https://www.virtualmin.com/node/39491

Replace the <virtual-server>, <username> and junk message names as necessary.

All dashes, spaces and punctuation are important.

Scan a message using the spamc client with the processing done by spamd and spamd child (both running as root):

spamc -u <username> < /home/<virtual-server>/homes/<username>/Maildir/.spam/cur/1454565258.32109_0.www

Scan a message as root:

spamassassin -D -t -p /home/<virtual-server>/homes/<username>/.spamassassin/user_prefs < /home/<virtual-server>/homes/<username>/Maildir/.spam/cur/1454619898.14251_0.www

Scan a message as a user:

su -s /bin/sh -c "spamassassin -D -t -p /home/<virtual-server>/homes/<username>/.spamassassin/user_prefs < /home/<virtual-server>/homes/<username>/Maildir/.spam/cur/1454565258.32109_0.www" - <username>

Train the Bayesian filter as a user:

su -s /bin/sh -c "sa-learn -D --spam --no-sync /home/<virtual-server>/homes/<username>/Maildir/.spam/{cur,new}" - <username>

Update rules (taken from /etc/cron.daily/spamassassin):

su - debian-spamd -c "sa-update -D --gpghomedir /var/lib/spamassassin/sa-update-keys"

Permissions on debian-spamd home directory:

chown -R debian-spamd.debian-spamd /var/lib/spamassassin
find /var/lib/spamassassin -type f -exec chmod 644 {} \;
find /var/lib/spamassassin -type d -exec chmod 775 {} \;
chmod -R o-rwx /var/lib/spamassassin/sa-update-keys/

reboot