Spamassassin cannot be managed through Plesk CP. If you try statrt it via Plesk CP the error occurs:
psaerror(string 'Unable to make action: Unable to manage service by spamd: Unable to start service.')
1. Make sure that SpamAssassin is enabled in "/etc/default/spamassassin". There should be the lines:
# Change to one to enable spamd
2. Try to run "$PRODUCT_ROOT_D/admin/sbin/packagemng" utility to update Plesk components list in the database.
3. Restart SpamAssassin and Plesk.