[exim.git] / doc / doc-txt / experimental-spec.txt

$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.9 2007/10/04 13:21:06 tom Exp $

From time to time, experimental features may be added to Exim.
While a feature  is experimental, there  will be a  build-time
option whose name starts  "EXPERIMENTAL_" that must be  set in
order to include the  feature. This file contains  information
about experimenatal  features, all  of which  are unstable and
liable to incompatibile change.


0. DKIM support
--------------------------------------------------------------

DKIM support is implemented via libdkim. A compatible version
is available here:

http://duncanthrax.net/exim-experimental/libdkim-1.0.15-tk.tar.gz

Build the lib according to the instructions in the enclosed
INSTALL file.

To build Exim with DKIM support, specify this in Local/Makefile:

EXPERIMENTAL_DKIM=yes
CFLAGS  += -I/home/tom/libdkim/include
LDFLAGS += -ldkim -lssl -lstdc++ -L/home/tom/libdkim/lib

Remember to tweak  the CFLAGS and  LDFLAGS lines to  match the
location of the libdomainkeys includes and lib on your system.

The current experimental implementation supports two independent
functions:

o Validate incoming DKIM-signed email.
o Sign outgoing email with DKIM.

The former is implemented in the ACLs for SMTP, the latter  as
an extension to the SMTP transport. That means both facilities
are limited to SMTP I/O.


1) Validate incoming email

Incoming messages are fed to the DKIM validation process as they
are received "on  the wire". This happens synchronously to Exim's
buffering of the message in the spool.

You must set "control = dkim_verify" in one of the ACLs preceding
DATA (you will typically use acl_smtp_rcpt), at a point where
non-local, non-relay, non-submission mail is processed. If that
control flag is not set, the message will NOT be verified.

Example:

warn log_message = Feeding message to DKIM validator.
     control = dk_verify

You can then check for DKIM signatures in the ACL after data
(acl_smtp_data), using the 'dkim' query-style lookup type. The
query string should be a domain or DKIM identity:

${lookup dkim{domain.example}}

Such a lookup will yield one of the following strings:

unverified: Exim did not (yet) verify the eventual DKIM
            signatures in this message. This may happen
            if a) You did not use control=dkim_verify
            or b) You are using the lookup before
            the DATA ACL.

unsigned:   The message does not have a signature from
            the specified domain.

good:       The message has a signature from the specified
            domain, and it verified successfully.

bad:        The message has a signature from the specified
            domain, but it did not verify.

defer:      A temporary DNS problem was encountered while
            trying to verify the signature.


2) Sign outgoing email with DKIM

Outgoing messages are  signed just before  Exim puts them  "on
the wire".  The only  thing that happens after DKIM signing is
eventual TLS encryption.

Signing is implemented by setting private options on the  SMTP
transport.  These   options  take   (expandable)  strings   as
arguments.

  dkim_domain = <expanded string> [MANDATORY]

    The domain you want to sign with. Should optimally match
    the domain in the "From:" header of the message, but
    does not necessarily have to. The result of this expanded
    option is put into the $dkim_domain expansion variable.

  dkim_selector = <expanded string> [MANDATORY]

    This  sets  the  key  selector  string.  You  can  use the
    $dkim_domain  expansion  variable  to  look  up  a  matching
    selector.  The result  is put  in the  expansion  variable
    $dkim_selector which  should be  used in  the dkim_private_key
    option along with $dkim_domain.

  dkim_private_key = <expanded string> [MANDATORY]

    This  sets the  private key to use. You can use the
    $dkim_domain and  $dkim_selector expansion variables to
    determine the private key to use. The result can either

      o be a valid RSA private key in ASCII armor, including
        line breaks.
      o start with a slash, in which case it is treated as
        a file that contains the private key.
      o be "0", "false" or the empty string, in which case
        the message will not be signed. This case will not
        result in an error, even if dkim_strict is set.

  dkim_canon = <expanded string> [OPTIONAL]

    This option sets the canonicalization method used when
    signing a message. The DKIM RFC currently supports two
    methods: "simple" and "relaxed".  The option defaults to
    "relaxed" when unset. Note: the current implementation
    only support using the same canonicalization method for
    both headers and body.

  dkim_strict = <expanded string> [OPTIONAL]

    This  option  defines  how  Exim  behaves  when  signing a
    message that should be signed fails for some reason.  When
    the expansion evaluates to either "1" or "true", Exim will
    defer. Otherwise Exim will send the message unsigned.  You
    can  use the $dkim_domain and $dkim_selector expansion
    variables here.

  dkim_sign_headers = <expanded string> [OPTIONAL]

    When set, this option must expand to (or be specified as)
    a colon-separated list of header names. These headers will
    be included in the message signature. When unspecified,
    the recommended headers will be used. Currently, these
    are:

    from:sender:reply-to:subject:date:
    message-id:to:cc:mime-version:content-type:
    content-transfer-encoding:content-id:
    content-description:resent-date:resent-from:
    resent-sender:resent-to:resent-cc:resent-message-id:
    in-reply-to:references:
    list-id:list-help:list-unsubscribe:
    list-subscribe:list-post:list-owner:list-archive


1. Yahoo DomainKeys support
--------------------------------------------------------------

DomainKeys  (DK)  support  is   built  into  Exim  using   the
"libdomainkeys"  reference   library  implementation.   It  is
available at

http://domainkeys.sf.net

You must build  this library on  your system and  compile Exim
against it. To build Exim with DK support, add these lines  to
your Local/Makefile:

EXPERIMENTAL_DOMAINKEYS=yes
CFLAGS  += -I/home/tom/exim-cvs/extra/libdomainkeys
LDFLAGS += -ldomainkeys -L/home/tom/exim-cvs/extra/libdomainkeys

Remember to tweak  the CFLAGS and  LDFLAGS lines to  match the
location of the libdomainkeys includes and lib on your system.

The   current   experimental   implementation   supports   two
independent functions:

o Validate incoming DK-signed email.
o Sign outgoing email with DK.

The former is implemented in the ACLs for SMTP, the latter  as
an extension to the SMTP transport. That means both facilities
are limited to SMTP I/O.


1) Validate incoming email

Incoming messages are fed to the DK validation process as they
are  received "on  the wire".  This happens  synchronously to
Exim's buffering of the message in the spool.

You  must  set  "control  =  dk_verify"  in  one  of  the ACLs
preceding DATA  (you will  typically use  acl_smtp_rcpt), at a
point  where  non-local,  non-relay,  non-submission  mail  is
processed. If that control flag  is not set, the message  will
NOT be verified.

Example:

warn log_message = Feeding message to DK validator.
     control = dk_verify

You can check for the outcome of the DK check in the ACL after
data (acl_smtp_data), using a number of ACL conditions  and/or
expansion variables.


1.1.) DK ACL conditions

  dk_sender_domains = <domain list>

    This   condition   takes  a   domainlist  as argument  and
    succeeds if the domain that DK has  been verifying  for is
    found in the list.


  dk_senders = <address list>

    This  condition  takes  an  addresslist  as argument   and
    succeeds  if  the address  that DK has been  verifying for
    is  found in the list.


  dk_sender_local_parts = <local part list>

    This  condition  takes   a local_part  list   as  argument
    and  succeeds   if  the   domain   that    DK   has   been
    verifying  for is found in the list.


  dk_status = <colon separated list of keywords>

    This condition takes a  list of keywords as  argument, and
    succeeds if one of the listed keywords matches the outcome
    of the DK check. The available keywords are:

    good            DK check succeeded, mail is verified.
    bad             DK check failed.
    no signature    Mail is not signed with DK.
    no key          Public key missing in target domain DNS.
    bad format      Public key available, but unuseable.
    non-participant Target domain states not to participate in DK.
    revoked         The signing key has been revoked by the domain.


  dk_policy = <colon separated list of keywords>

    This condition takes a  list of keywords as  argument, and
    succeeds if one of the listed keywords matches the  policy
    announced  by the  target domain.  The available  keywords
    are:

    signsall        The target domain signs all outgoing email.
    testing         The target domain is currently testing DK.


  dk_domain_source = <colon separated list of keywords>

    This condition takes a  list of keywords as  argument, and
    succeeds  if  one  of  the  listed  keywords  matches  the
    location where DK found the sender domain it verified for.
    The available keywords are:

    from            The domain came from the "From:" header.
    sender          The domain came from the "Sender:" header.
    none            DK was unable to find the responsible domain.


1.2.) DK verification expansion variables

  $dk_sender_domain

    Contains the domain that DK has verified for.


  $dk_sender

    Contains the address that DK has verified for.


  $dk_sender_local_part

    Contains the local part that DK has verified for.


  $dk_sender_source

    Contains the "source" of the above three variables, one of

      "from"    The address came from the "From:" header.
      "sender"  The address came from the "Sender:" header.

    When DK was unable to find a valid address, this variable
    is "0".


  $dk_signsall

    Is "1" if the target domain signs all outgoing email,
    "0" otherwise.


  $dk_testing

    Is "1" if the target domain is testing DK, "0" otherwise.


  $dk_is_signed

    Is "1" if the message is signed, "0" otherwise.


  $dk_status

    Contains the outcome of the DK check as a string, commonly
    used to add a "DomainKey-Status:" header to messages. Will
    contain one of:

    good            DK check succeeded, mail is verified.
    bad             DK check failed.
    no signature    Mail is not signed with DK.
    no key          Public key missing in target domain DNS.
    bad format      Public key available, but unuseable.
    non-participant Target domain states not to participate in DK.
    revoked         The signing key has been revoked by the domain.


  $dk_result

    Contains a  human-readable result  of the  DK check,  more
    verbose than $dk_status. Useful for logging purposes.


2) Sign outgoing email with DK

Outgoing messages are  signed just before  Exim puts them  "on
the wire".  The only  thing that  happens after  DK signing is
eventual TLS encryption.

Signing is implemented by setting private options on the  SMTP
transport.  These   options  take   (expandable)  strings   as
arguments.  The  most  important  variable  to  use  in  these
expansions is $dk_domain. It contains the domain that DK wants
to sign for.


  dk_selector = <expanded string> [MANDATORY]

    This  sets  the  key  selector  string.  You  can  use the
    $dk_domain  expansion  variable  to  look  up  a  matching
    selector.  The result  is put  in the  expansion  variable
    $dk_selector which  should be  used in  the dk_private_key
    option along with $dk_domain.


  dk_private_key = <expanded string> [MANDATORY]

    This  sets the  private key  to use.  You SHOULD  use  the
    $dk_domain   and  $dk_selector   expansion  variables   to
    determine the private key to use. The result can either

      o be a valid RSA private key in ASCII armor, including
        line breaks.
      o start with a slash, in which case it is treated as
        a file that contains the private key.
      o be "0", "false" or the empty string, in which case
        the message will not be signed. This case will not
        result in an error, even if dk_strict is set.


  dk_canon = <expanded string> [OPTIONAL]

    This  option sets  the canonicalization  method used  when
    signing a  message. The  DK draft  currently supports  two
    methods:  "simple"  and "nofws".  The  option defaults  to
    "simple" when unset.


  dk_strict = <expanded string> [OPTIONAL]

    This  option  defines  how  Exim  behaves  when  signing a
    message that should be signed fails for some reason.  When
    the expansion evaluates to either "1" or "true", Exim will
    defer. Otherwise Exim will send the message unsigned.  You
    can  and  should use  the  $dk_domain   and   $dk_selector
    expansion  variables here.


  dk_domain = <expanded string> [NOT RECOMMENDED]

    This  option overrides  DKs autodetection  of the  signing
    domain. You should only use  this option if you know  what
    you are doing. The result of the string expansion is  also
    put in $dk_domain.


2. Brightmail AntiSpam (BMI) suppport
--------------------------------------------------------------

Brightmail  AntiSpam  is  a  commercial  package.  Please  see
http://www.brightmail.com    for    more    information     on
the product. For  the sake of  clarity, we'll refer  to it  as
"BMI" from now on.


0) BMI concept and implementation overview

In  contrast  to   how  spam-scanning  with   SpamAssassin  is
implemented  in  exiscan-acl,  BMI  is  more  suited  for  per
-recipient  scanning of  messages. However,  each messages  is
scanned  only  once,  but  multiple  "verdicts"  for  multiple
recipients can be  returned from the  BMI server. The  exiscan
implementation  passes  the  message to  the  BMI  server just
before accepting it.  It then adds  the retrieved verdicts  to
the messages header file in the spool. These verdicts can then
be  queried  in  routers,  where  operation  is  per-recipient
instead  of per-message.  To use  BMI, you  need to  take the
following steps:

  1) Compile Exim with BMI support
  2) Set up main BMI options (top section of Exim config file)
  3) Set up ACL control statement (ACL section of the config
     file)
  4) Set up your routers to use BMI verdicts (routers section
     of the config file).
  5) (Optional) Set up per-recipient opt-in information.

These four steps are explained in more details below.

1) Adding support for BMI at compile time

  To compile with BMI support,  you need to link Exim  against
  the   Brighmail  client   SDK,  consisting   of  a   library
  (libbmiclient_single.so)  and  a  header  file  (bmi_api.h).
  You'll also need to explicitly set a flag in the Makefile to
  include BMI support in the Exim binary. Both can be achieved
  with  these lines in Local/Makefile:

  EXPERIMENTAL_BRIGHTMAIL=yes
  CFLAGS=-I/path/to/the/dir/with/the/includefile
  EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single

  If  you use  other CFLAGS  or EXTRALIBS_EXIM  settings then
  merge the content of these lines with them.

  Note for BMI6.x users: You'll also have to add -lxml2_single
  to the EXTRALIBS_EXIM line. Users of 5.5x do not need to  do
  this.

  You    should     also    include     the    location     of
  libbmiclient_single.so in your dynamic linker  configuration
  file   (usually   /etc/ld.so.conf)   and   run    "ldconfig"
  afterwards, or  else the  produced Exim  binary will  not be
  able to find the library file.


2) Setting up BMI support in the Exim main configuration

  To enable BMI  support in the  main Exim configuration,  you
  should set the path to the main BMI configuration file  with
  the "bmi_config_file" option, like this:

  bmi_config_file = /opt/brightmail/etc/brightmail.cfg

  This must go into section 1 of Exim's configuration file (You
  can  put it  right on  top). If  you omit  this option,  it
  defaults to /opt/brightmail/etc/brightmail.cfg.

  Note for BMI6.x users: This  file is in XML format  in V6.xx
  and its  name is  /opt/brightmail/etc/bmiconfig.xml. So  BMI
  6.x users MUST set the bmi_config_file option.


3) Set up ACL control statement

  To  optimize performance,  it makes  sense only  to process
  messages coming from remote, untrusted sources with the  BMI
  server.  To set  up a  messages for  processing by  the BMI
  server, you MUST set the "bmi_run" control statement in  any
  ACL for an incoming message.  You will typically do this  in
  an "accept"  block in  the "acl_check_rcpt"  ACL. You should
  use the "accept" block(s)  that accept messages from  remote
  servers for your own domain(s). Here is an example that uses
  the "accept" blocks from Exim's default configuration file:


  accept  domains       = +local_domains
          endpass
          verify        = recipient
          control       = bmi_run

  accept  domains       = +relay_to_domains
          endpass
          verify        = recipient
          control       = bmi_run

  If bmi_run  is not  set in  any ACL  during reception of the
  message, it will NOT be passed to the BMI server.


4) Setting up routers to use BMI verdicts

  When a message has been  run through the BMI server,  one or
  more "verdicts" are  present. Different recipients  can have
  different verdicts. Each  recipient is treated  individually
  during routing, so you  can query the verdicts  by recipient
  at  that stage.  From Exim's  view, a  verdict can  have the
  following outcomes:

  o deliver the message normally
  o deliver the message to an alternate location
  o do not deliver the message

  To query  the verdict  for a  recipient, the  implementation
  offers the following tools:


  - Boolean router  preconditions. These  can be  used in  any
    router. For a simple  implementation of BMI, these  may be
    all  that  you  need.  The  following  preconditions   are
    available:

    o bmi_deliver_default

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient is  to deliver  the message  normally. If  the
      message has not been  processed by the BMI  server, this
      variable defaults to TRUE.

    o bmi_deliver_alternate

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient  is to  deliver the  message to  an alternate
      location.  You  can  get the  location  string  from the
      $bmi_alt_location expansion variable if you need it. See
      further below. If the message has not been processed  by
      the BMI server, this variable defaults to FALSE.

    o bmi_dont_deliver

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient  is  NOT  to   deliver  the  message  to   the
      recipient. You will typically use this precondition in a
      top-level blackhole router, like this:

        # don't deliver messages handled by the BMI server
        bmi_blackhole:
          driver = redirect
          bmi_dont_deliver
          data = :blackhole:

      This router should be on top of all others, so  messages
      that should not be delivered do not reach other  routers
      at all. If   the  message  has  not  been  processed  by
      the  BMI server, this variable defaults to FALSE.


  - A list router  precondition to query  if rules "fired"  on
    the message for the recipient. Its name is "bmi_rule". You
    use  it  by  passing it  a  colon-separated  list of  rule
    numbers. You can use this condition to route messages that
    matched specific rules. Here is an example:

      # special router for BMI rule #5, #8 and #11
      bmi_rule_redirect:
        driver = redirect
        bmi_rule = 5:8:11
        data = postmaster@mydomain.com


  - Expansion variables. Several  expansion variables are  set
    during  routing.  You  can  use  them  in  custom   router
    conditions,  for  example.  The  following  variables  are
    available:

    o $bmi_base64_verdict

      This variable  will contain  the BASE64  encoded verdict
      for the recipient being routed. You can use it to add  a
      header to messages for tracking purposes, for example:

      localuser:
        driver = accept
        check_local_user
        headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
        transport = local_delivery

      If there is no verdict available for the recipient being
      routed, this variable contains the empty string.

    o $bmi_base64_tracker_verdict

      This variable  will contain  a BASE64  encoded subset of
      the  verdict  information  concerning  the  "rules" that
      fired  on the  message. You  can add  this string  to a
      header, commonly named "X-Brightmail-Tracker". Example:

      localuser:
        driver = accept
        check_local_user
        headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
        transport = local_delivery

      If there is no verdict available for the recipient being
      routed, this variable contains the empty string.

    o $bmi_alt_location

      If  the  verdict  is  to  redirect  the  message  to  an
      alternate  location,  this  variable  will  contain  the
      alternate location string returned by the BMI server. In
      its default configuration, this is a header-like  string
      that can be added to the message with "headers_add".  If
      there is  no verdict  available for  the recipient being
      routed, or if the  message is to be  delivered normally,
      this variable contains the empty string.

    o $bmi_deliver

      This is an additional integer variable that can be  used
      to query if the message should be delivered at all.  You
      should use router preconditions instead if possible.

      $bmi_deliver is '0': the message should NOT be delivered.
      $bmi_deliver is '1': the message should be delivered.


  IMPORTANT NOTE: Verdict inheritance.
  The  message  is passed  to  the BMI  server  during message
  reception,  using the  target addresses  from the  RCPT TO:
  commands in the SMTP transaction. If recipients get expanded
  or re-written (for example by aliasing), the new address(es)
  inherit the  verdict from  the original  address. This means
  that verdicts also apply to all "child" addresses  generated
  from top-level addresses that were sent to the BMI server.


5) Using per-recipient opt-in information (Optional)

  The  BMI server  features multiple  scanning "profiles"  for
  individual recipients.  These are  usually stored  in a LDAP
  server and are  queried by the  BMI server itself.  However,
  you can also  pass opt-in data  for each recipient  from the
  MTA to the  BMI server. This  is particularly useful  if you
  already look  up recipient  data in  Exim anyway  (which can
  also be  stored in  a SQL  database or  other source).  This
  implementation enables you  to pass opt-in  data to the  BMI
  server  in  the  RCPT   ACL.  This  works  by   setting  the
  'bmi_optin' modifier in  a block of  that ACL. If  should be
  set to a list  of comma-separated strings that  identify the
  features which the BMI server should use for that particular
  recipient. Ideally, you  would use the  'bmi_optin' modifier
  in the same  ACL block where  you set the  'bmi_run' control
  flag. Here is an example that will pull opt-in data for each
  recipient      from       a      flat       file      called
  '/etc/exim/bmi_optin_data'.

  The file format:

    user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
    user2@thatdomain.com: <OPTIN STRING3>


  The example:

    accept  domains       = +relay_to_domains
            endpass
            verify        = recipient
            bmi_optin     = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
            control       = bmi_run

  Of course,  you can  also use  any other  lookup method that
  Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
  as long as  the result is  a list of  colon-separated opt-in
  strings.

  For a list of available opt-in strings, please contact  your
  Brightmail representative.


3. Sender Policy Framework (SPF) support
--------------------------------------------------------------

To learn  more  about  SPF, visit   http://spf.pobox.com. This
document does   not explain  the SPF  fundamentals, you should
read and understand the implications of deploying SPF on  your
system before doing so.

SPF support is added via the libspf2 library. Visit

  http://www.libspf2.org/

to obtain  a copy,  then compile  and install  it. By default,
this will  put headers  in /usr/local/include  and the  static
library in /usr/local/lib.

To compile Exim with SPF support, set these additional flags in
Local/Makefile:

EXPERIMENTAL_SPF=yes
CFLAGS=-DSPF -I/usr/local/include
EXTRALIBS_EXIM=-L/usr/local/lib -lspf2

This assumes   that the   libspf2 files   are installed  in
their default locations.

You can now run SPF checks in incoming SMTP by using the "spf"
ACL condition  in either  the MAIL,  RCPT or  DATA ACLs.  When
using it in the RCPT ACL, you can make the checks dependend on
the RCPT  address (or  domain), so  you can  check SPF records
only  for   certain  target   domains.  This   gives  you  the
possibility  to opt-out  certain customers  that do  not want
their mail to be subject to SPF checking.

The spf condition  takes a list  of strings on  its right-hand
side. These strings describe the outcome of the SPF check  for
which the spf condition should succeed. Valid strings are:

  o pass      The SPF check passed, the sending host
              is positively verified by SPF.
  o fail      The SPF check failed, the sending host
              is NOT allowed to send mail for the domain
              in the envelope-from address.
  o softfail  The SPF check failed, but the queried
              domain can't absolutely confirm that this
              is a forgery.
  o none      The queried domain does not publish SPF
              records.
  o neutral   The SPF check returned a "neutral" state.
              This means the queried domain has published
              a SPF record, but wants to allow outside
              servers to send mail under its domain as well.
  o err_perm  This indicates a syntax error in the SPF
              record of the queried domain. This should be
              treated like "none".
  o err_temp  This indicates a temporary error during all
              processing, including Exim's SPF processing.
              You may defer messages when this occurs.

You can prefix each string with an exclamation mark to  invert
is meaning,  for example  "!fail" will  match all  results but
"fail".  The  string  list is  evaluated  left-to-right,  in a
short-circuit fashion.  When a  string matches  the outcome of
the SPF check, the condition  succeeds. If none of the  listed
strings matches the  outcome of the  SPF check, the  condition
fails.

Here is a simple example to fail forgery attempts from domains
that publish SPF records:

/* -----------------
deny message = $sender_host_address is not allowed to send mail from $sender_address_domain
     spf = fail
--------------------- */

You can also give special treatment to specific domains:

/* -----------------
deny message = AOL sender, but not from AOL-approved relay.
     sender_domains = aol.com
     spf = fail:neutral
--------------------- */

Explanation: AOL  publishes SPF  records, but  is liberal  and
still allows  non-approved relays  to send  mail from aol.com.
This will result in a "neutral" state, while mail from genuine
AOL servers  will result  in "pass".  The example  above takes
this into account and  treats "neutral" like "fail",  but only
for aol.com. Please note that this violates the SPF draft.

When the spf condition has run, it sets up several expansion
variables.

  $spf_header_comment
  This contains a human-readable string describing the outcome
  of the SPF check. You can add it to a custom header or use
  it for logging purposes.

  $spf_received
  This contains a complete Received-SPF: header that can be
  added to the message. Please note that according to the SPF
  draft, this header must be added at the top of the header
  list. Please see section 10 on how you can do this.

  $spf_result
  This contains the outcome of the SPF check in string form,
  one of pass, fail, softfail, none, neutral, err_perm or
  err_temp.

  $spf_smtp_comment
  This contains a string that can be used in a SMTP response
  to the calling party. Useful for "fail".


4. SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------

Exiscan  currently  includes SRS  support  via Miles  Wilton's
libsrs_alt library. The current version of the supported
library is 0.5.

In order to  use SRS, you  must get a  copy of libsrs_alt from

http://srs.mirtol.com/

Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set

EXPERIMENTAL_SRS=yes

in your Local/Makefile.


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