Setup on Debian for use with EXIM.

apt-get install spamassassin sa-compile

Edit /etc/spamassassin/local.cf to add local rules.

Compiling rules will speed up spamd but it will work otherwise.

compile-spamd.sh

#! /bin/sh
sa-compile
chown -R debian-spamd:debian-spamd /var/lib/spamassassin
chmod -R go-w,go+rX /var/lib/spamassassin/compiled
kill -HUP `cat /var/run/spamd.pid`
exit 0

Run from cron once per day to download new rules and recompile.

update-spamd.sh

#! /bin/sh
sa-update --nogpg
if test $? -eq 0; then
    compile-spamd.sh
fi
exit 0

#! /bin/sh
export TZ="UTC"
rm -f /var/run/spamd.pid
exec /usr/sbin/spamd --helper-home-dir=/var/lib/spamassassin --create-prefs --nouser-config -udebian-spamd -gdebian-spamd --socketpath=/var/run/spamd.sock --socketowner=debian-spamd --socketgroup=debian-spamd --socketmode=0777 --max-children 5 --pidfile /var/run/spamd.pid

Example plugin I wrote to catch fake mailing list spam.

:...skipping...
#! /usr/bin/perl
#
# By
#       Darron M Broad 2022
#
# See:
#       https://spamassassin.apache.org/full/3.4.x/doc/Mail_SpamAssassin_Plugin.html
#       https://spamassassin.apache.org/full/3.4.x/doc/Mail_SpamAssassin_Logger.html
#       https://spamassassin.apache.org/full/3.3.x/doc/Mail_SpamAssassin_PerMsgStatus.html
#
# /etc/spamassassin/local.cf:
#
#       loadplugin LocalPlugin /etc/spamassassin/local/plugin.pm
#
#       header          LOCAL_FAKE_LIST eval:check_for_fake_list()
#       score           LOCAL_FAKE_LIST 100
#       describe        LOCAL_FAKE_LIST From Fake List

package LocalPlugin;

use Mail::SpamAssassin::Plugin;
use Mail::SpamAssassin::Logger;
use Mail::SpamAssassin::PerMsgStatus;

our @ISA = qw(Mail::SpamAssassin::Plugin);

sub new
{
        my($class, $mailsa) = @_;

        $class = ref($class) || $class;
        my $self = $class->SUPER::new($mailsa);
        bless ($self, $class);

        $self->register_eval_rule("check_for_fake_list");

        info("LocalPlugin: REGISTERED");

        return $self;
}

sub check_for_fake_list
{
        my($self, $permsgstatus) = @_;

        $path = $permsgstatus->get("EnvelopeFrom");
        my @to = split /@/, $permsgstatus->get("To:addr"); # x@y.z
        my $name = $to[0];
        if ($name) {
                my $host = $to[1];
                $s = "mail-$name=$host\@mail\.";
                if ((rindex $path, $s, 0) == 0) {
                        info("LocalPlugin: SPAM $path");
                        return 1;
                }
                $s = "newsletter-$name=$host\@mail\.";
                if ((rindex $path, $s, 0) == 0) {
                        info("LocalPlugin: SPAM $path");
                        return 1;
                }
        }
    return 0;
}

1;