Exiscan-ACL's “spam” condition passes the message through either SpamAssassin or Brightmail, and triggers if these indicate that the message is junk. By default, it connects to a SpamAssassin daemon (spamd) running on localhost. The host address and port can be changed by adding a spamd_address setting in the main section of the Exim configuration file. For more information, see the exiscan-acl-spect.txt file included with the patch.

In our implementation, we are going to reject messages classified as spam. However, we would like to keep a copy of such messages in a separate mail folder, at least for the time being. This is so that the user can periodically scan for False Positives.

Exim offers controls that can be applied to a message that is accepted, such as freeze. The Exiscan-ACL patch adds one more of these controls, namely fakereject. This causes the following SMTP response:

550-FAKEREJECT id=message-id
550-Your message has been rejected but is being kept for evaluation.
550 If it was a legit message, it may still be delivered to the target recipient(s).

We can incorporate this feature into our implementation, by inserting the following snippet in acl_data, prior to the final accept statement:

  # Invoke SpamAssassin to obtain $spam_score and $spam_report.
  # Depending on the classification, $acl_m9 is set to "ham" or "spam".
  #
  # If the message is classified as spam, pretend to reject it. 
  #
  warn
    set acl_m9  = ham
    spam        = mail
    set acl_m9  = spam
    control     = fakereject
    logwrite    = :reject: Rejected spam (score $spam_score): $spam_report

  # Add an appropriate X-Spam-Status: header to the message.
  #
  warn
    message     = X-Spam-Status: \
                  ${if eq {$acl_m9}{spam}{Yes}{No}} (score $spam_score)\
                  ${if def:spam_report {: $spam_report}}
    logwrite    = :main: Classified as $acl_m9 (score $spam_score)

In this example, $acl_m9 is initially set to “ham”. Then SpamAssassin is invoked as the user mail. If the message is classified as spam, then $acl_m9 is set to “spam”, and the FAKEREJECT response above is issued. Finally, an X-Spam-Status: header is added to the message. The idea is that the Mail Delivery Agent or the recipient's Mail User Agent can use this header to filter junk mail into a separate folder.

By default, SpamAssassin presents its report in a verbose, table-like format, mainly suitable for inclusion in or attachment to the message body. In our case, we want a terse report, suitable for the X-Spam-Status: header in the example above. To do this, we add the following snippet in its site specific configuration file (/etc/spamassassin/local.cf, /etc/mail/spamassassin/local.cf, or similar):

### Report template
clear_report_template
report "_TESTSSCORES(, )_"

Also, a Bayesian scoring feature is built in, and is turned on by default. We normally want to turn this off, because it requires training that will be specific to each user, and thus is not suitable for system-wide SMTP time filtering:

### Disable Bayesian scoring
use_bayes 0

For these changes to take effect, you have to restart the SpamAssassin daemon (spamd).

Say you have a number of users that want to specify their individual SpamAssassin preferences, such as the spam threshold, acceptable languages and character sets, white/blacklisted senders, and so on. Or perhaps they really want to be able to make use of SpamAssassin's native Bayesian scoring (though I don't see why^[17]).

As discussed in the User Settings and Data section earlier in the document, there is a way for this to happen. We need to limit the number of recipients we accept per incoming mail delivery to one. We accept the first RCPT TO: command issued by the caller, then defer subsequent ones using a 451 SMTP response. As with greylisting, if the caller is a well-behaved MTA it will know how to interpret this response, and retry later.

  # Limit the number of recipients in each incoming message to one
  # to support per-user settings and data (e.g. for SpamAssassin).
  #
  # NOTE: Every mail sent to several users at your site will be
  #       delayed for 30 minutes or more per recipient.  This
  #       significantly slow down the pace of discussion threads
  #       involving several internal and external parties.
  #
  defer
    message      = We only accept one recipient at a time - please try later.
    condition    = $recipients_count

  # Invoke SpamAssassin to obtain $spam_score and $spam_report.
  # Depending on the classification, $acl_m9 is set to "ham" or "spam".
  #
  # We pass on the username specified in the recipient address,
  # i.e. the portion before any '=' or '@' character, converted
  # to lowercase.  Multiple recipients should not occur, since
  # we previously limited delivery to one recipient at a time.
  #
  # If the message is classified as spam, pretend to reject it. 
  #
  warn
    set acl_m9  = ham
    spam        = ${lc:${extract{1}{=@}{$recipients}{$value}{mail}}}
    set acl_m9  = spam
    control     = fakereject
    logwrite    = :reject: Rejected spam (score $spam_score): $spam_report