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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MARF Working Group J. Falk 3 Internet-Draft Return Path 4 Updates: 5965 (if approved) M. Kucherawy, Ed. 5 Intended status: Standards Track Cloudmark 6 Expires: August 10, 2012 February 7, 2012 8 Creation and Use of Email Feedback Reports: An Applicability Statement 9 for the Abuse Reporting Format (ARF) 10 draft-ietf-marf-as-07 12 Abstract 14 RFC 5965 defines an extensible, machine-readable format intended for 15 mail operators to report feedback about received email to other 16 parties. This document describes common methods for utilizing this 17 format for reporting both abuse and authentication failure events. 18 Mailbox Providers of any size, mail sending entities, and end users 19 can use these methods as a basis to create procedures that best suit 20 them. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on August 10, 2012. 39 Copyright Notice 41 Copyright (c) 2012 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 1. Introduction 56 The Abuse Reporting Format (ARF) was initially developed for two very 57 specific use cases. Initially, it was intended to be used for 58 reporting feedback between large email operators, or from large email 59 operators to end user network access operators, any of whom could be 60 presumed to have automated abuse-handling systems. Secondarily, it 61 is used by those same large mail operators to send those same reports 62 to other entities, including those involved in sending bulk email for 63 commercial purposes. In either case, the reports would be triggered 64 by direct end user action such as clicking on a "report spam" button 65 in their email client. 67 Though other uses for the format defined in [RFC5965] have been 68 discussed (and may be documented similarly in the future), abuse 69 remains the primary application, with a small amount of adoption of 70 extensions that enable authentication failure reporting. This memo 71 provides direction for both contexts. 73 The purpose for reporting abusive messages is to stop recurrences. 74 The methods described in this document focus on automating abuse 75 reporting as much as practical, so as to minimize the work of a 76 site's abuse team. There are further reasons why abuse feedback 77 generation is worthwhile, such as instruction of mail filters or 78 reputation trackers, or to initiate investigations of particularly 79 egregious abuses. These other applications are not discussed in this 80 memo. 82 Further introduction to this topic may be found in [RFC6449]. 84 2. Definitions 86 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 87 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 88 document are to be interpreted as described in [RFC2119], and are 89 intended to replace the Requirement Levels described in Section 3.3 90 of [RFC2026]. 92 Some of the terminology used in this document is taken from 93 [RFC5598]. 95 "Mailbox Provider" refers to an organization that accepts, stores, 96 and offers access to [RFC5322] messages ("email messages") for end 97 users. Such an organization has typically implemented SMTP 98 ([RFC5321]), and might provide access to messages through IMAP 99 ([RFC3501]), POP ([RFC1939]), a proprietary interface designed for 100 HTTP ([RFC2616]), or a proprietary protocol. 102 3. Applicability Statement 104 [RFC Editor: please remove this section prior to publication.] 106 NOTE TO IESG: This document is part of the experiment to reintroduce 107 Applicability Statements, as defined in Section 3.2 of [RFC2026], to 108 the Applications Area. 110 4. Discussion 112 [RFC Editor: please remove this section prior to publication.] 114 This document is being discussed within the IETF MARF Working Group, 115 on the marf@ietf.org mailing list. 117 5. Solicited and Unsolicited Reports 119 The original application of [RFC5965], and still by far the most 120 common, is when two mail systems make a private agreement to exchange 121 abuse reports, usually reports due to recipients manually reporting 122 messages as spam. We refer to these as solicited reports. 124 Other uses for ARF involve reports sent between parties that don't 125 know each other. These unsolicited reports are sent without prior 126 arrangement between the parties as to the context and meaning of the 127 reports, so the constraints on how these unsolicited reports need to 128 be structured such that the reports generated are likely to be useful 129 to the recipient, to what address(es) they can usefully be sent, what 130 issues the can be used to report, and how they can be handled by the 131 receiver of the report are very different. 133 6. Creating and Sending Complaint-Based Solicited Abuse Reports 135 [These numbered items are not intended to be in a paricular sequence. 136 The numbers are here during document development to make it easier to 137 idenify the items for discussion, and will be removed before 138 publication.] 139 1. A Mailbox Provider receives reports of abusive or unwanted mail 140 from its users, most often by providing a "report spam" button 141 (or similar nomenclature) in the MUA. The method of transferring 142 this message and any associated metadata from the MUA to the 143 Mailbox Provider's ARF processing system is not defined by any 144 standards document, but is discussed further in Section 3.2 of 145 [RFC6449]. Policy concerns related to the collection of this 146 data are discussed in Section 3.4 of [RFC6449]. 147 2. The Mailbox Provider SHOULD process the reports to improve its 148 spam filtering systems. The design of these systems is discussed 149 in [RFC2505] and elsewhere. 150 3. The Mailbox Provider SHOULD send reports to relevant parties who 151 have requested to receive such reports. To implement the 152 recommendations of this memo, the reports MUST be formatted per 153 [RFC5965], and transmitted as an email message ([RFC5322]), 154 typically using SMTP ([RFC5321]). The process whereby such 155 parties may request the reports is discussed in Section 3.5 of 156 [RFC6449]. 157 4. The reports SHOULD use "Feedback-Type: abuse", but MAY use other 158 types as appropriate. However, the Mailbox Provider generating 159 the reports SHOULD NOT assume that the operator receiving the 160 reports will treat different Feedback-Types differently. 161 5. The reports SHOULD include the following optional fields whenever 162 practical: Original-Mail-From, Arrival-Date, Source-IP, Original- 163 Rcpt-To. Other optional fields MAY be included, as the 164 implementer feels is appropriate. 165 6. Ongoing maintenance of an ARF processing system is discussed in 166 Section 3.6 of [RFC6449]. 167 7. Reports MAY be subjected to redaction of user-identifiable data 168 as described in [I-D.IETF-MARF-REDACTION]. 170 7. Receiving and Processing Complaint-Based Solicited Abuse Reports 172 [These numbered items are not intended to be in a paricular sequence. 173 The numbers are here during document development to make it easier to 174 idenify the items for discussion, and will be removed before 175 publication.] 177 1. At the time this document is being written, for the use cases 178 described here, mail operators need to proactively request a 179 stream of ARF reports from Mailbox Providers. Recommendations 180 for preparing to make that request are discussed in Section 4.1 181 of [RFC6449]. 182 2. Mail operators MUST be prepared to receive reports formatted per 183 [RFC5965] as email messages ([RFC5322]) over SMTP ([RFC5321]). 184 These and other types of email messages that may be received are 185 discussed in Section 4.2 of [RFC6449]. 187 3. Mail operators need to consider the idea of automating report 188 processing. Discussion of this can be found in Section 4.4 of 189 [RFC6449]. 190 4. An automated report processing system MUST accept all Feedback- 191 Types defined in [RFC5965] or extensions to it, but implementers 192 SHOULD NOT assume that Mailbox Providers will make use of any 193 Feedback-Type other than "abuse". Additional logic may be 194 required to separate different types of abuse reports after 195 receipt. 196 5. Implementers SHOULD NOT expect all Mailbox Providers to include 197 the same optional fields. 198 6. Actions that mail operators might take upon receiving a report 199 (or multiple reports) are discussed in Section 4.3 of [RFC6449]. 200 7. Reports MAY be subjected to redaction of user-identifiable data 201 as described in [I-D.IETF-MARF-REDACTION]. This is also 202 discussed in Section 4.4 of [RFC6449]. Although the end user 203 causing the report to be generated has been obscured, the report 204 processor SHOULD attempt to correlate and prioritize reports that 205 appear to have been caused by the same end user as it may be 206 indicative of a problem worthy of increased attention. 208 8. Generating and Handling Unsolicited Abuse Reports 210 [These numbered items are not intended to be in a paricular sequence. 211 The numbers are here during document development to make it easier to 212 idenify the items for discussion, and will be removed before 213 publication.] 215 The following advice is offered for the case of reports that are not 216 solicited: 217 1. Systems that generate unsolicited reports SHOULD ensure that the 218 criteria used to decide what messages to report accurately 219 identify messages that the reporting entity believes in good 220 faith are abusive. Such criteria might include direct complaint 221 submissions from MUAs, reports triggered by mail sent to "spam 222 trap" or "honeypot" addresses, reports of authentication 223 failures, and virus reports. (These applications might be 224 described in future IETF documents.) Systems SHOULD NOT report 225 all mail sent from a particular sender merely because some of it 226 is determined to be abusive. Mechanical reports of mail that 227 "looks like" spam, based solely on the results of inline content 228 analysis tools, SHOULD NOT be sent since, because of their 229 subjective nature, they are unlikely to provide a basis for the 230 recipient to take action. 231 2. MUAs SHOULD NOT generate abuse reports directly to entities 232 found in the message or by queries to WHOIS or other heuristic 233 means. Rather, the MUA should signal, by some means, the 234 mailbox provider to which it connects to generate such a report. 235 3. Report generators SHOULD send reports to recipients that are 236 both responsible for the messages and are able to do something 237 about them, and SHOULD NOT send reports to recipients that are 238 uninvolved or only peripherally involved. For example, they 239 SHOULD NOT send reports to the operator of every Autonomous 240 System in the path between the apparent originating system and 241 the operator generating the report. 242 4. Deciding where to send an unsolicited report will typically rely 243 on heuristics. Abuse addresses in WHOIS ([RFC3912]) records of 244 the IP address relaying the subject message and/or of the domain 245 name found in the results of a PTR ("reverse lookup") query on 246 that address are likely reasonable candidates, as is the 247 abuse@domain role address (see [RFC2142]) of related domains. 248 Unsolicited reports SHOULD NOT be sent to email addresses that 249 are not intended to handle abuse rpeorts, including any personal 250 or role address found in WHOIS records or on a web site that is 251 not either explicitly described as an abuse contact or is of the 252 form "abuse@domain". 253 5. A report generator MUST provide a way for a report recipient to 254 request no further reports be sent to that address and MAY 255 provide a way for recipients to change the address(es) to which 256 reports about them are sent. 257 6. Where an abusive message is signed using a domain-level 258 authentication technology such as DKIM ([RFC6376]) or SPF 259 ([RFC4408]), the domain that has been verified by the 260 authentication mechanism is likely a reasonable candidate for 261 receiving feedback about the message. However, this is not 262 universally true, since sometimes the domain thus verified 263 exists only to distinguish one stream of mail from another (see 264 Section 2.5 of [RFC6377]), and cannot actually receive email. 265 7. Recipients of unsolicited ARF reports SHOULD, in general, handle 266 them the same way as any other abuse reports. However, they MAY 267 take advantage of the standardized parts of the ARF format to 268 automate processing. Lacking knowledge about the sender of the 269 report, they SHOULD separate valid from invalid reports by, for 270 example, looking for references to IP ranges, domains, and 271 mailboxes for which the recipient organization is responsible in 272 the copy of the reported message, and by correlating multiple 273 reports of similar messages to identify bulk senders. 274 8. Reports SHOULD use "Feedback-Type: abuse", but MAY use other 275 types as appropriate. However, the Mailbox Provider generating 276 the reports SHOULD NOT assume that the operator receiving the 277 reports will treat different Feedback-Types differently. 278 9. Reports SHOULD include the following optional fields whenever 279 practical: Original-Mail-From, Arrival-Date, Source-IP, 280 Original-Rcpt-To. Other optional fields MAY be included, as the 281 implementer feels is appropriate. 283 10. Per Section 4.4 of [RFC6449], a network service provider MAY use 284 ARF data for automated forwarding of feedback messages to the 285 originating customer. 286 11. Published abuse mailbox addresses SHOULD NOT reject messages not 287 in the ARF format, as generation of ARF messages can 288 occasionally be unavailable or not applicable. 289 12. Experience suggests use of ARF is advisable in most contexts. 290 Automated recipient systems can handle abuse reports sent in ARF 291 format at least as well as any other format such as plain text, 292 with or without a copy of the message attached. That holds even 293 for systems that did not request ARF format reports, provided 294 that reports are generated with use by recipients not using 295 automated ARF parsing in mind. Anyone sending unsolicited 296 reports in ARF format can legitimately presume that some 297 recipients will only be able to access the human readable 298 (first, text/plain) part of it, and SHOULD include all 299 information needed also in this part. Further, they SHOULD 300 ensure that the report is readable when viewed as plain text, to 301 give low-end ticketing systems as much assistance as possible. 302 Finally, they need to be aware that the report could be 303 discarded or ignored due to failure to take these steps in the 304 most extreme cases. 305 13. Although [RFC6449] suggests that replying to feedback is not 306 useful, in the case of receipt of ARF reports where no feedback 307 arrangement has been established, a reply might be desirable to 308 indicate that the complaint will result in action, heading off 309 more severe filtering from the report generator. In addition, 310 using an address that cannot receive replies precludes any 311 requests for additional information, and increases the 312 likelihood that further reports will be discarded or blocked. 313 Thus, a report generator sending unsolicited reports SHOULD 314 ensure that a reply to such a report can be received. Where an 315 unsolicited report results in the establishment of contact with 316 a responsible and responsive party, this can be saved for future 317 complaint handling and possible establishment of a formal 318 (solicited) feedback arrangement. See Section 3.5 of [RFC6449] 319 for a discussion of establishment of feedback arrangements. 320 14. Unsolicited reports will have no meaning if sent to abuse 321 reporting addresses belonging to the abusive parties themselves. 322 Reports SHOULD NOT be sent to such addresses if they can be 323 identified beforehand. 324 15. Handling of unsolicited reports has a significant cost to the 325 receiver. Senders of unsolicited reports, especially those 326 sending large volumes of them automatically, need to be aware of 327 this and do all they reasonably can to avoid sending reports 328 that cannot be used as a basis for action by the recipient, 329 whether this is due to the report being sent about an incident 330 that is not abuse-related, the report being sent to an email 331 address that won't result in action, or the content or format of 332 the report being hard for the recipient to read or use. 334 9. Generating Automatic Authentication Failure Reports 336 There are some cases where report generation is caused by automation 337 rather than user request. A specific example of this is reporting, 338 using the ARF format (or extensions to it), of messages that fail 339 particular message authentication checks. Examples of this include 340 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]. 341 The considerations presented below apply in those cases. 343 The applicability statement for this use case is somewhat smaller as 344 many of the issues associated with abuse reports are not relevant to 345 reports about authentication failures. 347 1. Selection of the recipient(s) for reports that are automatically 348 generated MUST be done based on data provided by the report 349 recipient, and MUST NOT be done heuristically. Therefore these 350 reports are always solicited, though the means for doing so are 351 not specified in this memo. 352 2. If the message under evaluation by the Verifier is an ARF 353 ([RFC5965]) message, a report MUST NOT be generated. 354 3. In the case of transmitted reports in the form of a new message 355 (versus rejections during an SMTP ([RFC5321]) session), it is 356 necessary to construct the message so as to avoid amplification 357 attacks, deliberate or otherwise. The envelope sender address of 358 the report needs to be chosen so that these reports will not 359 generate mail loops. Similar to Section 2 of [RFC3464], the 360 envelope sender address of the report SHOULD be chosen to ensure 361 that no feedback reports will be issued in response to the report 362 itself. Therefore, when an SMTP transaction is used to send a 363 report, the MAIL FROM command SHOULD use the NULL return address, 364 i.e., "MAIL FROM:<>". 365 4. Reports SHOULD use "Feedback-Type: auth-failure", but MAY use 366 other types as appropriate. However, the Mailbox Provider 367 generating the reports SHOULD NOT assume that the operator 368 receiving the reports will treat different Feedback-Types 369 differently. 370 5. Reports SHOULD include the following optional fields whenever 371 practical: Original-Mail-From, Arrival-Date, Source-IP, Original- 372 Rcpt-To. Other optional fields MAY be included, as the 373 implementer feels is appropriate. 375 10. IANA Considerations 377 [RFC Editor: please remove this section prior to publication.] 379 This document has no IANA actions. 381 11. Security Considerations 383 11.1. In Other Documents 385 Implementers are strongly urged to review, at a minimum, the Security 386 Considerations sections of [RFC5965] and [RFC6449]. 388 11.2. Forgeries 390 Report generators that relay user complaints directly, rather than by 391 reference to a stored message (e.g., IMAP or POP), could be duped 392 into sending a complaint about a message that the complaining user 393 never actually received, as an attack on the purported originator of 394 the falsified message. Report generators need to be resilient to 395 such attack methods. 397 Also, these reports may be forged as easily as ordinary Internet 398 electronic mail. User agents and automatic mail handling facilities 399 (such as mail distribution list exploders) that wish to make 400 automatic use of reports of any kind should take appropriate 401 precautions to minimize the potential damage from denial-of-service 402 attacks. 404 Perhaps the simplest means of mitigating this threat is to assert 405 that these reports should themselves be signed with something like 406 DKIM. On the other hand, if there is a problem with the DKIM 407 infrastructure at the Verifier, signing DKIM failure reports may 408 produce reports that aren't trusted or even accepted by their 409 intended recipients. 411 11.3. Amplification Attacks 413 Failure to comply with the recommendations regarding selection of the 414 envelope sender can lead to amplification denial-of-service attacks. 416 11.4. Automatic Generation 418 ARF ([RFC5965]) reports have historically been generated individually 419 as a result of some kind of human request, such as someone clicking a 420 "Report Abuse" button in a mail reader. In contrast, the mechanisms 421 described in some extension documents (e.g., 423 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]) are 424 focused around automated reporting. This obviously implies the 425 potential for much larger volumes or frequency of messages, and thus 426 greater mail system load (both for report generators and report 427 receivers). 429 Those mechanisms are primarily intended for use in generating reports 430 to aid implementers of DKIM ([RFC6376]), ADSP ([RFC5617]), and SPF 431 ([RFC4408]), and other related protocols during development and 432 debugging. They are not generally intended for prolonged forensic 433 use, specifically because of these load concerns. However, extended 434 use is possible by ADMDs that want to keep a close watch for fraud or 435 infrastructure problems. It is important to consider the impact of 436 doing so on both report generators and the requesting ADMDs. 438 A sender requesting these reports can cause its mail servers to be 439 overwhelmed if it sends out signed messages whose signatures fail to 440 verify for some reason, provoking a large number of reports from 441 report generators. Similarly, a report generator could be 442 overwhelmed by a large volume of messages requesting reports whose 443 signatures fail to validate, as those now need to send reports back 444 to the signer. 446 Limiting the rate of generation of these messages may be appropriate 447 but threatens to inhibit the distribution of important and possibly 448 time-sensitive information. 450 In general ARF feedback loop terms, it is often suggested that report 451 generators only create these (or any) ARF reports after an out-of- 452 band arrangement has been made between two parties. These extension 453 mechanisms then become ways to adjust parameters of an authorized 454 abuse report feedback loop that is configured and activated by 455 private agreement rather than starting to send them automatically 456 based solely on data found in the messages, which may have unintended 457 consequences. 459 11.5. Reporting Multiple Incidents 461 If it is known that a particular host generates abuse reports upon 462 certain incidents, an attacker could forge a high volume of messages 463 that will trigger such a report. The recipient of the report could 464 then be innundated with reports. This could easily be extended to a 465 distributed denial-of-service attack by finding a number of report- 466 generating servers. 468 The incident count referenced in ARF ([RFC5965]) provides a limited 469 form of mitigation. The host generating reports can elect to send 470 reports only periodically, with each report representing a number of 471 identical or nearly-identical incidents. One might even do something 472 inverse-exponentially, sending reports for each of the first ten 473 incidents, then every tenth incident up to 100, then every 100th 474 incident up to 1000, etc., until some period of relative quiet after 475 which the limitation resets. 477 The use of this for "nearly-identical" incidents in particular causes 478 a degradation in reporting quality, however. If for example a large 479 number of pieces of spam arrive from one attacker, a reporting agent 480 could decide only to send a report about a fraction of those 481 messages. While this averts a flood of reports to a system 482 administrator, the precise details of each incident are similarly not 483 sent. 485 Other rate limiting provisions might be considered, including 486 detection of a temporary failure response from the report destination 487 and thus halting report generation to that destination for some 488 period, or simply imposing or negotiating a hard limit on the number 489 of reports to be sent to a particular receiver in a given time frame. 491 12. Acknowledgements 493 The author and editor wish to thank Steve Atkins, John Levine, Shmuel 494 Metz, and Alessandro Vesely for their contributions to this memo. 496 All of the Best Practices referenced by this document are found in 497 [RFC6449], written within the Collaboration Committee of the 498 Messaging Anti-Abuse Working Group (MAAWG). 500 Finally, the original author wishes to thank the doctors and staff at 501 the University of Texas MD Anderson Cancer Center for doing what they 502 do. 504 13. References 506 13.1. Normative References 508 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 509 Requirement Levels", BCP 14, RFC 2119, March 1997. 511 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 512 October 2008. 514 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 515 October 2008. 517 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 518 July 2009. 520 [RFC5965] Shafranovich, Y., Levine, J., and M. Kucherawy, "An 521 Extensible Format for Email Feedback Reports", RFC 5965, 522 August 2010. 524 13.2. Informative References 526 [I-D.IETF-MARF-DKIM-REPORTING] 527 Kucherawy, M., "Extensions to DKIM for Failure Reporting", 528 I-D draft-ietf-marf-dkim-reporting, January 2012. 530 [I-D.IETF-MARF-REDACTION] 531 Falk, JD. and M. Kucherawy, Ed., "Redaction of Potentially 532 Sensitive Data from Mail Abuse Reports", 533 I-D draft-ietf-marf-redaction, March 2011. 535 [I-D.IETF-MARF-SPF-REPORTING] 536 Kitterman, S., "SPF Authentication Failure Reporting using 537 the Abuse Report Format", 538 I-D draft-ietf-marf-spf-reporting, January 2012. 540 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 541 STD 53, RFC 1939, May 1996. 543 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 544 3", BCP 9, RFC 2026, October 1996. 546 [RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND 547 FUNCTIONS", RFC 2142, May 1997. 549 [RFC2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs", 550 BCP 30, RFC 2505, February 1999. 552 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 553 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 554 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 556 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 557 for Delivery Status Notifications", RFC 3464, 558 January 2003. 560 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 561 4rev1", RFC 3501, March 2003. 563 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 564 September 2004. 566 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 567 for Authorizing Use of Domains in E-Mail, Version 1", 568 RFC 4408, April 2006. 570 [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, 571 "DomainKeys Identified Mail (DKIM) Author Domain Signing 572 Practices (ADSP)", RFC 5617, August 2009. 574 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 575 Identified Mail (DKIM) Signatures", RFC 6376, 576 September 2011. 578 [RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and 579 Mailing Lists", BCP 167, RFC 6377, September 2011. 581 [RFC6449] Falk, J., "Complaint Feedback Loop Operational 582 Recommendations", RFC 6449, November 2011. 584 Authors' Addresses 586 J.D. Falk 587 Return Path 588 100 Mathilda Street, Suite 100 589 Sunnyvale, CA 94089 590 USA 592 Email: ietf@cybernothing.org 593 URI: http://www.returnpath.net/ 595 M. Kucherawy (editor) 596 Cloudmark 597 128 King St., 2nd Floor 598 San Francisco, CA 94107 599 US 601 Email: msk@cloudmark.com