Spam Assassin is a popular open source spam email filtering program used to scan and identify spam messages. The program uses a great deal of pre-set rules for scanning a message, these rules including checking email message header, message body, and sender email address of all email messages sending to your domain inbox. For further info on Spam Assassin program, please check the official website of Spam Assassin:
You may also want to check on Setup Email Filters for setting filters to automatically filter email messages tag as spam by spam assassin to trash or forward to a designated email account for further manual checking.
To enable Spam Assassin:
Click Spam Assassin link in the Mail option in your Cpanel hosting account control panel.
Click Enable Spam Assassin button to activate scanning of messages for your account.