Now it should be OK again. Problem is that if you update Plesk or make changes to the Mail server settings its possible for the mail handlers to go away.
I ran this command to re-add the handlers:
/usr/local/psa/admin/bin/mailmng --remove-handler --handler-name=sguardian --handler-type=global --hook=before-local > /dev/null
/usr/local/psa/admin/bin/mailmng --add-handler --handler-name=sguardian --handler-type=global --executable=/usr/bin/livesguardian --hook=before-local --context=sguardian --priority=2 > /dev/null