If there is a large spam attack, the antispam daemon spamd starts too many processes and the system can run out of resources. You can limit the number of simultaneously running SpamAssassin processes through the Plesk GUI. Use the The maximum number of worker spamd processes to run (1-5) option on the Server > Tools & Settings > SpamFilter page. You can retrieve this value by executing the following command:

#  mysql  -uadmin -p`cat /etc/psa/.psa.shadow ` psa -e "select * from misc where param='spamfilter_max_children'"