This is a brief guide to configuring SpamAssassin using a preinstalled version.
SpamAssassin is temporarily installed in /home/chris/spamassassin .
for a user named catuser, do the following.
(1) if you don't have a file in your home dir called .procmailrc create one and
include the following lines of code:
#------- code begins --------
VERBOSE=on
MAILDIR=$HOME/Maildir/
LOGFILE=$HOME/procmail.log
DEFAULT=$HOME/Maildir/
:0fw
| /home/chris/spamassassin/spamassassin -P -c /home/chris/spamassassin/rules
#------- code ends -----------
if you do already have .procmailrc set up, you only need to include the last two
lines above, prior to existing procmail rules.
(2) create a file called .qmail in your home dir (or add this line to an
existing file)
| preline procmail
(3) this configuration will just flag spam by changing the Subject field of
messages. for further info see the README file in the SpamAssassin dir or try
spamassassin.org .
chris (at) cat.org.au

Spam Assassin Update
Due to the update of spamassassin running some of the details need a change.
.procmailrc
should read
VERBOSE=on
MAILDIR=$HOME/Maildir/
LOGFILE=$HOME/procmail.log
DEFAULT=$HOME/Maildir/
:0fw
|/usr/bin/spamassassin
Yay no more spam, but you need to fiddle with the user options, This all came from here.
http://lists.cat.org.au/pipermail/catgeek/2003-June/009585.html