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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group T. Hansen, Ed. 3 Internet-Draft AT&T Laboratories 4 Obsoletes: 3798 (if approved) A. Melnikov, Ed. 5 Updates: 2046, 3461 (if approved) Isode Ltd 6 Intended status: Standards Track December 1, 2016 7 Expires: June 4, 2017 9 Message Disposition Notification 10 draft-ietf-appsawg-mdn-3798bis-16.txt 12 Abstract 14 This memo defines a MIME content-type that may be used by a mail user 15 agent (MUA) or electronic mail gateway to report the disposition of a 16 message after it has been successfully delivered to a recipient. 17 This content-type is intended to be machine-processable. Additional 18 message header fields are also defined to permit Message Disposition 19 Notifications (MDNs) to be requested by the sender of a message. The 20 purpose is to extend Internet Mail to support functionality often 21 found in other messaging systems, such as X.400 and the proprietary 22 "LAN-based" systems, and often referred to as "read receipts," 23 "acknowledgements", or "receipt notifications." The intention is to 24 do this while respecting privacy concerns, which have often been 25 expressed when such functions have been discussed in the past. 27 Because many messages are sent between the Internet and other 28 messaging systems (such as X.400 or the proprietary "LAN-based" 29 systems), the MDN protocol is designed to be useful in a multi- 30 protocol messaging environment. To this end, the protocol described 31 in this memo provides for the carriage of "foreign" addresses, in 32 addition to those normally used in Internet Mail. Additional 33 attributes may also be defined to support "tunneling" of foreign 34 notifications through Internet Mail. 36 This document obsoletes RFC 3798, moving it to Internet Standard. It 37 also updates RFC 2046 (message/partial Media Type handling) and RFC 38 3461 (Original-Recipient header field generation requirement). 40 Status of This Memo 42 This Internet-Draft is submitted in full conformance with the 43 provisions of BCP 78 and BCP 79. 45 Internet-Drafts are working documents of the Internet Engineering 46 Task Force (IETF). Note that other groups may also distribute 47 working documents as Internet-Drafts. The list of current Internet- 48 Drafts is at http://datatracker.ietf.org/drafts/current/. 50 Internet-Drafts are draft documents valid for a maximum of six months 51 and may be updated, replaced, or obsoleted by other documents at any 52 time. It is inappropriate to use Internet-Drafts as reference 53 material or to cite them other than as "work in progress." 55 This Internet-Draft will expire on June 4, 2017. 57 Copyright Notice 59 Copyright (c) 2016 IETF Trust and the persons identified as the 60 document authors. All rights reserved. 62 This document is subject to BCP 78 and the IETF Trust's Legal 63 Provisions Relating to IETF Documents 64 (http://trustee.ietf.org/license-info) in effect on the date of 65 publication of this document. Please review these documents 66 carefully, as they describe your rights and restrictions with respect 67 to this document. Code Components extracted from this document must 68 include Simplified BSD License text as described in Section 4.e of 69 the Trust Legal Provisions and are provided without warranty as 70 described in the Simplified BSD License. 72 Table of Contents 74 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 75 1.1. Purposes . . . . . . . . . . . . . . . . . . . . . . . . 3 76 1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . 4 77 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 78 2. Requesting Message Disposition Notifications . . . . . . . . 5 79 2.1. The Disposition-Notification-To Header . . . . . . . . . 5 80 2.2. The Disposition-Notification-Options Header . . . . . . . 7 81 2.3. The Original-Recipient Header Field . . . . . . . . . . . 8 82 2.4. Use with the Message/Partial Media Type . . . . . . . . . 9 83 3. Format of a Message Disposition Notification . . . . . . . . 10 84 3.1. The message/disposition-notification Media Type . . . . . 11 85 3.2. Message/disposition-notification Content Fields . . . . . 14 86 3.3. Extension-fields . . . . . . . . . . . . . . . . . . . . 21 87 4. Timeline of events . . . . . . . . . . . . . . . . . . . . . 21 88 5. Conformance and Usage Requirements . . . . . . . . . . . . . 22 89 6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 90 6.1. Forgery . . . . . . . . . . . . . . . . . . . . . . . . . 23 91 6.2. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 24 92 6.2.1. Disclosure of Product Information . . . . . . . . . . 25 93 6.2.2. MUA Fingerprinting . . . . . . . . . . . . . . . . . 25 94 6.3. Non-Repudiation . . . . . . . . . . . . . . . . . . . . . 25 95 6.4. Mail Bombing . . . . . . . . . . . . . . . . . . . . . . 25 96 7. Collected ABNF Grammar . . . . . . . . . . . . . . . . . . . 26 97 8. Guidelines for Gatewaying MDNs . . . . . . . . . . . . . . . 28 98 8.1. Gatewaying from other mail systems to MDNs . . . . . . . 28 99 8.2. Gatewaying from MDNs to other mail systems . . . . . . . 29 100 8.3. Gatewaying of MDN-requests to other mail systems . . . . 29 101 9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 102 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 103 10.1. Disposition-Notification-Options header field 104 disposition-notification-parameter names . . . . . . . . 32 105 10.2. Disposition modifier names . . . . . . . . . . . . . . . 33 106 10.3. MDN extension field names . . . . . . . . . . . . . . . 33 107 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33 108 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 109 12.1. Normative References . . . . . . . . . . . . . . . . . . 34 110 12.2. Informative References . . . . . . . . . . . . . . . . . 35 111 Appendix A. Changes from RFC 3798 . . . . . . . . . . . . . . . 36 112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 114 1. Introduction 116 This memo defines a media type [RFC2046] for message disposition 117 notifications (MDNs). An MDN can be used to notify the sender of a 118 message of any of several conditions that may occur after successful 119 delivery, such as display of the message contents, printing of the 120 message, deletion (without display) of the message, or the 121 recipient's refusal to provide MDNs. The "message/disposition- 122 notification" content-type defined herein is intended for use within 123 the framework of the "multipart/report" content type defined in RFC- 124 REPORT [RFC6522]. 126 This memo defines the format of the notifications and the RFC-MSGFMT 127 [RFC5322] header fields used to request them. 129 This memo is an update to RFC 3798 and is intended to be published at 130 Internet Standard Level. 132 1.1. Purposes 134 The MDNs defined in this memo are expected to serve several purposes: 136 a. Inform human beings of the disposition of messages after 137 successful delivery, in a manner that is largely independent of 138 human language; 140 b. Allow mail user agents to keep track of the disposition of 141 messages sent, by associating returned MDNs with earlier message 142 transmissions; 144 c. Convey disposition notification requests and disposition 145 notifications between Internet Mail and "foreign" mail systems 146 via a gateway; 148 d. Allow "foreign" notifications to be tunneled through a MIME- 149 capable message system and back into the original messaging 150 system that issued the original notification, or even to a third 151 messaging system; 153 e. Allow language-independent, yet reasonably precise, indications 154 of the disposition of a message to be delivered. 156 1.2. Requirements 158 These purposes place the following constraints on the notification 159 protocol: 161 a. It must be readable by humans, and must be machine-parsable. 163 b. It must provide enough information to allow message senders (or 164 their user agents) to unambiguously associate an MDN with the 165 message that was sent and the original recipient address for 166 which the MDN was issued (if such information is available), even 167 if the message was forwarded to another recipient address. 169 c. It must also be able to describe the disposition of a message 170 independent of any particular human language or of the 171 terminology of any particular mail system. 173 d. The specification must be extensible in order to accommodate 174 future requirements. 176 1.3. Terminology 178 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 179 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 180 document are to be interpreted as described in RFC-KEYWORDS 181 [RFC2119]. 183 All syntax descriptions use the ABNF specified by RFC-MSGFMT 184 [RFC5322], in which the lexical tokens (used below) are defined: 185 "CRLF", "FWS", "CFWS", "field-name", "mailbox-list", "msg-id", and 186 "text". The following lexical tokens are defined in RFC-SMTP 187 [RFC5321]: "atom". 189 2. Requesting Message Disposition Notifications 191 Message disposition notifications are requested by including a 192 Disposition-Notification-To header field in the message containing 193 one or more addresses specifying where dispositions should be sent. 194 Further information to be used by the recipient's Mail User Agent 195 (MUA) [RFC5598] in generating the MDN may be provided by also 196 including Original-Recipient and/or Disposition-Notification-Options 197 header fields in the message. 199 2.1. The Disposition-Notification-To Header 201 A request for the receiving user agent to issue message disposition 202 notifications is made by placing a Disposition-Notification-To header 203 field into the message. The syntax of the header field is 205 mdn-request-header = "Disposition-Notification-To" ":" mailbox-list CRLF 207 A Disposition-Notification-To header field can appear at most once in 208 a message. 210 The presence of a Disposition-Notification-To header field in a 211 message is merely a request for an MDN. The recipients' user agents 212 are always free to silently ignore such a request. 214 An MDN MUST NOT itself have a Disposition-Notification-To header 215 field. An MDN MUST NOT be generated in response to an MDN. 217 A user agent MUST NOT issue more than one MDN on behalf of each 218 particular recipient. That is, once an MDN has been issued on behalf 219 of a recipient, no further MDNs may be issued on behalf of that 220 recipient by the same user agent, even if another disposition is 221 performed on the message. However, if a message is forwarded, an MDN 222 may have been issued for the recipient doing the forwarding and the 223 recipient of the forwarded message may also cause an MDN to be 224 generated. 226 It is also possible that if the same message is being accessed by 227 multiple user agents (for example using POP3), then multiple 228 dispositions might be generated for the same recipient. User agents 229 SHOULD leverage support in the underlying message access protocol to 230 prevent multiple MDNs from being generated. In particular, when the 231 user agent is accessing the message using RFC-IMAP [RFC3501], it 232 SHOULD implement the procedures specified in RFC-IMAP-MDN [RFC3503]. 234 While Internet standards normally do not specify the behavior of user 235 interfaces, it is strongly recommended that the user agent obtain the 236 user's consent before sending an MDN. This consent could be obtained 237 for each message through some sort of prompt or dialog box, or 238 globally through the user's setting of a preference. The user might 239 also indicate globally that MDNs are to never be sent. The purpose 240 of obtaining user's consent is to protect user's privacy. The 241 default value should be not to send MDNs. 243 MDNs MUST NOT be sent automatically if the address in the 244 Disposition-Notification-To header field differs from the address in 245 the Return-Path header field (see RFC-MSGFMT [RFC5322]). In this 246 case, confirmation from the user MUST be obtained, if possible. If 247 obtaining consent is not possible (e.g., because the user is not 248 online at the time or the client is not an interactive email client), 249 then an MDN MUST NOT be sent. 251 Confirmation from the user MUST be obtained (or no MDN sent) if there 252 is no Return-Path header field in the message, or if there is more 253 than one distinct address in the Disposition-Notification-To header 254 field. 256 The comparison of the addresses is done using only the addr-spec 257 (local-part "@" domain) portion, excluding any angle brackets, phrase 258 and route. As prescribed by RFC 5322, the comparison is case- 259 sensitive for the local-part and case-insensitive for the domain 260 part. The local-part comparison SHOULD be done after performing 261 local-part canonicalization (i.e. after removing the surrounding 262 double-quote characters, if any, as well as any escaping "\" 263 characters. (See RFC-MSGFMT [RFC5322] for more details.) 264 Implementations MAY treat known domain aliases as equivalent for the 265 purpose of comparison. 267 Note that use of subaddressing (see [RFC5233]) can result in a 268 failure to match two local-parts and thus result in possible 269 suppression of the MDN. This document doesn't recommend special 270 handling for this case, as the receiving MUA can't reliably know 271 whether or not the sender is using subaddressing. 273 If the message contains more than one Return-Path header field, the 274 implementation may pick one to use for the comparison, or treat the 275 situation as a failure of the comparison. 277 The reason for not automatically sending an MDN if the comparison 278 fails or more than one address is specified is to reduce the 279 possibility of mail loops and of MDNs being used for mail bombing. 281 It's especially important that a message that contains a Disposition- 282 Notification-To header field also contain a Message-ID header field, 283 to permit user agents to automatically correlate MDNs with their 284 original messages. 286 If the request for message disposition notifications for some 287 recipients and not others is desired, two copies of the message 288 should be sent, one with a Disposition-Notification-To header field 289 and one without. Many of the other header fields of the message 290 (e.g., To, Cc) will be the same in both copies. The recipients in 291 the respective message envelopes determine from whom message 292 disposition notifications are requested and from whom they are not. 293 If desired, the Message-ID header field may be the same in both 294 copies of the message. Note that there are other situations (e.g., 295 Bcc) in which it is necessary to send multiple copies of a message 296 with slightly different header fields. The combination of such 297 situations and the need to request MDNs for a subset of all 298 recipients may result in more than two copies of a message being 299 sent, some with a Disposition-Notification-To header field and some 300 without. 302 If it is possible to determine that a recipient is a newsgroup, do 303 not include a Disposition-Notification-To header field for that 304 recipient. Similarly, if an existing message is resent or gatewayed 305 to a newsgroup, the agent doing resending/gatewaying SHOULD strip the 306 Disposition-Notification-To header field. See Section 5 for more 307 discussion. Clients that see an otherwise valid Disposition- 308 Notification-To header field in a newsgroup message SHOULD NOT 309 generate an MDN. 311 2.2. The Disposition-Notification-Options Header 313 Extensions to this specification may require that information be 314 supplied to the recipient's MUA for additional control over how and 315 what MDNs are generated. The Disposition-Notification-Options header 316 field provides an extensible mechanism for such information. The 317 syntax of this header field is as follows: 319 Disposition-Notification-Options = 320 "Disposition-Notification-Options" ":" [FWS] 321 disposition-notification-parameter-list CRLF 323 disposition-notification-parameter-list = 324 disposition-notification-parameter 325 *([FWS] ";" [FWS] disposition-notification-parameter) 327 disposition-notification-parameter = attribute [FWS] "=" 328 [FWS] importance [FWS] "," [FWS] value *([FWS] "," [FWS] value) 330 importance = "required" / "optional" 332 attribute = atom 334 value = word 336 A Disposition-Notification-Options header field can appear at most 337 once in a message. 339 An importance of "required" indicates that interpretation of the 340 disposition-notification-parameter is necessary for proper generation 341 of an MDN in response to this request. An importance of "optional" 342 indicates that an MUA that does not understand the meaning of this 343 disposition-notification-parameter MAY generate an MDN in response 344 anyway, ignoring the value of the disposition-notification-parameter. 346 No disposition-notification-parameter attribute names are defined in 347 this specification. Attribute names may be defined in the future by 348 later revisions or extensions to this specification. disposition- 349 notification-parameter attribute names MUST be registered with the 350 Internet Assigned Numbers Authority (IANA) using "Specification 351 required" registration policy. The "X-" prefix has historically been 352 used to denote unregistered "experimental" protocol elements, that 353 are assumed not to become common use. Deployment experience of this 354 and other protocols have shown that this assumption is often false. 355 This document allows the use of the "X-" prefix primarily to allow 356 the registration of attributes that are already in common use. The 357 prefix has no meaning for new attributes. Its use in substantially 358 new attributes may cause confusion and is therefore discouraged. 359 (See Section 10 for a registration form.) 361 2.3. The Original-Recipient Header Field 363 Since electronic mail addresses may be rewritten while the message is 364 in transit, it is useful for the original recipient address to be 365 made available by the delivering Message Transfer Agent (MTA) 366 [RFC5598]. The delivering MTA may be able to obtain this information 367 from the ORCPT parameter of the SMTP RCPT TO command, as defined in 368 RFC-SMTP [RFC5321] and RFC-DSN-SMTP [RFC3461]. 370 RFC-DSN-SMTP [RFC3461] is amended as follows: If the ORCPT 371 information is available, the delivering MTA SHOULD insert an 372 Original-Recipient header field at the beginning of the message 373 (along with the Return-Path header field). The delivering MTA MAY 374 delete any other Original-Recipient header fields that occur in the 375 message. The syntax of this header field is as follows: 377 original-recipient-header = 378 "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS 380 OWS = [CFWS] 381 ; Optional whitespace. 382 ; MDN generators SHOULD use "*WSP" 383 ; (typically a single space or nothing. 384 ; It SHOULD be nothing at the end of a field), 385 ; unless an RFC 5322 "comment" is required. 386 ; 387 ; MDN parsers MUST parse it as "[CFWS]". 389 The address-type and generic-address token are as specified in the 390 description of the Original-Recipient field in Section 3.2.3. 392 The purpose of carrying the original recipient information and 393 returning it in the MDN is to permit automatic correlation of MDNs 394 with the original message on a per-recipient basis. 396 2.4. Use with the Message/Partial Media Type 398 The use of the header fields Disposition-Notification-To, 399 Disposition-Notification-Options, and Original-Recipient with the 400 MIME message/partial content type (RFC-MIME-MEDIA [RFC2046]]) 401 requires further definition. 403 When a message is segmented into two or more message/partial 404 fragments, the three header fields mentioned in the above paragraph 405 SHOULD be placed in the "inner" or "enclosed" message (using the 406 terms of RFC-MIME-MEDIA [RFC2046]). If these header fields are found 407 in the header fields of any of the fragments, they are ignored. 409 When the multiple message/partial fragments are reassembled, the 410 following applies. If these header fields occur along with the other 411 header fields of a message/partial fragment message, they pertain to 412 an MDN that will be generated for the fragment. If these header 413 fields occur in the header fields of the "inner" or "enclosed" 414 message (using the terms of RFC-MIME-MEDIA [RFC2046]), they pertain 415 to an MDN that will be generated for the reassembled message. 416 Section 5.2.2.1 of RFC-MIME-MEDIA [RFC2046]) is amended to specify 417 that, in addition to the header fields specified there, the three 418 header fields described in this specification are to be appended, in 419 order, to the header fields of the reassembled message. Any 420 occurrences of the three header fields defined here in the header 421 fields of the initial enclosing message MUST NOT be copied to the 422 reassembled message. 424 3. Format of a Message Disposition Notification 426 A message disposition notification is a MIME message with a top-level 427 content-type of multipart/report (defined in RFC-REPORT [RFC6522]). 428 When multipart/report content is used to transmit an MDN: 430 a. The report-type parameter of the multipart/report content is 431 "disposition-notification". 433 b. The first component of the multipart/report contains a human- 434 readable explanation of the MDN, as described in RFC-REPORT 435 [RFC6522]. 437 c. The second component of the multipart/report is of content-type 438 message/disposition-notification, described in Section 3.1 of 439 this document. 441 d. If the original message or a portion of the message is to be 442 returned to the sender, it appears as the third component of the 443 multipart/report. The decision of whether or not to return the 444 message or part of the message is up to the MUA generating the 445 MDN. However, in the case of encrypted messages requesting MDNs, 446 if the original message or a portion thereof is returned, it MUST 447 be in its original encrypted form. 449 NOTE: For message disposition notifications gatewayed from foreign 450 systems, the header fields of the original message may not be 451 available. In this case, the third component of the MDN may be 452 omitted, or it may contain "simulated" RFC-MSGFMT [RFC5322] header 453 fields that contain equivalent information. In particular, it is 454 very desirable to preserve the subject and date fields from the 455 original message. 457 The MDN MUST be addressed (in both the message header field and the 458 transport envelope) to the address(es) from the Disposition- 459 Notification-To header field from the original message for which the 460 MDN is being generated. 462 The From header field of the MDN MUST contain the address of the 463 person for whom the message disposition notification is being issued. 465 The envelope sender address (i.e., SMTP "MAIL FROM") of the MDN MUST 466 be null (<>), specifying that no Delivery Status Notification 467 messages nor other messages indicating successful or unsuccessful 468 delivery are to be sent in response to an MDN. 470 A message disposition notification MUST NOT itself request an MDN. 471 That is, it MUST NOT contain a Disposition-Notification-To header 472 field. 474 The Message-ID header field (if present) for an MDN MUST be different 475 from the Message-ID of the message for which the MDN is being issued. 477 A particular MDN describes the disposition of exactly one message for 478 exactly one recipient. Multiple MDNs may be generated as a result of 479 one message submission, one per recipient. However, due to the 480 circumstances described in Section 2.1, it's possible that some of 481 the recipients for whom MDNs were requested will not generate MDNs. 483 3.1. The message/disposition-notification Media Type 485 The message/disposition-notification Media Type is defined as 486 follows: 488 Type name: message 490 Subtype name: disposition-notification 492 Required parameters: none 494 Optional parameters: none 496 Encoding considerations: "7bit" encoding is sufficient and MUST be 497 used to maintain readability when viewed by non- 498 MIME mail readers. 500 Security considerations: discussed in Section 6 of [RFCXXXX]. 502 Interoperability considerations: none 504 Published specification: [RFCXXXX] 506 Applications that use this media type: Mail Transfer Agents and 507 email clients that support multipart/report 508 generation and/or parsing. 510 Fragment identifier considerations: N/A 512 Additional information: 514 Deprecated alias names for this type: N/A 516 Magic number(s): none 518 File extension(s): .disposition-notification 520 Macintosh file type code(s): The 'TEXT' type 521 code is suggested as files of this type are 522 typically used for diagnostic purposes and 523 suitable for analysis in a text editor. A 524 uniform type identifier (UTI) of "public.utf8- 525 email-message-header" is suggested. This type 526 conforms to "public.plain-text". 528 Person & email address to contact for further information: ART Area 529 Mailing List 531 Intended usage: COMMON 533 Restrictions on usage: This media type contains textual data in the 534 US-ASCII charset, which is always 7-bit. 536 Author: See the Authors' Addresses section of [RFCXXXX] 538 Change controller: IETF 540 Provisional registration? no 541 (While the 7bit restriction applies to the message/disposition- 542 notification portion of the multipart/report content, it does not 543 apply to the optional third portion of the multipart/report content.) 545 The message/disposition-notification report type for use in the 546 multipart/report is "disposition-notification". 548 The body of a message/disposition-notification consists of one or 549 more "fields" formatted according to the ABNF of RFC-MSGFMT [RFC5322] 550 header "fields". The syntax of the message/disposition-notification 551 content is as follows: 553 disposition-notification-content = [ reporting-ua-field CRLF ] 554 [ mdn-gateway-field CRLF ] 555 [ original-recipient-field CRLF ] 556 final-recipient-field CRLF 557 [ original-message-id-field CRLF ] 558 disposition-field CRLF 559 *( error-field CRLF ) 560 *( extension-field CRLF ) 562 extension-field = extension-field-name ":" *([FWS] text) 564 extension-field-name = field-name 566 Note that the order of the above fields is recommended, but not 567 fixed. Extension fields can appear anywhere. 569 3.1.1. General conventions for fields 571 Since these fields are defined according to the rules of RFC-MSGFMT 572 [RFC5322], the same conventions for continuation lines and comments 573 apply. Notification fields may be continued onto multiple lines by 574 beginning each additional line with a SPACE or HTAB. Text that 575 appears in parentheses is considered a comment and not part of the 576 contents of that notification field. Field names are case- 577 insensitive, so the names of notification fields may be spelled in 578 any combination of upper and lower case letters. [RFC5322] comments 579 in notification fields may use the "encoded-word" construct defined 580 in RFC-MIME-HEADER [RFC2047]. 582 3.1.2. "*-type" subfields 584 Several fields consist of a "-type" subfield, followed by a semi- 585 colon, followed by "*text". 586 For these fields, the keyword used in the address-type or MTA-type 587 subfield indicates the expected format of the address or MTA-name 588 that follows. 590 The "-type" subfields are defined as follows: 592 a. An "address-type" specifies the format of a mailbox address. For 593 example, Internet Mail addresses use the "rfc822" address-type. 594 Other values can appear in this field as specified in the 595 "Address Types" IANA subregistry established by RFC-DSN-FORMAT 596 [RFC3464]. 598 address-type = atom 600 atom = 602 b. An "MTA-name-type" specifies the format of a mail transfer agent 603 name. For example, for an SMTP server on an Internet host, the 604 MTA name is the domain name of that host, and the "dns" MTA-name- 605 type is used. Other values can appear in this field as specified 606 in the "MTA Name Types" IANA subregistry established by RFC-DSN- 607 FORMAT [RFC3464]. 609 mta-name-type = atom 611 Values for address-type and mta-name-type are case-insensitive. 612 Thus, address-type values of "RFC822" and "rfc822" are equivalent. 614 The Internet Assigned Numbers Authority (IANA) maintains a registry 615 of address-type and mta-name-type values, along with descriptions of 616 the meanings of each, or a reference to one or more specifications 617 that provide such descriptions. (The "rfc822" address-type is 618 defined in RFC-DSN-SMTP [RFC3461].) Registration forms for address- 619 type and mta-name-type appear in RFC-DSN-FORMAT [RFC3464]. 621 3.2. Message/disposition-notification Content Fields 623 3.2.1. The Reporting-UA field 625 reporting-ua-field = "Reporting-UA" ":" OWS ua-name OWS [ ";" OWS ua-product OWS ] 627 ua-name = *text-no-semi 629 ua-product = *([FWS] text) 631 text-no-semi = %d1-9 / ; "text" characters excluding NUL, CR, 632 %d11 / %d12 / %d14-58 / %d60-127 ; LF, or semi-colon 634 The Reporting-UA field is defined as follows: 636 An MDN describes the disposition of a message after it has been 637 delivered to a recipient. In all cases, the Reporting-UA is the MUA 638 that performed the disposition described in the MDN. 640 The "Reporting-UA" field contains information about the MUA that 641 generated the MDN, which is often used by servers to help identify 642 the scope of reported interoperability problems, to work around or 643 tailor responses to avoid particular MUA limitations, and for 644 analytics regarding MUA or operating system use. A MUA SHOULD send a 645 "Reporting-UA" field unless specifically configured not to do so. 647 If the reporting MUA consists of more than one component (e.g., a 648 base program and plug-ins), this may be indicated by including a list 649 of product names. 651 A reporting MUA SHOULD limit generated product identifiers to what is 652 necessary to identify the product; a sender MUST NOT generate 653 advertising or other nonessential information within the product 654 identifier. 656 A reporting MUA SHOULD NOT generate a "Reporting-UA" field containing 657 needlessly fine-grained detail and SHOULD limit the addition of 658 subproducts by third parties. Overly long and detailed "Reporting- 659 UA" field values increase the risk of a user being identified against 660 their wishes ("fingerprinting"). 662 Likewise, implementations are encouraged not to use the product 663 tokens of other implementations in order to declare compatibility 664 with them, as this circumvents the purpose of the field. If a MUA 665 masquerades as a different MUA, recipients can assume that the user 666 intentionally desires to see responses tailored for that identified 667 MUA, even if they might not work as well for the actual MUA being 668 used. 670 Example: 672 Reporting-UA: Foomail 97.1 674 3.2.2. The MDN-Gateway field 676 The MDN-Gateway field indicates the name of the gateway or MTA that 677 translated a foreign (non-Internet) message disposition notification 678 into this MDN. This field MUST appear in any MDN that was translated 679 by a gateway from a foreign system into MDN format, and MUST NOT 680 appear otherwise. 682 mdn-gateway-field = "MDN-Gateway" ":" OWS mta-name-type OWS ";" OWS mta-name OWS 684 mta-name = *text 686 For gateways into Internet Mail, the MTA-name-type will normally be 687 "dns", and the mta-name will be the Internet domain name of the 688 gateway. 690 3.2.3. Original-Recipient field 692 The Original-Recipient field indicates the original recipient address 693 as specified by the sender of the message for which the MDN is being 694 issued. For Internet Mail messages, the value of the Original- 695 Recipient field is obtained from the Original-Recipient header field 696 from the message for which the MDN is being generated. If there is 697 an Original-Recipient header field in the message, or if information 698 about the original recipient is reliably available some other way, 699 then the Original-Recipient field MUST be included. Otherwise, the 700 Original-Recipient field MUST NOT be included. If there is more than 701 one Original-Recipient header field in the message, the MUA may 702 choose the one to use, or act as if no Original-Recipient header 703 field is present. 705 original-recipient-field = 706 "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS 708 generic-address = *text 710 The address-type field indicates the type of the original recipient 711 address. If the message originated within the Internet, the address- 712 type field will normally be "rfc822", and the address will be 713 according to the syntax specified in RFC-MSGFMT [RFC5322]. The value 714 "unknown" should be used if the Reporting MUA cannot determine the 715 type of the original recipient address from the message envelope. 716 This address is the same as that provided by the sender and can be 717 used to automatically correlate MDN reports with original messages on 718 a per recipient basis. 720 3.2.4. Final-Recipient field 722 The Final-Recipient field indicates the recipient for which the MDN 723 is being issued. This field MUST be present. 725 The syntax of the field is as follows: 727 final-recipient-field = 728 "Final-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS 730 The generic-address subfield of the Final-Recipient field SHOULD 731 contain the mailbox address of the recipient (which will be the same 732 as the From header field of the MDN) as it was when the MDN was 733 generated by the MUA. 735 One example of when this field might not contain the final 736 recipient address of the message is when an alias (e.g. "customer- 737 support@example.com") forwards mail to a specific personal address 738 (e.g. "bob@example.com"). Bob might want to be able to send MDNs, 739 but not give away his personal email address. In this case the 740 Final-Recipient field can be "customer-support@example.com" 741 instead of "bob@example.com". 743 The Final-Recipient address may differ from the address originally 744 provided by the sender, because it may have been transformed during 745 forwarding and gatewaying into a totally unrecognizable mess. 746 However, in the absence of the optional Original-Recipient field, the 747 Final-Recipient field and any returned content may be the only 748 information available with which to correlate the MDN with a 749 particular message recipient. 751 The address-type subfield indicates the type of address expected by 752 the reporting MTA in that context. Recipient addresses obtained via 753 SMTP will normally be of address-type "rfc822", but can be other 754 values from the "Address Types" subregistry of the "Delivery Status 755 Notification (DSN) Types" IANA registry. 757 Since mailbox addresses (including those used in the Internet) may be 758 case sensitive, the case of alphabetic characters in the address MUST 759 be preserved. 761 3.2.5. Original-Message-ID field 763 The Original-Message-ID field indicates the message-ID of the message 764 for which the MDN is being issued. It is obtained from the Message- 765 ID header field of the message for which the MDN is issued. This 766 field MUST be present if and only if the original message contained a 767 Message-ID header field. The syntax of the field is as follows: 769 original-message-id-field = 770 "Original-Message-ID" ":" msg-id 772 The msg-id token is as specified in RFC-MSGFMT [RFC5322]. 774 3.2.6. Disposition field 776 The Disposition field indicates the action performed by the 777 Reporting-MUA on behalf of the user. This field MUST be present. 779 The syntax for the Disposition field is: 781 disposition-field = 782 "Disposition" ":" OWS disposition-mode OWS ";" 783 OWS disposition-type 784 [ OWS "/" OWS disposition-modifier 785 *( OWS "," OWS disposition-modifier ) ] OWS 787 disposition-mode = action-mode OWS "/" OWS sending-mode 789 action-mode = "manual-action" / "automatic-action" 791 sending-mode = "MDN-sent-manually" / "MDN-sent-automatically" 793 disposition-type = "displayed" / "deleted" / "dispatched" / 794 "processed" 796 disposition-modifier = "error" / disposition-modifier-extension 798 disposition-modifier-extension = atom 800 The disposition-mode, disposition-type, and disposition-modifier 801 values may be spelled in any combination of upper and lower case US- 802 ASCII characters. 804 3.2.6.1. Disposition modes 806 Disposition mode consists of 2 parts: action mode and sending mode. 808 The following action modes are defined: 810 "manual-action" The disposition described by the disposition type 811 was a result of an explicit instruction by the 812 user rather than some sort of automatically 813 performed action. (This might include the case 814 when the user has manually configured her MUA to 815 automatically respond to valid MDN requests.) 816 Unless prescribed otherwise in a particular mail 817 environment, in order to preserve user's privacy, 818 this MUST be the default for MUAs. 820 "automatic-action" The disposition described by the disposition type 821 was a result of an automatic action, rather than 822 an explicit instruction by the user for this 823 message. This is typically generated by a Mail 824 Delivery Agent (e.g. MDN generations by Sieve 825 reject action [RFC5429], Fax-over-Email 826 [RFC3249], Voice Messaging System (VPIM) 827 [RFC3801] or upon delivery to a mailing list). 829 "Manual-action" and "automatic-action" are mutually exclusive. One 830 or the other MUST be specified. 832 The following sending modes are defined: 834 "MDN-sent-manually" The user explicitly gave permission for this 835 particular MDN to be sent. Unless prescribed 836 otherwise in a particular mail environment, in 837 order to preserve user's privacy, this MUST be 838 the default for MUAs. 840 "MDN-sent-automatically" The MDN was sent because the MUA had 841 previously been configured to do so 842 automatically. 844 "MDN-sent-manually" and "MDN-sent-automatically" are mutually 845 exclusive. One or the other MUST be specified. 847 3.2.6.2. Disposition types 849 The following disposition-types are defined: 851 "displayed" The message has been displayed by the MUA to 852 someone reading the recipient's mailbox. There 853 is no guarantee that the content has been read or 854 understood. 856 "dispatched" The message has been sent somewhere in some 857 manner (e.g., printed, faxed, forwarded) without 858 necessarily having been previously displayed to 859 the user. The user may or may not see the 860 message later. 862 "processed" The message has been processed in some manner 863 (i.e., by some sort of rules or server) without 864 being displayed to the user. The user may or may 865 not see the message later, or there may not even 866 be a human user associated with the mailbox. 868 "deleted" The message has been deleted. The recipient may 869 or may not have seen the message. The recipient 870 might "undelete" the message at a later time and 871 read the message. 873 3.2.6.3. Disposition modifiers 875 Only the extension disposition modifiers is defined: 877 disposition-modifier-extension 878 Disposition modifiers may be defined in the 879 future by later revisions or extensions to this 880 specification. MDN disposition value names MUST 881 be registered with the Internet Assigned Numbers 882 Authority (IANA) using "Specification required" 883 registration policy. (See Section 10 for a 884 registration form.) MDNs with disposition 885 modifier names not understood by the receiving 886 MUA MAY be silently ignored or placed in the 887 user's mailbox without special interpretation. 888 They MUST NOT cause any error message to be sent 889 to the sender of the MDN. 891 It is not required that an MUA be able to generate all of the 892 possible values of the Disposition field. 894 A user agent MUST NOT issue more than one MDN on behalf of each 895 particular recipient. That is, once an MDN has been issued on behalf 896 of a recipient, no further MDNs may be issued on behalf of that 897 recipient, even if another disposition is performed on the message. 898 However, if a message is forwarded, a "dispatched" MDN MAY be issued 899 for the recipient doing the forwarding and the recipient of the 900 forwarded message may also cause an MDN to be generated. 902 3.2.7. Error Field 904 The Error field is used to supply additional information in the form 905 of text messages when the "error" disposition modifier appear. The 906 syntax is as follows: 908 error-field = "Error" ":" *([FWS] text) 910 Note that syntax of these header fields doesn't include comments, so 911 "encoded-word" construct defined in RFC-MIME-HEADER [RFC2047] can't 912 be used to convey non ASCII text. Application that need to convey 913 non ASCII text in these fields should consider implementing message/ 914 global-disposition-notification media type specified in [RFC6533] 915 instead of this specification. 917 3.3. Extension-fields 919 Additional MDN fields may be defined in the future by later revisions 920 or extensions to this specification. MDN field names MUST be 921 registered with the Internet Assigned Numbers Authority (IANA) using 922 "Specification required" registration policy. (See Section 10 for a 923 registration form.) MDN Extension-fields may be defined for the 924 following reasons: 926 a. To allow additional information from foreign disposition reports 927 to be tunneled through Internet MDNs. The names of such MDN 928 fields should begin with an indication of the foreign environment 929 name (e.g., X400-Physical-Forwarding-Address). 931 b. To allow transmission of diagnostic information that is specific 932 to a particular mail user agent (MUA). The names of such MDN 933 fields should begin with an indication of the MUA implementation 934 that produced the MDN (e.g., Foomail-information). 936 4. Timeline of events 938 The following timeline shows when various events in the processing of 939 a message and generation of MDNs take place: 941 -- User composes message 943 -- User tells MUA to send message. 945 -- MUA passes message to Mail Submission Agent (MSA), original 946 recipient information passed along. 948 -- MSA sends message to next MTA. 950 -- Final MTA receives message. 952 -- Final MTA delivers message to recipient's mailbox (possibly 953 generating a Delivery Status Notification (DSN)). 955 -- (Recipient's) MUA discovers a new message in recipient's mailbox 956 and decides whether an MDN should be generated. If the MUA has 957 information that an MDN has already been generated for this 958 message, no further MDN processing described below is performed. 959 If MUA decides that no MDN can be generated, no further MDN 960 processing described below is performed. 962 -- MUA performs automatic processing and might generate corresponding 963 MDNs ("dispatched", "processed" or "deleted" disposition type with 964 "automatic-action" and "MDN-sent-automatically" disposition 965 modes). The MUA remembers that an MDN was generated. 967 -- MUA displays list of messages to user. 969 -- User selects a message and requests that some action be performed 970 on it. 972 -- MUA performs requested action; if an automatic MDN has not already 973 been generated, with user's permission, sends an appropriate MDN 974 ("displayed", "dispatched", "processed", or "deleted" disposition 975 type, with "manual-action" and "MDN-sent-manually" or "MDN-sent- 976 automatically" disposition mode). The MUA remembers that an MDN 977 was generated. 979 -- User possibly performs other actions on message, but no further 980 MDNs are generated. 982 5. Conformance and Usage Requirements 984 An MUA or gateway conforms to this specification if it generates MDNs 985 according to the protocol defined in this memo. It is not necessary 986 to be able to generate all of the possible values of the Disposition 987 field. 989 MUAs and gateways MUST NOT generate the Original-Recipient field of 990 an MDN unless the mail protocols provide the address originally 991 specified by the sender at the time of submission. Ordinary SMTP 992 does not make that guarantee, but the SMTP extension defined in RFC- 993 DSN-SMTP [RFC3461] permits such information to be carried in the 994 envelope if it is available. The Original-Recipient header field 995 defined in this document provides a way for the MTA to pass the 996 original recipient address to the MUA. 998 Each sender-specified recipient address may result in more than one 999 MDN. If an MDN is requested for a recipient that is forwarded to 1000 multiple recipients of an "alias" (as defined in RFC-DSN-SMTP 1001 [RFC3461], section 6.2.7.3), each of the recipients may issue an MDN. 1003 Successful distribution of a message to a mailing list exploder or 1004 gateway to Usenet newsgroup SHOULD be considered the final 1005 disposition of the message. A mailing list exploder MAY issue an MDN 1006 with a disposition type of "processed" and disposition modes of 1007 "automatic-action" and "MDN-sent-automatically" indicating that the 1008 message has been forwarded to the list. In this case, the request 1009 for MDNs is not propagated to the members of the list. 1011 Alternatively (if successful distribution of a message to a mailing 1012 list exploder/Usenet newsgroup is not considered the final 1013 disposition of the message), the mailing list exploder can issue no 1014 MDN and propagate the request for MDNs to all members of the list. 1015 The latter behavior is not recommended for any but small, closely 1016 knit lists, as it might cause large numbers of MDNs to be generated 1017 and may cause confidential subscribers to the list to be revealed. 1018 The mailing list exploder can also direct MDNs to itself, correlate 1019 them, and produce a report to the original sender of the message. 1021 This specification places no restrictions on the processing of MDNs 1022 received by user agents or mailing lists. 1024 6. Security Considerations 1026 The following security considerations apply when using MDNs: 1028 6.1. Forgery 1030 MDNs can be (and are, in practice) forged as easily as ordinary 1031 Internet electronic mail. User agents and automatic mail handling 1032 facilities (such as mail distribution list exploders) that wish to 1033 make automatic use of MDNs should take appropriate precautions to 1034 minimize the potential damage from denial-of-service attacks. 1036 Security threats related to forged MDNs include the sending of: 1038 a. A falsified disposition notification when the indicated 1039 disposition of the message has not actually occurred, 1041 b. Unsolicited MDNs 1043 Similarly, a forged spam or phishing email message can contain 1044 Disposition-Notification-To header field that can trick the recipient 1045 to send an MDN. MDN processing should only be invoked once 1046 authenticity of email message is verified. 1048 6.2. Privacy 1050 Another dimension of security is privacy. There may be cases in 1051 which a message recipient does not wish the disposition of messages 1052 addressed to him to be known, or is concerned that the sending of 1053 MDNs may reveal other sensitive information (e.g., when the message 1054 was read, using which email client, which OS was used). In this 1055 situation, it is acceptable for the MUA to silently ignore requests 1056 for MDNs. 1058 If the Disposition-Notification-To header field is passed on 1059 unmodified when a message is distributed to the subscribers of a 1060 mailing list, the subscribers to the list may be revealed to the 1061 sender of the original message by the generation of MDNs. 1063 Headers of the original message returned in part 3 of the multipart/ 1064 report, as well as content of the message/disposition-notification 1065 part could reveal confidential information about host names and/or 1066 network topology inside a firewall. 1068 Disposition mode (Section 3.2.6.1) can leak information about 1069 recipient's MUA configuration, in particular whether MDNs are 1070 acknowledged manually or automatically. If this is a concern, MUAs 1071 can return "manual-action/MDN-sent-manually" disposition mode in 1072 generated MDNs. 1074 In general, any optional MDN field may be omitted if the Reporting 1075 MUA site or user determines that inclusion of the field would impose 1076 too great a compromise of site confidentiality. The need for such 1077 confidentiality must be balanced against the utility of the omitted 1078 information in MDNs. 1080 In some cases, someone with access to the message stream may use the 1081 MDN request mechanism to monitor the mail reading habits of a target. 1082 If the target is known to generate MDN reports, they could add a 1083 disposition-notification-to field containing the envelope from 1084 address. This risk can be minimized by not sending MDN's 1085 automatically. 1087 6.2.1. Disclosure of Product Information 1089 The "Reporting-UA" field (Section 3.2.1) User-Agent (Section 5.5.3) 1090 and header fields often reveal information about the respective 1091 sender's software systems. In theory, this can make it easier for an 1092 attacker to exploit known security holes; in practice, attackers tend 1093 to try all potential holes regardless of the apparent software 1094 versions being used. Also note that the "Reporting-UA" field doesn't 1095 provide any new information in comparison to the "User-Agent" and/or 1096 (undocumented) "X-Mailer" header fields used by many MUAs. 1098 6.2.2. MUA Fingerprinting 1100 The "Reporting-UA" field (Section 3.2.1) might contain enough 1101 information to uniquely identify a specific device, usually when 1102 combined with other characteristics, particularly if the user agent 1103 sends excessive details about the user's system or extensions. Even 1104 when the guidance in Section 3.2.1 is followed to avoid 1105 fingerprinting, other sources of unique information may still be 1106 present, such as the Accept-Language header fields. 1108 6.3. Non-Repudiation 1110 MDNs do not provide non-repudiation with proof of delivery. Within 1111 the framework of today's Internet Mail, the MDNs defined in this 1112 document provide valuable information to the mail user; however, MDNs 1113 cannot be relied upon as a guarantee that a message was or was not 1114 seen by the recipient. Even if MDNs are not actively forged, they 1115 may be lost in transit. The recipient may bypass the MDN issuing 1116 mechanism in some manner. 1118 One possible solution for this purpose can be found in RFC-SEC- 1119 SERVICES [RFC2634]. 1121 6.4. Mail Bombing 1123 The MDN request mechanism introduces an additional way of mailbombing 1124 a mailbox. The MDN request notification provides an address to which 1125 MDN's should be sent. It is possible for an attacking agent to send 1126 a potentially large set of messages to otherwise unsuspecting third 1127 party recipients with a false "disposition-notification-to:" address. 1128 Automatic, or simplistic processing of such requests would result in 1129 a flood of MDN notifications to the target of the attack. 1130 Additionally, as generated MDN notifications can include full content 1131 of messages that caused them and thus they can be bigger than such 1132 messages, they can be used for bandwidth amplification attacks. Such 1133 an attack could overrun the storage capacity of the targeted mailbox 1134 and/or of the mail transport system, and deny service. 1136 For that reason, MDN's SHOULD NOT be sent automatically where the 1137 "disposition-notification-to:" address is different from the SMTP 1138 "MAIL FROM" address (which is carried in the Return-Path header 1139 field). See Section 2.1 for further discussion. 1141 7. Collected ABNF Grammar 1143 NOTE: The following lexical tokens are defined in RFC-MSGFMT 1144 [RFC5322]: CRLF, FWS, CFWS, field-name, mailbox-list, msg-id, text, 1145 comment, word. The following lexical tokens are defined in RFC-SMTP 1146 [RFC5321]: atom. (Note that RFC-MSGFMT [RFC5322] also defines 1147 "atom", but the version from RFC-SMTP [RFC5321] is more restrictive 1148 and this more restrictive version is used in this document.) 1149 "encoded-word" construct defined in RFC-MIME-HEADER [RFC2047] is 1150 allowed everywhere where RFC-MSGFMT [RFC5322] "comment" is used, for 1151 example in CFWS. 1153 OWS = [CFWS] 1154 ; Optional whitespace. 1155 ; MDN generators SHOULD use "*WSP" 1156 ; (typically a single space or nothing. 1157 ; It SHOULD be nothing at the end of a field), 1158 ; unless an RFC 5322 "comment" is required. 1159 ; 1160 ; MDN parsers MUST parse it as "[CFWS]". 1162 Message header fields: 1163 mdn-request-header = 1164 "Disposition-Notification-To" ":" mailbox-list CRLF 1166 Disposition-Notification-Options = 1167 "Disposition-Notification-Options" ":" [FWS] 1168 disposition-notification-parameter-list CRLF 1170 disposition-notification-parameter-list = 1171 disposition-notification-parameter 1172 *([FWS] ";" [FWS] disposition-notification-parameter) 1174 disposition-notification-parameter = attribute [FWS] "=" [FWS] 1175 importance [FWS] "," [FWS] value *([FWS] "," [FWS] value) 1177 importance = "required" / "optional" 1179 attribute = atom 1180 value = word 1182 original-recipient-header = 1183 "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS CRLF 1185 Report content: 1186 disposition-notification-content = 1187 [ reporting-ua-field CRLF ] 1188 [ mdn-gateway-field CRLF ] 1189 [ original-recipient-field CRLF ] 1190 final-recipient-field CRLF 1191 [ original-message-id-field CRLF ] 1192 disposition-field CRLF 1193 *( error-field CRLF ) 1194 *( extension-field CRLF ) 1196 address-type = atom 1198 mta-name-type = atom 1200 reporting-ua-field = "Reporting-UA" ":" OWS ua-name OWS [ ";" OWS ua-product OWS ] 1202 ua-name = *text-no-semi 1204 ua-product = *([FWS] text) 1206 text-no-semi = %d1-9 / ; "text" characters excluding NUL, CR, 1207 %d11 / %d12 / %d14-58 / %d60-127 ; LF, or semi-colon 1209 mdn-gateway-field = "MDN-Gateway" ":" OWS mta-name-type OWS ";" OWS mta-name 1211 mta-name = *text 1213 original-recipient-field = 1214 "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS 1216 generic-address = *text 1218 final-recipient-field = 1219 "Final-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS 1221 original-message-id-field = "Original-Message-ID" ":" msg-id 1223 disposition-field = 1224 "Disposition" ":" OWS disposition-mode OWS ";" 1225 OWS disposition-type 1226 [ OWS "/" OWS disposition-modifier 1227 *( OWS "," OWS disposition-modifier ) ] OWS 1229 disposition-mode = action-mode OWS "/" OWS sending-mode 1231 action-mode = "manual-action" / "automatic-action" 1233 sending-mode = "MDN-sent-manually" / "MDN-sent-automatically" 1235 disposition-type = "displayed" / "deleted" / "dispatched" / 1236 "processed" 1238 disposition-modifier = "error" / disposition-modifier-extension 1240 disposition-modifier-extension = atom 1242 error-field = "Error" ":" *([FWS] text) 1244 extension-field = extension-field-name ":" *([FWS] text) 1246 extension-field-name = field-name 1248 8. Guidelines for Gatewaying MDNs 1250 NOTE: This section provides non-binding recommendations for the 1251 construction of mail gateways that wish to provide semi-transparent 1252 disposition notifications between the Internet and another electronic 1253 mail system. Specific MDN gateway requirements for a particular pair 1254 of mail systems may be defined by other documents. 1256 8.1. Gatewaying from other mail systems to MDNs 1258 A mail gateway may issue an MDN to convey the contents of a "foreign" 1259 disposition notification over Internet Mail. When there are 1260 appropriate mappings from the foreign notification elements to MDN 1261 fields, the information may be transmitted in those MDN fields. 1262 Additional information (such as might be needed to tunnel the foreign 1263 notification through the Internet) may be defined in extension MDN 1264 fields. (Such fields should be given names that identify the foreign 1265 mail protocol, e.g., X400-* for X.400 protocol elements). 1267 The gateway must attempt to supply reasonable values for the 1268 Reporting-UA, Final-Recipient, and Disposition fields. These will 1269 normally be obtained by translating the values from the foreign 1270 notification into their Internet-style equivalents. However, some 1271 loss of information is to be expected. 1273 The sender-specified recipient address and the original message-id, 1274 if present in the foreign notification, should be preserved in the 1275 Original-Recipient and Original-Message-ID fields. 1277 The gateway should also attempt to preserve the "final" recipient 1278 address from the foreign system. Whenever possible, foreign protocol 1279 elements should be encoded as meaningful printable ASCII strings. 1281 For MDNs produced from foreign disposition notifications, the name of 1282 the gateway MUST appear in the MDN-Gateway field of the MDN. 1284 8.2. Gatewaying from MDNs to other mail systems 1286 It may be possible to gateway MDNs from the Internet into a foreign 1287 mail system. The primary purpose of such gatewaying is to convey 1288 disposition information in a form that is usable by the destination 1289 system. A secondary purpose is to allow "tunneling" of MDNs through 1290 foreign mail systems in case the MDN may be gatewayed back into the 1291 Internet. 1293 In general, the recipient of the MDN (i.e., the sender of the 1294 original message) will want to know, for each recipient: the closest 1295 available approximation to the original recipient address, and the 1296 disposition (displayed, printed, etc.). 1298 If possible, the gateway should attempt to preserve the Original- 1299 Recipient address and Original-Message-ID (if present) in the 1300 resulting foreign disposition report. 1302 If it is possible to tunnel an MDN through the destination 1303 environment, the gateway specification may define a means of 1304 preserving the MDN information in the disposition reports used by 1305 that environment. 1307 8.3. Gatewaying of MDN-requests to other mail systems 1309 By use of the separate disposition-notification-to request header 1310 field, this specification offers a richer functionality than most, if 1311 not all, other email systems. In most other email systems, the 1312 notification recipient is identical to the message sender as 1313 indicated in the "from" address. There are two interesting cases 1314 when gatewaying into such systems: 1316 1. If the address in the disposition-notification-to header field is 1317 identical to the address in the SMTP "MAIL FROM", the expected 1318 behavior will result, even if the disposition-notification-to 1319 information is lost. Systems should propagate the MDN request. 1321 2. If the address in the disposition-notification-to header field is 1322 different from the address in the SMTP "MAIL FROM", gatewaying 1323 into a foreign system without a separate notification address 1324 will result in unintended behavior. This is especially important 1325 when the message arrives via a mailing list expansion software 1326 that may specifically replace the SMTP "MAIL FROM" address with 1327 an alternate address. In such cases, the MDN request should not 1328 be gatewayed and should be silently dropped. This is consistent 1329 with other forms of non-support for MDN. 1331 9. Example 1333 NOTE: This example is provided as illustration only, and is not 1334 considered part of the MDN protocol specification. If the example 1335 conflicts with the protocol definition above, the example is wrong. 1337 Likewise, the use of *-type subfield names or extension fields in 1338 this example is not to be construed as a definition for those type 1339 names or extension fields. 1341 This is an MDN issued after a message has been displayed to the user 1342 of an Internet Mail user agent. 1344 Date: Wed, 20 Sep 1995 00:19:00 (EDT) -0400 1345 From: Joe Recipient 1346 Message-Id: <199509200019.12345@example.com> 1347 Subject: Disposition notification 1348 To: Jane Sender 1349 MIME-Version: 1.0 1350 Content-Type: multipart/report; report-type=disposition-notification; 1351 boundary="RAA14128.773615765/example.com" 1353 --RAA14128.773615765/example.com 1355 The message sent on 1995 Sep 19 at 13:30:00 (EDT) -0400 to Joe 1356 Recipient with subject "First draft of 1357 report" has been displayed. 1358 This is no guarantee that the message has been read or understood. 1360 --RAA14128.773615765/example.com 1361 content-type: message/disposition-notification 1363 Reporting-UA: joes-pc.cs.example.com; Foomail 97.1 1364 Original-Recipient: rfc822;Joe_Recipient@example.com 1365 Final-Recipient: rfc822;Joe_Recipient@example.com 1366 Original-Message-ID: <199509192301.23456@example.org> 1367 Disposition: manual-action/MDN-sent-manually; displayed 1369 --RAA14128.773615765/example.com 1370 content-type: message/rfc822 1372 [original message optionally goes here] 1374 --RAA14128.773615765/example.com-- 1376 10. IANA Considerations 1378 There are two actions for IANA: 1380 1. IANA is asked to update the registration template for the 1381 message/disposition-notification media type to the one in 1382 Section 3.1 of this document, and to update the reference for 1383 that media type to point to this document instead of to RFC 3798. 1385 2. The registries specified here already exist, and this section is 1386 updating their documentation. IANA is asked to change the 1387 reference document for the three Message Disposition Notification 1388 Parameters registries to point to this document instead of to RFC 1389 3798. 1391 This document specifies three types of parameters that must be 1392 registered with the Internet Assigned Numbers Authority (IANA). All 1393 of them use [RFC5226] "Specification required" IANA registration 1394 policy. 1396 The forms below are for use when registering a new disposition- 1397 notification-parameter name for the Disposition-Notification-Options 1398 header field, a new disposition modifier name, or a new MDN extension 1399 field. Each piece of information required by a registration form may 1400 be satisfied either by providing the information on the form itself, 1401 or by including a reference to a published, publicly available 1402 specification that includes the necessary information. IANA MAY 1403 reject registrations because of incomplete registration forms or 1404 incomplete specifications. 1406 To register, complete the following applicable form and send it via 1407 electronic mail to . 1409 10.1. Disposition-Notification-Options header field disposition- 1410 notification-parameter names 1412 A registration for a Disposition-Notification-Options header field 1413 disposition-notification-parameter name MUST include the following 1414 information: 1416 a. The proposed disposition-notification-parameter name. 1418 b. The syntax for disposition-notification-parameter values, 1419 specified using BNF, ABNF, regular expressions, or other non- 1420 ambiguous language. 1422 c. If disposition-notification-parameter values are not composed 1423 entirely of graphic characters from the US-ASCII repertoire, a 1424 specification for how they are to be encoded as graphic US-ASCII 1425 characters in a Disposition-Notification-Options header field. 1427 d. A reference to a permanent and readily available public 1428 specification that describes the semantics of the disposition- 1429 notification-parameter values. 1431 10.2. Disposition modifier names 1433 A registration for a disposition-modifier name (used in the 1434 Disposition field of a message/disposition-notification) MUST include 1435 the following information: 1437 a. The proposed disposition-modifier name. 1439 b. A reference to a permanent and readily available public 1440 specification that describes the semantics of the disposition 1441 modifier. 1443 10.3. MDN extension field names 1445 A registration for an MDN extension-field name MUST include the 1446 following information: 1448 a. The proposed extension field name. 1450 b. The syntax for extension values, specified using BNF, ABNF, 1451 regular expressions, or other non-ambiguous language. 1453 c. If extension-field values are not composed entirely of graphic 1454 characters from the US-ASCII repertoire, a specification for how 1455 they are to be encoded as graphic US-ASCII characters in a 1456 Disposition-Notification-Options header field. 1458 d. A reference to a permanent and readily available public 1459 specification that describes the semantics of the extension 1460 field. 1462 11. Acknowledgements 1464 The contributions of Bruce Lilly, Alfred Hoenes, Barry Leiba, Ben 1465 Campbell, Pete Resnick, Donald Eastlake and Alissa Cooper are 1466 gratefully acknowledged for this revision. 1468 The contributions of Roger Fajman and Greg Vaudreuil to earlier 1469 versions of this document are also gratefully acknowledged. 1471 12. References 1473 12.1. Normative References 1475 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1476 DOI 10.17487/RFC5321, October 2008, 1477 . 1479 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1480 DOI 10.17487/RFC5322, October 2008, 1481 . 1483 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1484 Extensions (MIME) Part One: Format of Internet Message 1485 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 1486 . 1488 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1489 Extensions (MIME) Part Two: Media Types", RFC 2046, 1490 DOI 10.17487/RFC2046, November 1996, 1491 . 1493 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 1494 Part Three: Message Header Extensions for Non-ASCII Text", 1495 RFC 2047, DOI 10.17487/RFC2047, November 1996, 1496 . 1498 [RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for 1499 the Reporting of Mail System Administrative Messages", 1500 STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012, 1501 . 1503 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 1504 Extension for Delivery Status Notifications (DSNs)", 1505 RFC 3461, DOI 10.17487/RFC3461, January 2003, 1506 . 1508 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 1509 for Delivery Status Notifications", RFC 3464, 1510 DOI 10.17487/RFC3464, January 2003, 1511 . 1513 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1514 Requirement Levels", BCP 14, RFC 2119, 1515 DOI 10.17487/RFC2119, March 1997, 1516 . 1518 [RFC3503] Melnikov, A., "Message Disposition Notification (MDN) 1519 profile for Internet Message Access Protocol (IMAP)", 1520 RFC 3503, DOI 10.17487/RFC3503, March 2003, 1521 . 1523 12.2. Informative References 1525 [RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME", 1526 RFC 2634, DOI 10.17487/RFC2634, June 1999, 1527 . 1529 [RFC3249] Cancio, V., Moldovan, M., Tamura, H., and D. Wing, 1530 "Implementers Guide for Facsimile Using Internet Mail", 1531 RFC 3249, DOI 10.17487/RFC3249, September 2002, 1532 . 1534 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1535 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 1536 . 1538 [RFC3801] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet 1539 Mail - version 2 (VPIMv2)", RFC 3801, 1540 DOI 10.17487/RFC3801, June 2004, 1541 . 1543 [RFC5233] Murchison, K., "Sieve Email Filtering: Subaddress 1544 Extension", RFC 5233, DOI 10.17487/RFC5233, January 2008, 1545 . 1547 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1548 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1549 DOI 10.17487/RFC5226, May 2008, 1550 . 1552 [RFC5429] Stone, A., Ed., "Sieve Email Filtering: Reject and 1553 Extended Reject Extensions", RFC 5429, 1554 DOI 10.17487/RFC5429, March 2009, 1555 . 1557 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 1558 DOI 10.17487/RFC5598, July 2009, 1559 . 1561 [RFC6533] Hansen, T., Ed., Newman, C., and A. Melnikov, 1562 "Internationalized Delivery Status and Disposition 1563 Notifications", RFC 6533, DOI 10.17487/RFC6533, February 1564 2012, . 1566 Appendix A. Changes from RFC 3798 1568 Changed IANA registration for different subregistries to 1569 "Specification Required" to match what is already used by IANA. 1571 Updated IANA registration template for message/disposition- 1572 notification. 1574 "X-" fields no longer reserved for experimental use and can now be 1575 registered in compliance with RFC 6648. 1577 Fixed the default MTA-name-type used in "MDN-Gateway" to be "dns". 1579 Strengthen requirements on obtaining user consent in order to protect 1580 user privacy. 1582 Removed discussion of using source routes with MDNs, as source route 1583 is a deprecated Email feature. 1585 The values of "dispatched" and "processed" were lost from the ABNF 1586 for "disposition-type". (Erratum #691) 1588 Because the warning disposition modifier was previously removed, 1589 warning-field has also been removed. (Erratum #692) 1591 Because the failed disposition type was previously removed, failure- 1592 field has also been removed. 1594 The ABNF for ua-name and ua-product included semi-colon, which could 1595 not be distinguished from *text in the production. The ua-name was 1596 restricted to not include semi-colon. Semi-colon can still appear in 1597 the ua-product. 1599 Removed recommendation to include the MUA DNS host name in the 1600 "Reporting-UA" MDN field. 1602 The ABNF did not indicate all places that whitespace was allowable, 1603 in particular folding whitespace, although all implementations allow 1604 whitespace and folding in the header fields just like any other 1605 RFC5322 [RFC5322]-formatted header field. There were also a number 1606 of places in the ABNF that inconsistently permitted comments and 1607 whitespace in one leg of the production and not another. The ABNF 1608 now specifies FWS and CFWS in several places that should have already 1609 been specified by the grammar. 1611 Extension-field was defined in the collected grammar but not in the 1612 main text. 1614 The comparison of mailboxes in Disposition-Notification-To to the 1615 Return-Path addr-spec was clarified. 1617 The use of the grammar production "parameter" was confusing with the 1618 RFC2045 [RFC2045] production of the same name, as well as other uses 1619 of the same term. These have been clarified. 1621 A clarification was added on the extent of the 7bit nature of MDNs. 1623 Uses of the terms "may" and "might" were clarified. 1625 A clarification was added on the order of the fields in the message/ 1626 disposition-notification content. 1628 Authors' Addresses 1630 Tony Hansen (editor) 1631 AT&T Laboratories 1632 200 Laurel Ave. South 1633 Middletown, NJ 07748 1634 USA 1636 Email: tony@att.com 1638 Alexey Melnikov (editor) 1639 Isode Ltd 1640 14 Castle Mews 1641 Hampton, Middlesex TW12 2NP 1642 UK 1644 Email: Alexey.Melnikov@isode.com