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7 1. Abstract
9 This document describes an extension to the SMTP service [RFC1425],
10 called Extended DATA Reply. This is an extended format of the SMTP
11 response to the DATA verb which provides recipient-specific status
12 information. This allows mail recipients to use individual mail
13 filters as part of the Simple Mail Transfer Protocol. This is a very
14 desirable feature because of escalating amounts of unwanted junk E-
15 mail sent over the Internet.
17 2. Introduction
19 The reply to the SMTP [RFC821] DATA verb is a status code indicating
20 if the message was accepted or rejected. The status code also
21 indicates if the rejection is temporary or permanent. Additional
22 delivery attempts will be made for messages that are temporarily
25 SMTP allows a message to have multiple recipients, but the reply to
26 the DATA verb cannot indicate if the message was successfully
27 accepted for some recipients only and rejected for the remaining
28 recipients (permanently or temporarily). It can only indicate if the
29 message was accepted or rejected for every one of the listed
32 The only way to handle this condition within the current protocol is
33 to send a status code indicating that the message was accepted, then
34 immediately generate a non-delivery report for undeliverable
35 recipients. This imposes additional resources and overhead to
36 generate and transmit the non-delivery report.
38 The ability to have recipient-specific status in the reply to the
39 DATA verb is desirable for the following reasons:
41 A) The receiving SMTP server does not have to generate and send
42 a non-delivery report.
44 B) Recipients can install individual mail filters that
45 selectively block unwanted E-mail, and the mail filters will
46 be able to examine the entire contents of the message before
47 deciding to reject the message. Currently, recipient-
48 specific filtering is only possible in response to the "RCPT
49 TO:" verb. But the only available information at that point
50 is the network address of the sending SMTP agent and return
51 address of the message. Filtering unwanted junk mail based
52 only on the network address and the return address is of very
53 limited benefit.
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63 There are several other mail enhancements that involve recipient-
64 specific processing. [RFC2033] defines a related, but a completely
65 different, mail delivery protocol which features recipient-specific
66 processing. There was also another draft memo published, draft-ietf-
67 fax-smtp-session, which defined an extension for recipient-specific
68 mail processing, however its scope is different from the scope of
69 this memo.
71 3. Framework for the EXDATA SMTP transport extension
73 This SMTP transport extension [RFC1425] is laid out as follows.
75 (1) The name of the SMTP transport extension defined here is
76 Extended Data Reply (EXDATA).
78 (2) The EHLO keyword associated with this extension is EXDATA.
80 (3) The EXDATA EHLO keyword takes no parameters.
82 (4) One optional ESMTP keyword EXDATA is associated with the MAIL
83 FROM command. This parameter takes no values.
85 (5) No additional ESMTP verbs are defined by this extension.
87 (6) The next sections specify how support for this extension
88 affects the behavior of a server and client SMTP server.
90 4. The EXDATA SMTP extension
92 The receiving SMTP server indicates that it is capable of supporting
93 the Extended Data Reply SMTP extension by including the EXDATA
94 keyword in the EHLO list.
96 The sending SMTP agent indicates that it supports this extension by
97 including the EXDATA keyword in the MAIL FROM command.
99 If the sending SMTP agent includes the EXDATA keyword in the first
100 MAIL FROM command, it MUST include the keyword in every MAIL FROM
101 command used in the same SMTP session. If the sending SMTP agent
102 does not include the EXDATA keyword in the first MAIL FROM command,
103 it MUST NOT include the keyword in any MAIL FROM command used in the
104 same SMTP session.
106 The receiving SMTP server SHOULD NOT send extended replies to the
107 DATA verb if the EXDATA keyword is not included in the MAIL FROM
110 An extended DATA verb reply MAY be sent instead of the second status
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119 code reply to the DATA verb. It MUST NOT be sent as the first reply
120 to the DATA verb (before the message is transmitted). Using the
121 extended format in the second reply to the DATA verb is optional, and
122 not required. The receiving SMTP server is always permitted to send
123 regular status replies.
125 5. Format of an extended SMTP DATA reply
127 The extended SMTP DATA reply uses the 558 numeric status code which
128 is explicitly reserved for this purpose. When a 558 numeric status
129 code is the second reply to the DATA verb, the reply MUST be
130 formatted as follows, where <SP>, <CR> and <LF> represent the ASCII
131 space, carriage return, and line feed characters:
133 ex-reply-code: ex-reply-recipient-list
135 ex-reply-recipient-list: non-final-recipient ex-reply-recipient-list
136 | final-recipient
138 non-final-recipient: non-term-line non-final-recipient
139 | non-final-recipient-term-line
141 non-term-line: "558-" result-code "-" result-text
143 non-final-recipient-term-line: "558-" result-code <SP> result-text
145 final-recipient: non-term-line final-recipient
146 | final-recipient-term-line
148 final-recipient-term-line: "558" <SP> result-code <SP> result-text
150 result-code: digit digit digit
152 result-text: 0 or more characters, except <CR> and <LF>, terminated
153 by the <CR> <LF> sequence
155 digit: Characters "0" through "9"
158 The extended DATA reply is formatted like a standard multiline SMTP
159 reply with a 558 numeric status code. The extended information is
160 contained within the text portion of the 558 multiline reply. The
161 extended information consists of one or more individual replies,
162 where each reply itself is formatted like an SMTP reply. There will
163 be exactly one reply for each recipient. Each individual reply may
164 be a multiline reply itself.
166 6. Restrictions on the extended SMTP DATA reply
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175 Extended replies MUST contain exactly one individual reply for each
176 recipient whose RCPT TO command was acknowledged with a 2xx result
177 code. Extended replies MUST NOT have any replies for recipients
178 whose RCPT TO commands were previously rejected with a 4xx or 5xx
179 result code.
181 For example, if a message had three recipients, and the second RCPT
182 TO command was rejected, the extended reply MUST contain two
183 individual replies: the first one is for the first recipient, the
184 second one for the last recipient.
186 Technically, an extended SMTP DATA reply with a 2xx numeric code in
187 every individual reply is technically equivalent to a regular 2xx
188 reply to the DATA transaction. The "be conservative with what you
189 send, and be liberal with what you receive" principle would dictate
190 that the sending SMTP agent MUST be prepared to properly handle
191 either reply, and the receiving SMTP agent SHOULD use a regular 2xx
192 response in this situation.
194 7. Examples
196 In this example the first recipient of a two-recipient message is
197 accepted. The second recipient is rejected:
199 558-250 Message accepted
200 558-550-Access denied:
201 558 550 Insufficient permission
203 In this example the first recipient is rejected. The second recipient
204 is accepted:
206 558-550-Access denied
207 558-550 Insufficient permission
208 558-250-Message accepted
209 558 250 Queue ID is 120
212 7.1 Parsing extended replies
214 Extended replies can be parsed rather easily by noting that extended
215 replies are wrapped inside a standard SMTP multiline reply with a 558
216 status code. If the numeric code of the second status reply to the
217 DATA verb is 558, and if the receiving server listed EXDATA in the
218 EHLO keyword list, the "558-" and "558<SP>" characters are removed
219 from the start of every line in the multiline response. The remaining
220 text is now interpreted as a series of individual SMTP replies, one
221 reply per recipient. Each individual reply may be a multiline reply
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231 7.2 Parsing incomplete replies
233 If the client receives an incomplete 558 reply, the client should
234 process normally any of this reply’s subreplies which were received
235 in their entirety, and should process any incomplete or unreceived
236 subreplies as if they had been 451 subreplies.
238 While receiving a 558 reply, the client must implement a timeout of
239 at least 10 minutes per subreply. It MUST use per- subreply timeouts
240 rather than trying to time the entire 558 reply. This means that the
241 overall timeout is approximately proportional to the number of
244 Rationale: The 10 minute limit is chosen because that’s the suggested
245 minimum timeout for processing a message body, in the middle of
246 section 188.8.131.52 of RFC 2821. The second and third sentences above
247 are adapted from the first paragraph of that same section of the RFC.
249 While sending a 558 reply, the server should take no more than about
250 5 minutes to send each subreply. If individual users administer
251 their own filtering software, it would be advisable to use a timeout
252 mechanism to force a decision after 5 minutes.
254 Rationale: if a user’s filter always takes too long, and triggers the
255 client’s 10 minute timeout, other users might not ever be able to
256 receive the message.
258 8. Security Concerns
260 8.1 Transitional behavior
262 The stated benefit of this extension is to allow recipients to use
263 individual mail filters to reject mail during an SMTP transaction.
264 However this extension must be widely implemented for this to happen.
265 Furthermore, the sending SMTP agent can always omit the EXDATA
266 keyword from the MAIL FROM: verb.
268 It follows that recipient-specific mail filtering is still a very
269 desireable feature if the sending SMTP agent does not state its
270 support for the EXDATA extension. One approach is to reply to the
271 DATA verb with an accepted status, then apply individual filters and
272 generate a non-delivery report for recipients whose filters rejected
273 the message. But this would be exactly how individual mail filtering
274 is currently implemented, and it carries with it the same
275 disadvantages and setbacks.
277 Another possible approach is to go ahead and issue an extended reply,
278 which will be interpreted as a permanent failure by the sending SMTP
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287 agent. This approach is usually unacceptable because the sending
288 SMTP agent will correctly conclude that every recipient was
289 undeliverable. If a message comes from a mailing list, the mailing
290 list software can end up removing all recipients from the mailing
291 list only because of a malfunctioning mail filter used by one
292 recipient only.
294 Here’s an approach that attempts to achieve a compromise between
295 having recipient-specific mail filtering, and avoiding unwanted
296 disruptions of this kind.
298 A) Recipients must explicitly enable the ability to set
299 filtering rules for their mailboxes
301 B) Allow each recipient to specify "whitelisted" senders and/or
302 network addresses. The mail server will not do any filtering
303 on white-listed mail, and will never send EXDATA-formatted
304 replies to those sources.
306 C) When the first valid RCPT TO: verb is received (after zero or
307 more RCPT TO: verbs that were deemed to be unacceptable for
308 any other reason), determine if the first recipient has this
309 sender whitelisted.
311 D) If the message is whitelisted by the first accepted
312 recipient, any additional recipients MUST also have the
313 message similarly whitelisted. If not, reject the
314 corresponding RCPT TO: verb with a temporary 421 status code.
315 The sending SMTP agent is expected to retransmit the message
316 for all non whitelisted recipients after a small delay.
318 E) If the message is not whitelisted by the first recipient, any
319 additional recipients MUST NOT have this message whitelisted
320 If they do, reject the corresponding RCPT TO: verb with a
321 temporary 421 status code. The sending SMTP agent is
322 expected to retransmit the message for all whitelisted
323 recipients after a small delay.
325 E) This logic results in separate delivery attempts for
326 whitelisted and non whitelisted recipients (with a small
327 delay in between). Individual recipients can decide whether
328 to whitelist specific sources of mail (such as mailing
329 lists). By whitelisting known mail sources, the recipients
330 can make sure that anyone else’s mail filter will not affect
331 their mail delivery.
333 8.2 Response time to SMTP DATA
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343 Implementations should try to avoid any lengthy delays due to mail
344 filters being used, before returning a reply to the SMTP DATA. Many
345 SMTP clients wait for a response for only a short period of time,
346 before giving up. This can lead to a message duplication problem,
347 described in [RFC1047].
349 9. Comments
351 An experimental patch was made available in September 1997 (with
352 various revisions that followed) to a relatively popular mail server.
353 The patch used an XEXDATA EHLO keyword. The experimental patch was
354 mainly used to implement recipient-specific mail filtering:
355 permitting individual recipients of a message to selectively reject
356 incoming E-mail traffic based on the contents of the message. The
357 patch implemented the XEXDATA extension only for the receiving side,
358 not for the sending side.
360 No issues have been reported concerning the theoretical design of the
361 extended result code.
363 10. References
365 [RFC1425] Klensin, J., Freed, N., Rose, M., Stefferud, E.,
366 Crocker, D. "SMTP Service Extensions", RFC 1425, United
367 Nations University, Innosoft International, Inc., Dover Beach
368 Consulting, Inc., Network Management Associates, Inc., The
369 Branch Office, February 1993
371 [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
372 821, USC/Information Sciences Institute, August 1982.
374 [RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
375 Carnegie Mellon, October 1996.
377 [RFC1047] Partridge, C., "DUPLICATE MESSAGES AND SMTP", RFC 1047,
378 CIC at BBN Labs, February 1988.
380 11. Author’s address
382 Sam Varshavchik
383 Double Precision, Inc.
384 402 Main Street Suite 100-241
385 Metuchen, NJ 08840
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