The built-in Sophos antivirus application is integrated into Kerio Control and you will need a special license to run it.

In Configuration → Content Filtering → Antivirus, on the Antivirus Engine tab, allow antivirus scan.

Kerio Control allows to select protocols which antivirus check will be applied to. The HTTP, FTP scanning, Email scanning and SSL-VPN scanning, tabs enable detailed configuration of scanning of individual protocols. Usually, the default settings are convenient.