Filtering spam in Postfix is pretty simple.  There’s some advanced techniques you can use, but simply setting up Spamassassin will suit many people.  One downside is seeing all the ***** SPAM ***** mails in your inbox.  It took a while to come up with a solution, but the best fit so far has been implementing Maildrop to automatically move those files to a Junk folder.  Here’s the steps to set this up on a Debian 5.0 system with Postfix and Spamassassin.

First, setup your /etc/maildroprc file:

# commands and variables for making the mail directories
maildirmake=/usr/bin/maildirmake
mkdir=/bin/mkdir
rmdir=/bin/rmdir
MAILDIR=$DEFAULT

# make the user's mail directory if it doesn't exist
`test -e $MAILDIR`
if ($RETURNCODE != 0)
{
`$mkdir -p $MAILDIR`
`$rmdir $MAILDIR`
`$maildirmake $MAILDIR`
}

# make the .Junk folder if it doesn't exist
JUNK_FOLDER=.Junk
_JUNK_DEST=$MAILDIR/$JUNK_FOLDER/
`test -d $_JUNK_DEST`
if ($RETURNCODE != 0 )
{
`$maildirmake $_JUNK_DEST`
#auto subscribe. the following works for courier-imap
`echo INBOX.Junk >> $MAILDIR/courierimapsubscribed`
}

# If the Spam-Flag is set, move the mail to the Junk folder
if (/^X-Spam-Flag:.*YES/)
{
exception {
to $DEFAULT/.Junk/
}
}

The comments clearly state what’s going on there.

Once that’s setup, you will go into your /etc/postfix/master.cf and make sure the

maildrop  unix  -       n       n       -       -       pipe
  flags=DRhu user=vmail argv=/usr/bin/maildrop -d ${recipient}

is not commented out.

Next you will have to set the /usr/bin/maildrop file setuid root. This is so maildrop can interact with authdaemon and the mail folders.

#chmod +s /usr/bin/maildrop

Then you have to add this to your /etc/postfix/main.cf file:

virtual_transport = maildrop
maildrop_destination_recipient_limit = 1

If there is another virtual_transport line, be sure to comment that out first.

Last, set the permissions on the authdaemon so that maildrop can access it.

chown vmail /var/run/courier/authdaemon

And that’s all. Nice and simple way to handle all that Junk mail.