.1. General 599 Changes that merely add new header fields, such as those specified by 600 [LIST-ID], [LIST-URLS] and [MAIL], are generally the most friendly to 601 a DKIM-participating email infrastructure. Their addition by an MLM 602 will not affect any existing DKIM signatures unless those fields were 603 already present and covered by a signature's hash, or a signature was 604 created specifically to disallow their addition (see the note about 605 "h=" in Section 3.5 of [DKIM]). 607 However, the practice of applying headers and footers to message 608 bodies is common and not expected to fade regardless of what 609 documents this or any standards body might produce. This sort of 610 change will invalidate the signature on a message where the body hash 611 covers the entire message. Thus, the following sections also discuss 612 and suggest other processing alternatives. 614 A possible mitigation to this incompatibility is use of the "l=" tag 615 to bound the portion of the body covered by the DKIM body hash, but 616 this is not workable for [MIME] messages; moreover, it has security 617 considerations (see Section 3.5 of [DKIM]). Its use is therefore 618 discouraged. 620 Expressions of list-specific policy (e.g., rules for participation, 621 small advertisements, etc.) are often added to outgoing messages by 622 MLM operators. There is currently no header field proposed for 623 relaying such general operational MLM details apart from what 624 [LIST-URLS] already supports. This sort of information is commonly 625 included footer text appended to the body of the message, or header 626 text prepended above the original body. It is RECOMMENDED that 627 periodic, automatic mailings to the list are sent to remind 628 subscribers of list policy. It is also RECOMMENDED that the use of 629 standard header fields to express list operation parameters be 630 applied rather than body changes. These periodic mailings will be 631 repetitive, of course, but by being generally the same each time they 632 can be easily filtered if desired. 634 5.2. DKIM Author Domain Signing Practices 636 ADSP presents a particular challenge. An author domain posting a 637 policy of "discardable" imposes a very tight restriction on the use 638 of mailing lists, essentially constraining that domain's users to 639 lists operated by aliasing MLMs only; any MLM that alters a message 640 from such a domain or removes its signature subjects the message to 641 severe action by verifiers or receivers. A resending MLM SHOULD 642 reject outright any mail from an author whose domain posts such a 643 policy, as those messages likely to be discarded or rejected by any 644 ADSP-aware recipients. See also the discussion in Section 5.3. 646 Where such rejection of "discardable" mail is not enforced, and such 647 mail arrives to a verifier that applies ADSP checks which fail, the 648 message SHOULD either be discarded (i.e. accept the message at the 649 [SMTP] level but discard it without delivery) or rejected by 650 returning a 5xx error code. In the latter case, some advice for how 651 to conduct the rejection in a potentially meaningful way can be found 652 in Section 5.11. 654 The reason for these recommendations is best illustrated by example. 655 Suppose the following: 657 o users U1 and U2 are subscribers of list L; 659 o U1 is within an ADMD that advertises a "discardable" policy using 660 ADSP; 662 o L alters submissions prior to re-sending in a way that invalidates 663 the DKIM signature added by U1's ADMD; 665 o U2's ADMD enforces ADSP at the border by issuing an SMTP error 666 code; and 668 o L is configured to remove subscribers whose mail is bouncing. 670 It follows then that a submission to L from U1 will be received at 671 U2, but since the DKIM signature fails to verify, U2's ADMD will 672 reject it based on the ADSP protocol. That rejection is received at 673 L, which proceeds to remove U2 from the list. 675 See also Appendix B.5 of [ADSP] for further discussion. 677 5.3. Subscriptions 679 At subscription time, an ADSP-aware MLM SHOULD check for a published 680 ADSP record for the new subscriber's domain. If the policy specifies 681 "discardable", the MLM SHOULD disallow the subscription or present a 682 warning that the subscriber's submissions to the mailing list might 683 not be deliverable to some recipients because of the subscriber's 684 ADMD's published policy. 686 Of course, such a policy record could be created after subscription, 687 so this is not a universal solution. An MLM implementation MAY do 688 periodic checks of its subscribers and issue warnings where such a 689 policy is detected, or simply check upon each submission. 691 5.4. Exceptions To ADSP Recommendations 693 Where an ADMD has established some out-of-band trust agreement with 694 another ADMD such that an Authentication-Results field applied by one 695 is trusted by the other, the above recommendations for MLM operation 696 with respect to ADSP do not apply because it is then possible to 697 establish whether or not a valid author signature can be inferred 698 even if one is not present on receipt. 700 5.5. Author-Related Signing 702 An important consideration is that authors rarely have any direct 703 influence over the management of an MLM. Specifically, the behavior 704 of an intermediary (e.g., an MLM that is not careful about filtering 705 out junk mail or being diligent about unsubscription requests) can 706 trigger recipient complaints that reflect back on those agents that 707 appear to be responsible for the message, in this case an author via 708 the address found in the RFC5322.From field. In the future, as DKIM 709 signature outputs (i.e., the signing domain) are used as inputs to 710 reputation modules, there may be a desire to insulate one's 711 reputation from influence by the unknown results of sending mail 712 through an MLM. In that case, authors SHOULD create a mail stream 713 specifically used for generating signatures when sending traffic to 714 MLMs. 716 This suggestion can be made more general. Mail that is of a 717 transactional or generally end-to-end nature, and not likely to be 718 forwarded around either by MLMs or users, SHOULD be signed with a 719 different mail stream identifier from a stream that serves more 720 varied uses. 722 5.6. Verification Outcomes at MLMs 724 MLMs typically attempt to authenticate messages posted through them. 725 They usually do this through the trivial (and insecure) means of 726 verifying the RFC5322.From field email address (or, less frequently, 727 the RFC5321.MailFrom parameter) against a list subscription registry. 728 DKIM enables a stronger form of authentication: The MLM can require 729 that messages using a given RFC5322.From address also have a DKIM 730 signature with a corresponding "d=" domain. This feature would be 731 somewhat similar to using ADSP, except that the requirement for it 732 would be imposed by the MLM and not the author's organization. 734 (Note, however, that this goes beyond DKIM's documented semantics. 735 It is presented as a possible workable enhancement.) 736 As described, the MLM might conduct DKIM verification of a signed 737 message to attempt to confirm the identity of the author. Although 738 it is a common and intuitive conclusion, few signed messages will 739 include an author signature (see [ADSP]). MLM implementers adding 740 such support would have accommodate this. For example, an MLM might 741 be designed to accommodate a list of possible signing domains (the 742 "d=" portion of a DKIM signature) for a given author, and determine 743 at verification time if any of those are present. This enables a 744 more reliable method of authentication at the expense of having to 745 store a mapping of authorized signing domains for subscribers and 746 trusting that it will be kept current. 748 A message that cannot be thus authenticated MAY be held for 749 moderation or rejected outright. 751 This logic could apply to any list operation, not just list 752 submission. In particular, this improved authentication MAY apply to 753 subscription, unsubscription, and/or changes to subscriber options 754 that are sent via email rather than through an authenticated, 755 interactive channel such as the web. 757 In the case of verification of signatures on submissions, MLMs SHOULD 758 add an [AUTH-RESULTS] header field to indicate the signature(s) 759 observed on the submission as it arrived at the MLM and what the 760 outcome of the evaluation was. Downstream agents might or might not 761 trust the content of that header field depending on their own a 762 priori knowledge of the operation of the ADMD generating (and, 763 preferably, signing) that header field. See [AUTH-RESULTS] for 764 further discussion. 766 5.7. Signature Removal Issues 768 A message that arrives signed with DKIM means some domain prior to 769 MLM Input has made a claim of some responsibility for the message. 770 An obvious benefit to leaving the input-side signatures intact, then, 771 is to preserve that original assertion of responsibility for the 772 message so that the receivers of the final message have an 773 opportunity to evaluate the message with that information available 774 to them. 776 However, if the MLM is configured to make changes to the message 777 prior to re-posting that would invalidate the original signature(s), 778 further action is RECOMMENDED to prevent invalidated signatures from 779 arriving at final recipients, possibly triggering unwarranted filter 780 actions. (Note, however, that such filtering actions are plainly 781 wrong; [DKIM] stipulates that an invalid signature is to be treated 782 as no signature at all.) 783 A possible solution would be to: 785 1. Attempt verification of all DKIM signatures present on the input 786 message; 788 2. Apply local policy to authenticate the identity of the author; 790 3. Remove all existing [AUTH-RESULTS] fields (optional); 792 4. Add an [AUTH-RESULTS] header field to the message to indicate the 793 results of the above; 795 5. Remove all previously-evaluated DKIM signatures; 797 6. Affix a new signature that includes in in its hashes the entire 798 message on the output side, including the Authentication-Results 799 header field just added (see Section 5.8). 801 Removing the original signature(s) seems particularly appropriate 802 when the MLM knows it is likely to invalidate any or all of them due 803 to the nature of the reformatting it will do. This avoids false 804 negatives at the list's subscribers in their roles as receivers of 805 the message; although [DKIM] stipulates that an invalid signature is 806 the same as no signature, it is anticipated that there will be some 807 implementations that ignore this advice. 809 The MLM could re-evaluate existing signatures after making its 810 message changes to determine whether or not any of them have been 811 invalidated. The cost of this is reduced by the fact that, 812 presumably, the necessary public keys have already been downloaded 813 and one or both of the message hashes could be reused. 815 Per the discussion in [AUTH-RESULTS], a receiver's choice to put any 816 faith in the veracity of that header field requires an a priori 817 assessment of the agent that created it. Absent that assessment, a 818 receiver cannot interpret the field as valid. Thus, the final 819 recipients of the message have no way to verify on their own the 820 authenticity of the author's identity on that message. However, if 821 that field is the only one on the message when the verifier gets it, 822 and the verifier explicitly trusts the signer that included the 823 Authentication-Results field in its header hash (in this case, the 824 MLM), the verifier is in a position to believe that a valid author 825 signature was present on the message. 827 This can be generalized as follows: A receiver SHOULD consider only 828 [AUTH-RESULTS] fields bearing an authserv-id that appears in a list 829 of sites the receiver trusts and which is also included in the header 830 hash of a [DKIM] signature added by a domain in the same trusted 831 list. 833 Since an aliasing MLM makes no substantive changes to a message, it 834 need not consider the issue of signature removal as the original 835 signatures should arrive at least to the next MTA unmodified. It is 836 possible that future domain-based reputations would prefer a more 837 rich data set on receipt of a message, and in that case signature 838 removal would be undesirable. 840 An authoring MLM is closed to outside submitters, thus much of this 841 discussion does not apply in that case. 843 5.8. MLM Signatures 845 DKIM-aware resending MLMs and authoring MLMs SHOULD affix their own 846 signatures when distributing messages. The MLM is responsible for 847 the alterations it makes to the original messages it is re-sending, 848 and should express this via a signature. This is also helpful for 849 getting feedback from any FBLs that might be set up so that undesired 850 list mail can generate appropriate action. 852 MLM signatures will likely be used by recipient systems to recognize 853 list mail, and they give the MLM's ADMD an opportunity to develop a 854 good reputation for the list itself. 856 A signing MLM is, as any other MLM, free to omit redistribution of a 857 message if that message was not signed in accordance with its own 858 local configuration or policy. It could also redistribute but not 859 sign such mail. However, selective signing is NOT RECOMMENDED; 860 essentially that would create two message streams from the MLM, one 861 signed and one not, which can confuse DKIM-aware verifiers and 862 receivers. 864 A signing MLM could add a List-Post: header field (see [LIST-URLS]) 865 using that DNS domain matching the one used in the "d=" tag of the 866 DKIM signature that is added by the MLM. This can be used by 867 verifiers or receivers to identify the DKIM signature that was added 868 by the MLM. This is not required, however; it is believed the 869 reputation of the signer will be a more critical data point rather 870 than this suggested binding. Furthermore, this is not a binding 871 recognized by any current specification document. 873 A DKIM-aware resending MLM SHOULD sign the entire message after the 874 message is prepared for distribution (i.e. the "MLM Output" from 875 Section 3.2). Any other configuration might generate signatures that 876 will not validate. 878 DKIM-aware authoring MLMs MUST sign the mail they send according to 879 the regular signing guidelines given in [DKIM]. 881 One concern is that having an MLM apply its signature to unsigned 882 mail might cause some verifiers or receivers to interpret the 883 signature as conferring more authority or authenticity to the message 884 content than is defined by [DKIM]. This is an issue beyond MLMs and 885 primarily entails receive-side processing outside of the scope of 886 [DKIM]. It is nevertheless worth noting here. 888 5.9. Verification Outcomes at Final Receiving Sites 890 In general, verifiers and receivers SHOULD treat a signed message 891 from an MLM like any other signed message; indeed, it would be 892 difficult to discern any difference since specifications such as 893 [LIST-URLS] and [LIST-ID] are not universally deployed and can be 894 trivially spoofed. 896 However, because the author domain will commonly be different from 897 the MLM's signing domain, there may be a conflict with [ADSP] as 898 discussed in Section 4.3 and Section 5.7, especially where an ADMD 899 has misused ADSP. 901 5.10. Use With FBLs 903 An FBL operator might wish to act on a complaint from a user about a 904 message sent to a list. Some FBLs could choose to generate feedback 905 reports based on DKIM verifications in the subject message. Such 906 operators SHOULD send a report to each domain with a valid signature 907 that has an FBL agreement established, as DKIM signatures are claims 908 of some responsibility for that message. Because authors generally 909 have limited control over the operation of a list, this point makes 910 MLM signing all the more important. 912 MLM operators SHOULD register with FBLs from major service providers. 913 In the context of DKIM, there SHOULD be an exchange of information 914 with the FBL provider including what signing domain the MLM will use, 915 if any. 917 Where the FBL wishes to be more specific, it MAY act solely on a DKIM 918 signature where the signing domain matches the DNS domain found in a 919 List-Post: header field (or similar). 921 Use of FBLs in this way SHOULD be made explicit to list subscribers. 922 For example, if it is the policy of the MLM's ADMD to handle an FBL 923 item by unsubscribing the user that was the apparent sender of the 924 offending message, advising subscribers of this in advance would help 925 to avoid surprises later. 927 A DKIM-signed message sent to an MLM, and then distributed to all of 928 a list's recipients, could result in a complaint from one of the 929 final recipients for some reason. This could be an actual complaint 930 from some subscriber that finds the message abusive or otherwise 931 undesirable, or it could be an automated complaint such as receiver 932 detection of an invalidated DKIM signature or some other condition. 933 It could also be a complaint that results from antagonistic 934 behaviour, such as is common when a subscriber to a list is having 935 trouble unsubscribing, and then begins issuing complaints about all 936 submissions to the list. This would result in a complaint being 937 generated in the context of an FBL report back to the message author. 938 However, the original author has no involvement in operation of the 939 MLM itself, meaning the FBL report is not actionable, and is thus 940 undesirable. 942 5.11. Handling Choices at Receivers 944 A recipient that explicitly trusts signatures from a particular MLM 945 MAY wish to extend that trust to an [AUTH-RESULTS] header field 946 signed by that MLM. The recipient MAY then do additional processing 947 of the message, using the results recorded in the Authentication- 948 Results header field instead of the original author's DKIM signature. 949 This includes possibly processing the message as per ADSP 950 requirements. 952 Receivers SHOULD ignore or remove all unsigned externally-applied 953 Authentication-Results header fields, and those not signed by an ADMD 954 that can be trusted by the receiver. See Section 5 and Section 7 of 955 [AUTH-RESULTS] for further discussion. 957 Upon DKIM and ADSP evaluation during an SMTP session (a common 958 implementation), an agent MAY decide to reject a message during an 959 SMTP session. If this is done, use of an [SMTP] failure code not 960 normally used for "user unknown" (550) is preferred; therefore, 554 961 SHOULD be used. If the rejecting SMTP server supports [ENHANCED] 962 status codes, it SHOULD make a distinction between messages rejected 963 deliberately due to policy decisions rather than those rejected 964 because of other delivery issues. In particular, a policy rejection 965 SHOULD be relayed using a 5.7.1 enhanced status code and some 966 appropriate wording in the text part of the reply, in contrast to a 967 code of 5.1.1 indicating the user does not exist. Those MLMs that 968 automatically attempt to remove users with prolonged delivery 969 problems (such as account deletion) SHOULD thus detect the difference 970 between policy rejection and other delivery failures, and act 971 accordingly. SMTP servers doing so SHOULD also use appropriate 972 wording in the text portion of the reply, perhaps explicitly using 973 the string "ADSP" to facilitate searching of relevant data in logs. 975 The preceding paragraph does not apply to an [ADSP] policy of 976 "discardable". In such cases where the submission fails that test, 977 the receiver or verifier SHOULD discard the message but return an 978 SMTP success code, i.e. accept the message but drop it without 979 delivery. An SMTP rejection of such mail instead of the requested 980 discard action causes more harm than good. 982 6. DKIM Reporting 984 As mechanisms become available for reporting forensic details about 985 DKIM verification failures, MLMs will benefit from their use. 987 MLMs SHOULD apply DKIM failure reporting mechanisms as a method for 988 providing feedback to signers about issues with DKIM infrastructure. 989 This is especially important for MLMs that implement DKIM 990 verification as a mechanism for authentication of list configuration 991 commands and submissions from subscribers. 993 7. IANA Considerations 995 This document includes no IANA actions. It should be removed prior 996 to publication. 998 8. Security Considerations 1000 This document provides suggested or best current practices for use 1001 with DKIM, and as such does not introduce any new technologies for 1002 consideration. However, the following security issues should be 1003 considered when implementing the above practices. 1005 8.1. Security Considerations from DKIM and ADSP 1007 Readers should be familiar with the material in the Security 1008 Considerations in [DKIM], [ADSP] and [AUTH-RESULTS] as appropriate. 1010 8.2. Authentication Results When Relaying 1012 Section 5 advocates addition of an [AUTH-RESULTS] header field to 1013 indicate authentication status of a message received as MLM Input. 1014 Per Section 7.2 of [AUTH-RESULTS], receivers generally should not 1015 trust such data without a good reason to do so, such as an a priori 1016 agreement with the MLM's ADMD. 1018 Such agreements are strongly advised to include a requirement that 1019 those header fields be covered by a [DKIM] signature added by the 1020 MLM's ADMD. 1022 9. References 1024 9.1. Normative References 1026 [ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM 1027 Sender Signing Practises", RFC 5617, August 2009. 1029 [AUTH-RESULTS] 1030 Kucherawy, M., "Message Header Field for Indicating 1031 Message Authentication Status", RFC 5451, April 2009. 1033 [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 1034 Identified Mail (DKIM) Signatures", 1035 I-D draft-ietf-dkim-rfc4871bis, April 2011. 1037 [EMAIL-ARCH] 1038 Crocker, D., "Internet Mail Architecture", RFC 5598, 1039 July 2009. 1041 [KEYWORDS] 1042 Bradner, S., "Key words for use in RFCs to Indicate 1043 Requirement Levels", BCP 14, RFC 2119, March 1997. 1045 [MAIL] Resnick, P., "Internet Message Format", RFC 5322, 1046 October 2008. 1048 9.2. Informative References 1050 [ARF] Shafranovich, Y., Levine, J., and M. Kucherawy, "An 1051 Extensible Format for Email Feedback Reports", RFC 5965, 1052 August 2010. 1054 [DKIM-DEPLOYMENT] 1055 Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, 1056 "DomainKeys Identified Mail (DKIM) Development, Deployment 1057 and Operations", I-D DRAFT-IETF-DKIM-DEPLOYMENT, 1058 January 2010. 1060 [DKIM-OVERVIEW] 1061 Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys 1062 Identified Mail (DKIM) Service Overview", RFC 5585, 1063 July 2009. 1065 [ENHANCED] 1066 Vaudreuil, G., "Enhanced Mail System Status Codes", 1067 RFC 3463, January 2003. 1069 [IODEF] Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident 1070 Object Description Exchange Format", RFC 5070, 1071 December 2007. 1073 [LIST-ID] Chandhok, R. and G. Wenger, "List-Id: A Structured Field 1074 and Namespace for the Identification of Mailing Lists", 1075 RFC 2919, March 2001. 1077 [LIST-URLS] 1078 Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax 1079 for Core Mail List Commands and their Transport through 1080 Message Header Fields", RFC 2369, July 1998. 1082 [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1083 Extensions (MIME) Part One: Format of Internet Message 1084 Bodies", RFC 2045, November 1996. 1086 [MIME-TYPES] 1087 Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1088 Extensions (MIME) Part Two: Media Types", RFC 2046, 1089 November 1996. 1091 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1092 October 2008. 1094 Appendix A. Acknowledgements 1096 The author wishes to acknowledge the following for their review and 1097 constructive criticism of this document: Serge Aumont, Daniel Black, 1098 Dave Crocker, J.D. Falk, Tony Hansen, Eliot Lear, Charles Lindsey, 1099 John Levine, Jeff Macdonald, S. Moonesamy, Rolf E. Sonneveld, and 1100 Alessandro Vesely. 1102 Appendix B. Example Scenarios 1104 This section describes a few MLM-related DKIM scenarios that were 1105 part of the impetus for this work, and the recommended resolutions 1106 for each. 1108 B.1. MLMs and ADSP 1110 Problem: 1112 o author ADMD advertises an ADSP policy of "dkim=discardable" 1114 o author sends DKIM-signed mail to a non-participating MLM, which 1115 invalidates the signature 1117 o receiver MTA checks DKIM and ADSP at SMTP time, and is configured 1118 to reject ADSP failures, so rejects this message 1120 o process repeats a few times, after which the MLM unsubscribes the 1121 receiver 1123 Solution: MLMs should refuse mail from domains advertising ADSP 1124 policies of "discardable" unless the MLMs are certain they make no 1125 changes that invalidate DKIM signatures. 1127 B.2. MLMs and FBLs 1129 Problem: 1131 o subscriber sends signed mail to a non-participating MLM that does 1132 not invalidate the signature 1134 o a recipient reports the message as spam 1136 o FBL at recipient ADMD sends report to contributor rather than list 1137 manager 1139 Solution: MLMs should sign mail they send and might also strip 1140 existing signatures; FBLs should report to list operators instead of 1141 subscribers where such can be distinguished, otherwise to all parties 1142 with valid signatures. 1144 Author's Address 1146 Murray S. Kucherawy 1147 Cloudmark 1148 128 King St., 2nd Floor 1149 San Francisco, CA 94107 1150 US 1152 Phone: +1 415 946 3800 1153 Email: msk@cloudmark.com