7Network Working Group G. White
8Request for Comments: 4865 Independent
9Updates: 3463, 3464 G. Vaudreuil
10Category: Standards Track Alcatel-Lucent
14 SMTP Submission Service Extension for Future Message Release
18 This document specifies an Internet standards track protocol for the
19 Internet community, and requests discussion and suggestions for
20 improvements. Please refer to the current edition of the "Internet
21 Official Protocol Standards" (STD 1) for the standardization state
22 and status of this protocol. Distribution of this memo is unlimited.
26 Copyright (C) The IETF Trust (2007).
30 This memo defines an extension to the SMTP submission protocol for a
31 client to indicate a future time for the message to be released for
32 delivery. This extension permits a client to use server-based
33 storage for a message that should be held in queue until an appointed
34 time in the future. This is useful for clients which do not have
35 local storage or are otherwise unable to release a message for
36 delivery at an appointed time.
40 There is a widely used feature within the voice messaging community
41 to compose and send a message for delivery in the future. This is
42 useful for sending announcements to be heard at the beginning of a
43 work day, to send birthday greetings a day or so ahead, or to use as
44 a lightweight facility to build a personal reminder service.
46 This extension uses the SMTP submission protocol [n3] to allow a
47 client, when submitting a message, to indicate a future time for the
48 message to be released for delivery.
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60RFC 4865 SMTP Future Message Release May 2007
65 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
66 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
67 document are to be interpreted as described in [n1].
71 The Future Message Release service extension for SMTP submission uses
72 the SMTP service extension mechanism [n4] to extend the SMTP
73 submission protocol [n3]. The following SMTP submission service
74 extension is hereby defined:
76 The name of the SMTP submission service extension is "Future Message
79 1) The Extended Hello (EHLO) keyword associated with this service
80 extension is "FUTURERELEASE".
82 2) Two required parameters, the max-future-release-interval and the
83 max-future-release-date-time, are combined with the EHLO keyword in
84 the manner specified in [n4].
86 The max-future-release-interval is a positive integer indicating the
87 maximum amount of time for which the message submission server (MSA)
88 will hold messages for future release.
90 Using ABNF [n2], the syntax of this parameter is as follows:
93 ; integer in the range 1-999999999
96 max-future-release-interval = future-release-integer
99 Universal Coordinated Time (UTC), indicating the most remote date
100 and time in the future until which the MSA will hold messages for
103 Using ABNF [n2], the syntax of this parameter is as follows:
105 max-future-release-date-time = date-time
107 where the format of date-time is defined in [n10].
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116RFC 4865 SMTP Future Message Release May 2007
119 3) When forming the portion of the EHLO reply containing the
120 FUTURERELEASE keyword, the keyword is followed by the max-future-
121 release-interval, and then the max-future-release-date-time. The
122 keyword and two values are delimited by spaces.
124 For example, the ABNF for a continuation line in the EHLO response
125 that contains the FUTURERELEASE keyword is:
128 SP max-future-release-date-time
130 4) One required parameter, the hold-param, is added to the MAIL
131 command using either the keyword "HOLDFOR" or the keyword
134 The HOLDFOR parameter value is a future-release-interval, which is
135 a positive integer indicating the amount of time the message is to
136 be held by the MSA before release.
138 The HOLDUNTIL parameter value is a future-release-date-time, which
139 is a timestamp, normalized to UTC, indicating the future date and
140 time until which the message is to be held by the MSA before
143 Using ABNF [n2], the syntax of this parameter is as follows:
145 future-release-interval = future-release-integer
149 hold-for-param = "HOLDFOR=" future-release-interval
151 hold-until-param = "HOLDUNTIL=" future-release-date-time
153 hold-param = hold-for-param / hold-until-param
155 The absence of this parameter on the MAIL command does not imply a
156 default value for this parameter.
158 5) The maximum length of a MAIL command is increased by 34 characters
159 by the possible addition of the hold-param.
161 6) No additional SMTP verbs are defined by this extension.
164 protocol [n3]. This service extension is not appropriate for
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172RFC 4865 SMTP Future Message Release May 2007
177 It is unfortunate to define two seemingly identical ways to indicate
178 a future message release time. When the client has both accurate
179 time and accurate time zone information, either interval or date-time
180 can be trivially calculated from the other. However, in the current
181 world of clients, there are clients with accurate local time but no
182 indication of their time zone, and clients without a suitably
183 accurate clock. Based on the limited facilities available to these
184 time-challenged clients, it is likely that only one or the other of
185 these mechanisms will be useful.
187 It is believed that servers will have accurate time, and can
188 trivially convert between these mechanisms. It is also accepted that
189 the protocol and implementation overhead of offering these two
190 mechanisms is low, and that few interoperability challenges are
195 1) An SMTP client preparing to use Future Message Release MUST first
196 verify that the MSA supports this extension.
198 2) An SMTP client using Future Message Release MUST include one, and
199 only one, hold-param with the MAIL command.
201 3) An SMTP client using Future Message Release with the "for" option
202 of the hold-param MUST ensure that the future-release-interval is
203 less than or equal to the max-future-release-interval advertised
206 4) An SMTP client using Future Message Release with the "until"
207 option of the hold-param MUST ensure that the future-release-
208 date-time is earlier than or equal to the max-future-release-
209 date-time advertised by the MSA.
213 1) An MSA supporting Future Message Release MUST comply with the SMTP
214 submission protocol as described in [n3].
216 2) An MSA supporting Future Message Release MUST NOT advertise this
217 support (i.e. include the FUTURERELEASE keyword in its EHLO reply)
218 on any port other than the submission port.
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228RFC 4865 SMTP Future Message Release May 2007
231 3) An MSA supporting Future Message Release MUST include the
232 FUTURERELEASE keyword, and associated max-future-release-interval
233 and max-future-release-date-time parameters, in its reply to the
236 4) An MSA supporting Future Message Release MUST accept a MAIL
237 command containing a valid hold-param, given that the MAIL command
238 contains no other errors.
240 5) An MSA that accepts a message with a request for Future Message
241 Release indicating the "for" option MUST NOT release the message
242 until the amount of time specified in the future-release-interval
245 6) An MSA that accepts a message with a request for Future Message
246 Release indicating the "until" option MUST NOT release the message
247 until the date and time indicated by the future-release-date-time
251 command containing the "for" option specifying a value that is
252 greater than the advertised max-future-release-interval, or
256 command containing the "until" option specifying a value that is
257 later than the advertised max-future-release-date-time, or
261 command containing more than one hold-param.
264 command per items 7, 8, or 9, above, SHOULD supply the reply code
265 501 (syntax error in parameters or arguments [n4]) in the reply.
268 command per items 7, 8, or 9, above, SHOULD supply the Enhanced
269 Mail System Status Code 5.5.4 (invalid command arguments [i1]) in
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2875. Protocol Interactions
2895.1. Interaction with the DSN SMTP Service Extensions
291 The Delivery Status Notification (DSN) service extension is described
292 in [n7], and DSN message format is described in [n8].
2945.1.1. SMTP Client Interaction with DSN
296 1) An SMTP client MUST NOT request Future Message Release when
297 sending a DSN to the MSA.
2995.1.2. MSA Interaction with DSN
301 1) If an MSA generates a DSN for a message that includes a Future
302 Message Release request, the MSA MUST include an Arrival-Date
303 field in the machine-readable body part of the DSN.
306 Message Release request, the MSA MUST include a Future-Release-
307 Request field in the machine-readable body part of the DSN. The
308 value of this field is the value of the HOLD parameter contained
309 in the MAIL command of the original message.
311 The Future-Release-Request field is an extension to the set of DSN
312 per-message fields described in [n8]. Using ABNF [n2], the syntax
313 of this new field is as follows:
316 ("until;" future-release-date-time)
317 ; this is the value of the HOLD param from
318 ; the MAIL command of the original message
321 orig-hold-param-value
3235.2. Interaction with the DELIVERBY SMTP Service Extension
325 If an MSA supports the Future Message release and Deliver By service
326 extensions, it is possible for an SMTP client to make simultaneous
327 requests for future message release and deliver-by times when
328 submitting a message. A problem will occur if the future message
329 release time is farther in the future than the deliver-by time. In
330 order to honor the deliver-by request, the future message release
331 request has to be ignored. In order to honor the future message
332 release request, the deliver-by request has to be ignored. This
333 section addresses that problem. The Deliver By extension is
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340RFC 4865 SMTP Future Message Release May 2007
3435.2.1. SMTP Client Interaction with DELIVERBY
345 1) When an SMTP client wishes to use the Future Message Release and
346 Deliver By extensions with the same message, the client MUST
347 ensure that the specified deliver-by time is farther in the future
348 than the specified ("until" option) or implied ("for" option)
349 future message release time.
3515.2.2. MSA Interaction with DELIVERBY
353 1) If an MSA supports Future Message Release and Deliver By
354 extensions, and receives a message requesting the use of both
355 extensions, the MSA MUST reject the MAIL command if it determines
356 that the future message release time is farther in the future than
359 2) When an MSA is rejecting a MAIL command per item 1, above, it
360 SHOULD supply the reply code 501 (syntax error in parameters or
361 arguments [n4]) in the reply.
363 3) When an MSA is rejecting a MAIL command per item 1, above, it
364 SHOULD supply the Enhanced Mail System Status Code 5.5.4 (invalid
365 command arguments [i1]) in the reply.
3675.3. Interaction with the MDN Function
369 The Message Disposition Notification (MDN) function is described in
3725.3.1. SMTP Client Interaction with MDN
374 1) An SMTP client MUST NOT request Future Message Release when
375 sending an MDN to the MSA.
3776. Security Considerations
379 The Future Message Release service extension presents a number of
380 security considerations:
382 1) Unauthorized future-release messages provide a means to overwhelm
383 the storage of an MSA. The authorization mechanisms required for
384 the base mail submission protocol [n3] are expected to provide
385 appropriate defense against such attacks.
388 also provide a way to overwhelm an MSA's storage. An MSA's future
389 release message storage SHOULD be subject to a per-user quota.
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396RFC 4865 SMTP Future Message Release May 2007
399 3) If an MSA is imposing a per-user quota on future-release message
400 storage, and detects that an incoming future-release message will
401 exceed the user's future-release message storage quota, the MSA
402 MUST reject the MAIL command.
404 4) When an MSA is rejecting a MAIL command per 5.3, it SHOULD supply
405 the reply code 552 (requested mail action aborted: exceeded
406 storage allocation [n4]) in the reply.
408 5) When an MSA is rejecting a MAIL command per 5.3, it SHOULD supply
409 the new Enhanced Mail System Status Code defined for this purpose.
410 This new status code updates [i1].
414 There is insufficient per-user quota to queue the message for
415 future release. This code suggests the client can submit again
416 only after the per-user queue has drained.
420 There is insufficient system quota to queue the message for
421 future release. This code suggests the client can submit again
422 after the system queue has drained.
424 6) Inaccurate time on the MSA may result in premature or delayed
425 release of messages. Both HOLDUNTIL and HOLDFOR request
426 mechanisms are sensitive to inaccurate or changing clocks on the
429 7) Some element of deception is inherent in the future message
430 release concept. The message release time is intentionally
431 delayed past the time it would otherwise be released; hence, the
432 message delivery time is delayed past the time it would otherwise
433 be delivered. This extension provides no mechanism for hiding
434 this from the message recipient. The RFC 2822 [n5] message
435 header, and specifically the Date field, remain unchanged after
436 submission. While a sending client MAY elect to place the
437 future-message-release-time as the date in the Date field, there
438 is no requirement or expectation that the Received fields and
439 other trace information be modified by the transport system to
440 further this deception.
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4557. IANA Considerations
457 This extension has been added to the list of SMTP Service Extensions
458 on the Mail Parameters Web page.
462 Much of the credit for this document is due to the LEMONADE working
463 group. Through many revisions, the discussion resulted in
464 fundamental new understandings of this protocol and corresponding
465 refinement of the implied requirements and protocol. Special thanks
466 to those who patiently lead the WG to understand that doing both
467 interval and date-time was the pragmatically correct approach to the
468 needs of diverse clients.
4709. Normative References
472 [n1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
473 Levels", BCP 14, RFC 2119, March 1997.
475 [n2] Crocker, D. and P. Overell, "Augmented BNF for Syntax
476 Specifications: ABNF", RFC 4234, October 2005.
478 [n3] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC
481 [n4] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
484 [n5] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
486 [n6] Newman, D., "Deliver By SMTP Service Extension", RFC 2852, June
489 [n7] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
490 Extension for Delivery Status Notifications (DSNs)", RFC 3461,
493 [n8] Moore, K. and G. Vaudreuil, "An Extensible Message Format for
494 Delivery Status Notifications", RFC 3464, January 2003.
496 [n9] Hansen, T. and G. Vaudreuil, "Message Disposition
497 Notification", RFC 3798, May 2004.
499 [n10] Klyne, G. and C. Newman, "Date and Time on the Internet:
500 Timestamps", RFC 3339, July 2002
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51110. Informative References
513 [i1] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
522 EMail: g.a.white@tx.rr.com
529 EMail: GregV@ieee.org
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564RFC 4865 SMTP Future Message Release May 2007
567Full Copyright Statement
569 Copyright (C) The IETF Trust (2007).
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