This option determines if spam filtering is done using the standalone `spamassassin` program, or the `spamc` lightweight client. In the latter case, your system must be running the `spamd` server, or no spam classification will be done.

Using `spamc` is typically faster and consumes less memory and CPU. However, it will take away the ability to have a different SpamAssassin configuration for each virtual server, as the `spamd` server can only read a single set of configuration files.