Ben's blog

This assumes that you have postfix installed and running as your SMTP server

First, make sure that you’re root

whoami

I probably shouldn’t have to explain that if you’re reading this but just in case; if that last command returned something else than ‘root’ issue the following command

sudo su

and enter your password

step 1: Let’s install the packages we’re gonna need

apt-get update
apt-get install spamassassin spamc

step 2: Now we configure spamassasin

cat /etc/default/spamassassin | sed -e 's#ENABLED=0#ENABLED=1#g' > /etc/default/spamassassin
cat /etc/default/spamassassin | sed -e 's#CRON=0#CRON=1#g' > /etc/default/spamassassin
cat /etc/spamassassin/local.cf | set -e 's## rewrite_header Subject *****SPAM*****#rewrite_header Subject [*****SPAM*****] > /etc/spamassassin/local.cf

and we start/restart it

/etc/init.d/spamassassin restart

step 3: We create a little script that will take desired action upon spamassassin flagging

create a user called spamassassin (or whatever you want as long as you keep it consistent)

useradd -m spamassassin

then edit the script file /home/spamassassin/spamcheck and throw the following in it

 # variables
 
SENDMAIL="/usr/sbin/sendmail -i"
 
EGREP=/bin/egrep
 
SPAMLIMIT=10
 
# exit codes from <sysexits.h>
 
EX_UNAVAILABLE=69
 
# clean up when done or when aborting.
 
trap "rm -f /tmp/out.$$" 0 1 2 3 15
 
# pipe message to spamc
 
cat | /usr/bin/spamc -u spamd > /tmp/out.$$
 
# are there more than $SPAMLIMIT stars in X-Spam-Level header? :
 
if $EGREP -q "^X-Spam-Level: *{$SPAMLIMIT,}" < /tmp/out.$$
 
then
 
# option 1: move spam messages to sideline dir so a human can look at them later:
 
mv /tmp/out.$$ /home/spamassassin/`date +%Y-%m-%d_%R`-$$
 
# option 2: divert spam message to an alternate e-mail address:
 
#$SENDMAIL xyz@xxxx.xx < /tmp/out.$$
 
# option 3: just delete the spam message
 
# rm -f /tmp/out.$$
 
# option 4: still relay the email to the recipient with the subject of the email now containing [*****SPAM*****]
 
# $SENDMAIL "$@" < /tmp/out.$$
 
else
 
$SENDMAIL "$@" < /tmp/out.$$
 
fi
 
# Postfix returns the exit status of the Postfix sendmail command.
 
exit $?

make sure that you

chown spamassassin:spamassassin /home/spamassassin/spamcheck
chmod 750 /home/spamassassin/spamcheck

step 4: Ok, so we got spamassassin going and a little script that will take an email and throw it in /home/spamassassin if it’s spam (if you chose option1) now we just need to tell postfix to pass all messages to that script

edit /etc/postfix/master.cf and replace

smtp      inet  n       -       -       -       -       smtpd

with

smtp      inet  n       -       n       -       -       smtpd -o content_filter=spamcheck:dummy

also add the following 2 lines at the bottom of the file (the indentation is important)

spamcheck   unix  -       n       n       -       10      pipe

flags=Rq user=spamassassin argv=/bin/spamcheck -f ${sender} -- ${recipient}

We’re almost there, just restart postfix and you’re good to go!

/etc/init.d/postfix restart

If you wanna test that out, watch the log while you send emails to your servers

tail -f /var/log/syslog

send a clean mail, make sure that it reaches destination, then send something you know will get flagged as spam and make sure it ends up in /home/spamassasin instead of the intended recipient.

The reason we choose option 1 here is because there’s no point in still relaying a flagged email as it will still clog the recipient’s mailbox. On the other hand we don’t want to just delete it if spamassassin makes a mistake we want to play it safe and keep every emails should something arise, we quarantine the bad ones in /home/spamassassin

Lastly, as long as you have postfix just feeding the emails to a script like we just did, it’s easy to become fancier and do all kinds of processing to the email, on my server I actually call a php script that throws emails in a DB.