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.