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********************************
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* SHOULD YOU USE THIS PACKAGE? *
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********************************
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Since version 4.50, Exim has the content-scanning extension formerly
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known as "exiscan" built-in. It has a number of advantages and
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disadvantages compared to SA-Exim.
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Advantages of built-in content-scanning interface:
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 * One less configuration file to edit.
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 * Spam control policy integrates better with Exim's ACL system.
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 * It's possible to tell SA which user to scan for (the -u parameter of
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   spamc). SA-Exim can't do that (yet).
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 * Finer control over the mail header is possible, but not in a clean
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   way (it involves putting all header fields you might possibly want
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   to add in the report template, and using rather complicated
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   expansion expressions to extract the wanted ones from
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   $spam_report). At any rate, you can choose a prefix different from
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   "X-Spam-".
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Advantages of SA-Exim:
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 * It is possible to use the report_safe feature, which turns mail
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   deemed to be spam into a message/rfc822 attachment of a report
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   message. (Note however that if you do, then any X-SA-* fields added
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   to help the greylisting module can't be removed.)
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 * All the add_header and rewrite_header options in
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   /etc/spamassassin/local.cf will be obeyed. In other words,
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   everything will be *almost* as if you filtered the mail through
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   spamassassin on the command line.
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 * So-called teergrubing ("tarpitting") is possible in a way that
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   isn't possible with exiscan (I'm not in any way saying that it
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   works as a counterattack against spammers).
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 * You can simply add the sa-exim package to a standard exim4
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   installation and it should, in principle, instantly work (except
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   you have to uncomment one line in sa-exim.conf).
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Both alternatives enable you to defer, greylist, reject, and blackhole
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mail, optionally saving copies, at configurable score levels.
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*****************
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* CONFIGURATION *
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*****************
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This version of the sa-exim package defaults to placing a configuration
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sniplet in /etc/exim4/conf.d/. Depending on what you have answered to the
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DebConf questions while configuring Exim4, the module will be loaded
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automatically, or human intervention is required.
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To find out what configuration file Exim4 is using, issue:
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  $ exim4 -bV | tail -1
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  Configuration file is /path/to/configfile
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If /path/to/configfile shows:
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  - /etc/exim4/exim4.conf
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    You are using the hand-crafted configuration file.
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	See the 'HAND-CRAFTED' section below.
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  - /var/lib/exim4/config.autogenerated
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    You are using the debianized configuration scheme - with either
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    'split' or 'unsplit' configuration file.
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	See the 'DEBIANIZED' section below.
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HAND-CRAFTED
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------------
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Use 'grep "local_scan_path" /etc/exim4/exim4.conf" to see if the sa-exim
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line is included in the configuration. If grep returns something, check
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if it matches the following line. If grep returns nothing, you have to
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manually add the following line to the exim4.conf file and restart exim4.
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    local_scan_path = /usr/lib/exim4/local_scan/sa-exim.so
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Change or add the line above and manually restart exim4 by issuing
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'invoke-rc.d exim4 reload' or '/etc/init.d/exim4 reload' as root.
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DEBIANIZED
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----------
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Use 'grep "local_scan_path" /var/lib/exim4/config.autogenerated' to
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see if the sa-exim line is included in the configuration. If grep
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returns something, you're set and already using the sa-exim module. If
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grep returns nothing, we need to figure out a few things:
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Issue:
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	$ grep "use_split_config" /etc/exim4/update-exim4.conf.conf
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      dc_use_split_config='true'
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If your result shows 'false' where mine shows 'true', then you're
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using the unsplit configuration, generated from
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/etc/exim4/exim4.conf.template.  If you haven't customized that file
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you could edit /etc/exim4/update-exim4.conf.conf by hand, change the
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'false' to 'true' and issue 'update-exim4.conf' as root. Then, check
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again if the sa-exim module line is included. It should. If it still
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isn't: mail me. If it is, restart exim4 by issuing 'invoke-rc.d exim4
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restart' or '/etc/init.d/exim4 restart' as root. If you *have*
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customized /etc/exim4/exim4.conf.template, then you'd better stick
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with the unsplit configuration scheme and add the local_scan_path
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setting by hand, like with the hand-crafted configuration file.
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***************
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* GREYLISTING *
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***************
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Greylisting is implemented as a SpamAssassin module. To enable it you
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need to add the following five lines to your SpamAssassin
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configuration:
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loadplugin Greylisting /usr/share/perl5/Mail/SpamAssassin/Plugin/Greylisting.pm
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header GREYLIST_ISWHITE eval:greylisting("( 'dir' => '/var/spool/sa-exim/tuplets'; 'method' => 'dir'; 'greylistsecs' => '1800'; 'dontgreylistthreshold' => 11; 'connectiphdr' => 'X-SA-Exim-Connect-IP'; 'envfromhdr' => 'X-SA-Exim-Mail-From'; 'rcpttohdr' => 'X-SA-Exim-Rcpt-To'; 'greylistnullfrom' => 1; 'greylistfourthbyte' => 0 )")
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describe GREYLIST_ISWHITE The incoming server has been whitelisted for this recipient and sender
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score GREYLIST_ISWHITE  -1.5
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priority GREYLIST_ISWHITE 99999
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(It is a long-standing bug that the module is installed in the wrong
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directory, which is why the full path has to be specified on the
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loadplugin line, but fixing it is probably not worth the disruption of
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existing installations.)
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If two messages from the same /24 IPv4 network or /64 IPv6 network (or
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individual IP address, depending on greylistfourthbyte), with the same
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sender, with the same list of recipient, and with a score below
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dontgreylistthreshold are seen at least greylistsecs apart, the
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triplet will be whitelisted and the GREYLIST_ISWHITE rule will be
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considered to match thenceforth. That will signal to the local_scan
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library to raise SAtempreject to let the message through, in addition
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to the negative spam score it carries.
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Notice that messages can be permanently rejected (score above
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SApermreject) and still get a triplet whitelisted if the score is
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below dontgreylistthreshold. If dontgreylistthreshold or SAtempreject
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+ SAgreylistraisetempreject are less than SApermreject, some mail may
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be temporarily rejected indefinitely.
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See README.Greylisting for more details.
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***********************
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* SPAMD CONFIGURATION *
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***********************
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By default, spamd runs as root and assumes the identity of the user it
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is told it is scanning mail on behalf of by whoever connects to it
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(see README.spamd.gz in the spamassassin package for a discussion on
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security). When SA-Exim runs spamc, this user will normally be
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Debian-exim. You can set the SAspamcUser option in sa-exim.conf to
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override this, but since a mail can have multiple recipients and is
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only scanned once, per-user setups are problematic. Also, the
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greylisting module won't work unless all users can write to the
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tuplets directory.
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Thus, when using SpamAssassin together with SA-Exim you may want to
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run spamd under a specific system account by modifying the OPTIONS
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variable in /etc/default/spamassassin to include a --username option.
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However, if you ONLY use SpamAssassin with SA-Exim this is in practice
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not strictly necessary.
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You should NOT run spamd as the "nobody" user and/or the "nogroup"
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group if you configure SpamAssassin to use sa-exim's greylisting
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module, the bayesian classifier, or any helper module that needs to
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write files, because nobody/nogroup should be completely unprivileged
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and thus not own any files. Instead you should create a dedicated
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account to run spamd under. You can then adjust the ownership of
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/var/spool/sa-exim/tuplets and the username in
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/etc/cron.d/greylistclean accordingly.
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***********************************
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* PROBLEMS WITH BAYES AUTO-EXPIRY *
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***********************************
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When scanning mail during the SMTP dialogue there is somewhat limited
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time before the remote host gives up, even if they should wait for at
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least ten minutes. To avoid Exim returning a temporary error status,
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or the remote host giving up prematurely and in some cases for good,
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SA-Exim overrides Exim's timeout handler and accepts the message if
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SpamAssassin takes too long, by default 240 seconds.
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Using SpamAssassin's Bayesian learning module means that it will
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automatically expire old tokens when its database has grown too large.
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That can take several minutes. If it takes too long, SA-Exim will
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abort it, meaning that SpamAssassin will run auto-expiry again next
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time, and be aborted, and so on...
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If this happens, you have a few remedies:
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1) Set SAtimeout to a higher value in /etc/exim4/sa-exim.conf.
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2) Run sa-learn --force-expire periodically. How you run it depends on
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   how you've configured SpamAssassin. Running it as Debian-exim may
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   be sufficient.
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2 a) In addition, you can add
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   bayes_auto_expire 0
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   to /etc/spamassassin/local.cf. This may not be a good idea if
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   SpamAssassin, for whatever reason, is also used as a more
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   traditional filter from e.g. .procmailrc, as all users will need to
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   run sa-learn --force-expire then.
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2 b) If you get a lot of mail, consider adding
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   bayes_learn_to_journal 1
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   to local.cf. See the Mail::SpamAssassin::Conf(3) manual page for
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   more information.
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**********************************
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* NOTICE ABOUT SPAMC CONFIG FILE *
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**********************************
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Recent versions of spamc can read command-line parameters and switches
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from a configuration file called /etc/spamassassin/spamc.conf. If that
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file specifies conflicting options, it will prevent SA-Exim from
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working. For now, you'll have to make sure that it doesn't.
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 -- Magnus Holmgren <holmgren@debian.org>, Fri, 22 Jul 2016 09:58:32 +0200