http://www.ols.es/exim/dlext/

pipe_message

   Runs program injecting current message or file into it's stdin and
   returning the stdout output.

   Usage: ${dlfunc{/usr/local/exim/exim-ext.so}{pipe_message}{app}{file}}
   If {file} is ommited, the current message is piped to the application.
   returns: application stdout output.

   DSpam usage example::

 # the trick here is to call dspam in deliver=summary mode, which will
 # return only the dspam header, so you only need to add this header
 # to the message (no need to reinject it):

   warn    set acl_m13    = ${dlfunc{/usr/local/exim/exim-ext.so}{pipe_message}\
                             {/usr/local/bin/dspam --deliver=summary --client \
                              --mode=teft --user global}}
           set acl_m14    = ${extract {result}{$acl_m13}}
           set acl_m15    = ${extract {signature}{$acl_m13}}

 # now you get in acl_m13 the dspam header, the signature in acl_m15
 # (if you need it for logging or something else) and the result
 # in acl_m14

 # you could optionally retrain it if it comes from a very trusted sender:

 warn      condition      = [VERY TRUSTED SENDER]
           condition      = ${if match {$acl_m14}{Spam}}
           set acl_m13    = ${dlfunc{/usr/local/exim/exim-ext.so}{pipe_message}\
                            {/usr/local/bin/dspam --user global --client \
                            --class=innocent --source=error \
                            --deliver=summary --signature=$acl_m15}}
           set acl_m14    = Innocent (retrained)
           set acl_m15    =

 # deny at SMTP time if it's spam:

 deny      condition      = ${if match {$acl_m14}{Spam}}
           message        = Spam not welcome

 # and if accepted add the proper header so it could be latter retrained
 # if need:

   warn    condition      = ${if def:acl_m14}
           add_header     = $acl_m13


   This enables exim to run dspam and add the result header to the message
   (both at data acl or at transport/router time) without having to run ugly
   transports which reinject the message again into exim.