Integration into Courier-MTA using maildrop

First, install SpamAssassin and Courier MTA according to their installation docs. Make sure they are functioning on their own. You may want to use spamc/spamd for better performance. What's left is the connection between Courier and SpamAssassin.

If you have virtual users (as opposed to plain *nix users) you may have to let Spamassassin know where to find per-user preferences. This can be done by passing the username as an option to spamc using Courier's maildroprc config file as shown below, and then using the spamd --virtual-config-dir option. You can omit this if using only system-wide preferences.

To let Courier MTA use maildrop as default for delivery, edit /etc/courier/courierd and set DEFAULTDELIVERY to "| /usr/bin/maildrop" (or wherever your maildrop binary is). Then edit /etc/courier/maildroprc, the central maildrop script:

import USER
if ($LOGNAME ne "")
{
 xfilter "/usr/bin/vendor_perl/spamc -u $LOGNAME"
}
else
{
 xfilter "/usr/bin/vendor_perl/spamc -u $USER"
}

Via the xfilter line, maildrop pipes its contents through spamc before delivery.