SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam. SpamAssassin uses a variety of mechanisms including header and text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases. SpamAssassin runs on a server, and filters spam before it reaches your mailbox.
This tool is included in standard Cpanel control panel.
SpamAssassin assign a score to each email, if the email scores higher than user specified max score, then it will be flagged as SPAM by SpamAssassin.
Most of the configuration, updates are done by WHM or the hosting company, as a user of cpanel and email service, users doesn’t need to do anything except ENABLE the service and set a max score before it is flagged as spam. Default value is 5, higher value will be more conservative.
User may also set the SpamAssassin to automatically delete all spam emails from cpanel control panel. The default value is disabled.
Check out list of host uses cpanel.