[Exim] Filtering by subject

Hello,

I have checked for docs and am a little confused/or unable to find the
right ones. Here is simply what I want to do:

exim version 4.12

local_delivery:
driver = appendfile
file = /var/spool/exim/$local_part
delivery_date_add
envelope_to_add
return_path_add

spam_delivery:
local_delivery:
driver = appendfile
file = /var/spool/spam/$local_part
delivery_date_add
envelope_to_add
return_path_add


How am I able to get it to search the subject line and if it contains
the phrase: ***SPAM***

It will use spam_delivery, otherwise use local_delivery ?

Is there a better way to do this? I pretty much want 2 spool
directories, on of spam for each user, one for regular mail.

We have a webmail (squirrelmail) setup to go to the spam folder, so the
user can view the spam via a webbrowser.

Any help or a point in the right direction would be appreciated.

Thanks a lot!

eximlist@???