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 The connection between Courier and SpamAssassin.If you have virtual users (as opposed to plain *nix users), it is vital that you make sure that courier-mta (as well as courier-imap and courier-pop) uses the authdaemon as authentication module. That means that in /etc/courier/esmtpd, imapd and pop3d the variable AUTHMODULES should be set to "authdaemon". Keep in mind that your courier config files may be in another directory. The authdaemon has to be configured to use the right authentication modules (mysql, userdb, whatever). Then patch SpamAssassin according to http://da.andaka.org/Doku/courier-spamassassin.html so that SpamAssassin can ask authdaemon for the whereabouts of the maildirs of virtual users (and plain users too — but if you only have plain users, you may skip this step) is made by using maildrop as the MDA and spamd/spamc to perform the spam checking using Courier's mail filtering language.

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

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

You must then decide what to do with any spam mail. For example, an instruction in the the users .mailfilter file:

if(/^X-Spam-Status:    includeYES/)
{
 to "$HOME/Maildir/.mailfilterJunk/"
}

If you have virtual users (as opposed to plain *nix users) you may want to let Spamassassin know where to find per-user preferences. Do this by passing the username to spamc with the -u option and then using the spamd --virtual-config-dir option, for example in a systemd service file (where vmail is the virtual mail user):

ExecStart=/usr/bin/vendor_perl/spamd -x -u vmail -g vmail --virtual-config-dir=/home/vmail/%%d/%%l

You can omit this if using only system-wide preferencesVia the xfilter line, maildrop pipes its contents through spamc; via the include line, the user's mailfilter script is called.