Only run one spamassasin process at a time?
When this field is set to No as it is by default, Virtualmin will run multiple concurrent spamassassin processes if several email messages arrive at the same time. However, this may use excessive CPU and RAM on a small system. If you select Yes, any virtual servers with spam filtering enabled will have their Procmail configuration files updated to run only one copy of spamassassin at a time.

This restriction applies across all virtual servers, so even if two messages to different domains arrive at the same time, there will only ever be one spamassassin process running.