Second SPF fix, moved to where type is correct.
[exim.git] / doc / doc-txt / experimental-spec.txt
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1From time to time, experimental features may be added to Exim.
2While a feature is experimental, there will be a build-time
3option whose name starts "EXPERIMENTAL_" that must be set in
4order to include the feature. This file contains information
5about experimenatal features, all of which are unstable and
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6liable to incompatibile change.
7
8
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9OCSP Stapling support
10--------------------------------------------------------------
11
12X509 PKI certificates expire and can be revoked; to handle this, the
13clients need some way to determine if a particular certificate, from a
14particular Certificate Authority (CA), is still valid. There are three
15main ways to do so.
16
17The simplest way is to serve up a Certificate Revocation List (CRL) with
18an ordinary web-server, regenerating the CRL before it expires. The
19downside is that clients have to periodically re-download a potentially
20huge file from every certificate authority it knows of.
21
22The way with most moving parts at query time is Online Certificate
23Status Protocol (OCSP), where the client verifies the certificate
24against an OCSP server run by the CA. This lets the CA track all
25usage of the certs. This requires running software with access to the
26private key of the CA, to sign the responses to the OCSP queries. OCSP
27is based on HTTP and can be proxied accordingly.
28
29The only widespread OCSP server implementation (known to this writer)
30comes as part of OpenSSL and aborts on an invalid request, such as
31connecting to the port and then disconnecting. This requires
32re-entering the passphrase each time some random client does this.
33
34The third way is OCSP Stapling; in this, the server using a certificate
35issued by the CA periodically requests an OCSP proof of validity from
36the OCSP server, then serves it up inline as part of the TLS
37negotiation. This approach adds no extra round trips, does not let the
38CA track users, scales well with number of certs issued by the CA and is
39resilient to temporary OCSP server failures, as long as the server
40starts retrying to fetch an OCSP proof some time before its current
41proof expires. The downside is that it requires server support.
42
43If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
44then it gains one new option: "tls_ocsp_file".
45
46The file specified therein is expected to be in DER format, and contain
47an OCSP proof. Exim will serve it as part of the TLS handshake. This
48option will be re-expanded for SNI, if the tls_certificate option
49contains $tls_sni, as per other TLS options.
50
51Exim does not at this time implement any support for fetching a new OCSP
52proof. The burden is on the administrator to handle this, outside of
53Exim. The file specified should be replaced atomically, so that the
54contents are always valid. Exim will expand the "tls_ocsp_file" option
55on each connection, so a new file will be handled transparently on the
56next connection.
57
58Exim will check for a validity next update timestamp in the OCSP proof;
59if not present, or if the proof has expired, it will be ignored.
60
61At this point in time, we're gathering feedback on use, to determine if
62it's worth adding complexity to the Exim daemon to periodically re-fetch
63OCSP files and somehow handling multiple files.
64
65
66
67
0b23848a 68Brightmail AntiSpam (BMI) suppport
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69--------------------------------------------------------------
70
71Brightmail AntiSpam is a commercial package. Please see
72http://www.brightmail.com for more information on
73the product. For the sake of clarity, we'll refer to it as
74"BMI" from now on.
75
76
770) BMI concept and implementation overview
78
79In contrast to how spam-scanning with SpamAssassin is
80implemented in exiscan-acl, BMI is more suited for per
81-recipient scanning of messages. However, each messages is
82scanned only once, but multiple "verdicts" for multiple
83recipients can be returned from the BMI server. The exiscan
84implementation passes the message to the BMI server just
85before accepting it. It then adds the retrieved verdicts to
86the messages header file in the spool. These verdicts can then
87be queried in routers, where operation is per-recipient
88instead of per-message. To use BMI, you need to take the
89following steps:
90
91 1) Compile Exim with BMI support
3ec3e3bb 92 2) Set up main BMI options (top section of Exim config file)
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93 3) Set up ACL control statement (ACL section of the config
94 file)
95 4) Set up your routers to use BMI verdicts (routers section
96 of the config file).
97 5) (Optional) Set up per-recipient opt-in information.
98
8ff3788c 99These four steps are explained in more details below.
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100
1011) Adding support for BMI at compile time
102
103 To compile with BMI support, you need to link Exim against
104 the Brighmail client SDK, consisting of a library
105 (libbmiclient_single.so) and a header file (bmi_api.h).
106 You'll also need to explicitly set a flag in the Makefile to
107 include BMI support in the Exim binary. Both can be achieved
108 with these lines in Local/Makefile:
109
110 EXPERIMENTAL_BRIGHTMAIL=yes
47bbda99 111 CFLAGS=-I/path/to/the/dir/with/the/includefile
ee161e8f 112 EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
8ff3788c 113
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114 If you use other CFLAGS or EXTRALIBS_EXIM settings then
115 merge the content of these lines with them.
116
7c0c8547 117 Note for BMI6.x users: You'll also have to add -lxml2_single
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118 to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do
119 this.
8ff3788c 120
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121 You should also include the location of
122 libbmiclient_single.so in your dynamic linker configuration
123 file (usually /etc/ld.so.conf) and run "ldconfig"
124 afterwards, or else the produced Exim binary will not be
125 able to find the library file.
126
127
3ec3e3bb 1282) Setting up BMI support in the Exim main configuration
ee161e8f 129
3ec3e3bb 130 To enable BMI support in the main Exim configuration, you
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131 should set the path to the main BMI configuration file with
132 the "bmi_config_file" option, like this:
8ff3788c 133
ee161e8f 134 bmi_config_file = /opt/brightmail/etc/brightmail.cfg
8ff3788c 135
3ec3e3bb 136 This must go into section 1 of Exim's configuration file (You
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137 can put it right on top). If you omit this option, it
138 defaults to /opt/brightmail/etc/brightmail.cfg.
139
140 Note for BMI6.x users: This file is in XML format in V6.xx
141 and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI
142 6.x users MUST set the bmi_config_file option.
8ff3788c 143
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144
1453) Set up ACL control statement
146
147 To optimize performance, it makes sense only to process
148 messages coming from remote, untrusted sources with the BMI
149 server. To set up a messages for processing by the BMI
150 server, you MUST set the "bmi_run" control statement in any
151 ACL for an incoming message. You will typically do this in
152 an "accept" block in the "acl_check_rcpt" ACL. You should
153 use the "accept" block(s) that accept messages from remote
154 servers for your own domain(s). Here is an example that uses
3ec3e3bb 155 the "accept" blocks from Exim's default configuration file:
8ff3788c 156
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157
158 accept domains = +local_domains
159 endpass
160 verify = recipient
161 control = bmi_run
162
163 accept domains = +relay_to_domains
164 endpass
165 verify = recipient
166 control = bmi_run
8ff3788c 167
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168 If bmi_run is not set in any ACL during reception of the
169 message, it will NOT be passed to the BMI server.
170
171
1724) Setting up routers to use BMI verdicts
173
174 When a message has been run through the BMI server, one or
175 more "verdicts" are present. Different recipients can have
176 different verdicts. Each recipient is treated individually
177 during routing, so you can query the verdicts by recipient
3ec3e3bb 178 at that stage. From Exim's view, a verdict can have the
ee161e8f 179 following outcomes:
8ff3788c 180
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181 o deliver the message normally
182 o deliver the message to an alternate location
183 o do not deliver the message
8ff3788c 184
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185 To query the verdict for a recipient, the implementation
186 offers the following tools:
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187
188
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189 - Boolean router preconditions. These can be used in any
190 router. For a simple implementation of BMI, these may be
191 all that you need. The following preconditions are
192 available:
8ff3788c 193
ee161e8f 194 o bmi_deliver_default
8ff3788c 195
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196 This precondition is TRUE if the verdict for the
197 recipient is to deliver the message normally. If the
198 message has not been processed by the BMI server, this
199 variable defaults to TRUE.
8ff3788c 200
ee161e8f 201 o bmi_deliver_alternate
8ff3788c 202
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203 This precondition is TRUE if the verdict for the
204 recipient is to deliver the message to an alternate
205 location. You can get the location string from the
206 $bmi_alt_location expansion variable if you need it. See
207 further below. If the message has not been processed by
208 the BMI server, this variable defaults to FALSE.
8ff3788c 209
ee161e8f 210 o bmi_dont_deliver
8ff3788c 211
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212 This precondition is TRUE if the verdict for the
213 recipient is NOT to deliver the message to the
214 recipient. You will typically use this precondition in a
215 top-level blackhole router, like this:
8ff3788c 216
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217 # don't deliver messages handled by the BMI server
218 bmi_blackhole:
219 driver = redirect
220 bmi_dont_deliver
221 data = :blackhole:
8ff3788c 222
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223 This router should be on top of all others, so messages
224 that should not be delivered do not reach other routers
225 at all. If the message has not been processed by
226 the BMI server, this variable defaults to FALSE.
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227
228
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229 - A list router precondition to query if rules "fired" on
230 the message for the recipient. Its name is "bmi_rule". You
231 use it by passing it a colon-separated list of rule
232 numbers. You can use this condition to route messages that
233 matched specific rules. Here is an example:
8ff3788c 234
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235 # special router for BMI rule #5, #8 and #11
236 bmi_rule_redirect:
237 driver = redirect
238 bmi_rule = 5:8:11
239 data = postmaster@mydomain.com
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240
241
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242 - Expansion variables. Several expansion variables are set
243 during routing. You can use them in custom router
244 conditions, for example. The following variables are
245 available:
8ff3788c 246
ee161e8f 247 o $bmi_base64_verdict
8ff3788c 248
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249 This variable will contain the BASE64 encoded verdict
250 for the recipient being routed. You can use it to add a
251 header to messages for tracking purposes, for example:
8ff3788c 252
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253 localuser:
254 driver = accept
255 check_local_user
256 headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
257 transport = local_delivery
8ff3788c 258
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259 If there is no verdict available for the recipient being
260 routed, this variable contains the empty string.
8ff3788c 261
ee161e8f 262 o $bmi_base64_tracker_verdict
8ff3788c 263
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264 This variable will contain a BASE64 encoded subset of
265 the verdict information concerning the "rules" that
266 fired on the message. You can add this string to a
267 header, commonly named "X-Brightmail-Tracker". Example:
8ff3788c 268
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269 localuser:
270 driver = accept
271 check_local_user
272 headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
273 transport = local_delivery
8ff3788c 274
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275 If there is no verdict available for the recipient being
276 routed, this variable contains the empty string.
8ff3788c 277
ee161e8f 278 o $bmi_alt_location
8ff3788c 279
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280 If the verdict is to redirect the message to an
281 alternate location, this variable will contain the
282 alternate location string returned by the BMI server. In
283 its default configuration, this is a header-like string
284 that can be added to the message with "headers_add". If
285 there is no verdict available for the recipient being
286 routed, or if the message is to be delivered normally,
287 this variable contains the empty string.
8ff3788c 288
ee161e8f 289 o $bmi_deliver
8ff3788c 290
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291 This is an additional integer variable that can be used
292 to query if the message should be delivered at all. You
293 should use router preconditions instead if possible.
8ff3788c 294
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295 $bmi_deliver is '0': the message should NOT be delivered.
296 $bmi_deliver is '1': the message should be delivered.
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297
298
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299 IMPORTANT NOTE: Verdict inheritance.
300 The message is passed to the BMI server during message
301 reception, using the target addresses from the RCPT TO:
302 commands in the SMTP transaction. If recipients get expanded
303 or re-written (for example by aliasing), the new address(es)
304 inherit the verdict from the original address. This means
305 that verdicts also apply to all "child" addresses generated
306 from top-level addresses that were sent to the BMI server.
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307
308
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3095) Using per-recipient opt-in information (Optional)
310
311 The BMI server features multiple scanning "profiles" for
312 individual recipients. These are usually stored in a LDAP
313 server and are queried by the BMI server itself. However,
314 you can also pass opt-in data for each recipient from the
315 MTA to the BMI server. This is particularly useful if you
3ec3e3bb 316 already look up recipient data in Exim anyway (which can
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317 also be stored in a SQL database or other source). This
318 implementation enables you to pass opt-in data to the BMI
319 server in the RCPT ACL. This works by setting the
320 'bmi_optin' modifier in a block of that ACL. If should be
321 set to a list of comma-separated strings that identify the
322 features which the BMI server should use for that particular
323 recipient. Ideally, you would use the 'bmi_optin' modifier
324 in the same ACL block where you set the 'bmi_run' control
325 flag. Here is an example that will pull opt-in data for each
326 recipient from a flat file called
327 '/etc/exim/bmi_optin_data'.
8ff3788c 328
ee161e8f 329 The file format:
8ff3788c 330
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331 user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
332 user2@thatdomain.com: <OPTIN STRING3>
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333
334
ee161e8f 335 The example:
8ff3788c 336
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337 accept domains = +relay_to_domains
338 endpass
339 verify = recipient
340 bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
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341 control = bmi_run
342
ee161e8f 343 Of course, you can also use any other lookup method that
3ec3e3bb 344 Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
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345 as long as the result is a list of colon-separated opt-in
346 strings.
8ff3788c 347
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348 For a list of available opt-in strings, please contact your
349 Brightmail representative.
ee161e8f 350
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351
352
353
0b23848a 354Sender Policy Framework (SPF) support
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355--------------------------------------------------------------
356
f413481d 357To learn more about SPF, visit http://www.openspf.org. This
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358document does not explain the SPF fundamentals, you should
359read and understand the implications of deploying SPF on your
360system before doing so.
361
8ff3788c 362SPF support is added via the libspf2 library. Visit
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363
364 http://www.libspf2.org/
8ff3788c 365
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366to obtain a copy, then compile and install it. By default,
367this will put headers in /usr/local/include and the static
368library in /usr/local/lib.
369
3ec3e3bb 370To compile Exim with SPF support, set these additional flags in
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371Local/Makefile:
372
373EXPERIMENTAL_SPF=yes
374CFLAGS=-DSPF -I/usr/local/include
375EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
376
377This assumes that the libspf2 files are installed in
378their default locations.
379
380You can now run SPF checks in incoming SMTP by using the "spf"
381ACL condition in either the MAIL, RCPT or DATA ACLs. When
382using it in the RCPT ACL, you can make the checks dependend on
383the RCPT address (or domain), so you can check SPF records
384only for certain target domains. This gives you the
385possibility to opt-out certain customers that do not want
386their mail to be subject to SPF checking.
387
388The spf condition takes a list of strings on its right-hand
389side. These strings describe the outcome of the SPF check for
390which the spf condition should succeed. Valid strings are:
391
392 o pass The SPF check passed, the sending host
393 is positively verified by SPF.
394 o fail The SPF check failed, the sending host
395 is NOT allowed to send mail for the domain
396 in the envelope-from address.
397 o softfail The SPF check failed, but the queried
398 domain can't absolutely confirm that this
399 is a forgery.
400 o none The queried domain does not publish SPF
401 records.
402 o neutral The SPF check returned a "neutral" state.
403 This means the queried domain has published
404 a SPF record, but wants to allow outside
405 servers to send mail under its domain as well.
406 o err_perm This indicates a syntax error in the SPF
407 record of the queried domain. This should be
408 treated like "none".
409 o err_temp This indicates a temporary error during all
3ec3e3bb 410 processing, including Exim's SPF processing.
ee161e8f 411 You may defer messages when this occurs.
8ff3788c 412
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413You can prefix each string with an exclamation mark to invert
414is meaning, for example "!fail" will match all results but
415"fail". The string list is evaluated left-to-right, in a
416short-circuit fashion. When a string matches the outcome of
417the SPF check, the condition succeeds. If none of the listed
418strings matches the outcome of the SPF check, the condition
419fails.
420
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421Here is an example to fail forgery attempts from domains that
422publish SPF records:
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423
424/* -----------------
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425deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. \
426 Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
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427 spf = fail
428--------------------- */
429
430You can also give special treatment to specific domains:
431
432/* -----------------
433deny message = AOL sender, but not from AOL-approved relay.
434 sender_domains = aol.com
435 spf = fail:neutral
436--------------------- */
437
438Explanation: AOL publishes SPF records, but is liberal and
439still allows non-approved relays to send mail from aol.com.
440This will result in a "neutral" state, while mail from genuine
441AOL servers will result in "pass". The example above takes
442this into account and treats "neutral" like "fail", but only
443for aol.com. Please note that this violates the SPF draft.
444
445When the spf condition has run, it sets up several expansion
446variables.
447
448 $spf_header_comment
449 This contains a human-readable string describing the outcome
450 of the SPF check. You can add it to a custom header or use
451 it for logging purposes.
8ff3788c 452
ee161e8f 453 $spf_received
8fe685ad 454 This contains a complete Received-SPF: header that can be
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455 added to the message. Please note that according to the SPF
456 draft, this header must be added at the top of the header
457 list. Please see section 10 on how you can do this.
8ff3788c 458
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459 Note: in case of "Best-guess" (see below), the convention is
460 to put this string in a header called X-SPF-Guess: instead.
461
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462 $spf_result
463 This contains the outcome of the SPF check in string form,
464 one of pass, fail, softfail, none, neutral, err_perm or
465 err_temp.
8ff3788c 466
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467 $spf_smtp_comment
468 This contains a string that can be used in a SMTP response
469 to the calling party. Useful for "fail".
8ff3788c 470
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471In addition to SPF, you can also perform checks for so-called
472"Best-guess". Strictly speaking, "Best-guess" is not standard
473SPF, but it is supported by the same framework that enables SPF
474capability. Refer to http://www.openspf.org/FAQ/Best_guess_record
475for a description of what it means.
476
477To access this feature, simply use the spf_guess condition in place
478of the spf one. For example:
479
480/* -----------------
481deny message = $sender_host_address doesn't look trustworthy to me
482 spf_guess = fail
483--------------------- */
484
485In case you decide to reject messages based on this check, you
486should note that although it uses the same framework, "Best-guess"
487is NOT SPF, and therefore you should not mention SPF at all in your
488reject message.
489
490When the spf_guess condition has run, it sets up the same expansion
491variables as when spf condition is run, described above.
492
493Additionally, since Best-guess is not standarized, you may redefine
494what "Best-guess" means to you by redefining spf_guess variable in
495global config. For example, the following:
496
497/* -----------------
498spf_guess = v=spf1 a/16 mx/16 ptr ?all
499--------------------- */
500
501would relax host matching rules to a broader network range.
8ff3788c 502
ee161e8f 503
0b23848a 504SRS (Sender Rewriting Scheme) Support
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505--------------------------------------------------------------
506
507Exiscan currently includes SRS support via Miles Wilton's
8ff3788c 508libsrs_alt library. The current version of the supported
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509library is 0.5.
510
511In order to use SRS, you must get a copy of libsrs_alt from
512
513http://srs.mirtol.com/
514
515Unpack the tarball, then refer to MTAs/README.EXIM
516to proceed. You need to set
517
518EXPERIMENTAL_SRS=yes
519
520in your Local/Makefile.
521
522
523--------------------------------------------------------------
524End of file
525--------------------------------------------------------------