To get to this page, to to Services → SMTP Server → Filtering

The Filtering tab lets you specify filtering on incoming SMTP connections.

The Edit SMTP Rules button lets you view & edit SMTP Rules which apply to this SMTP service. SMTP Rules are filtering conditions which look at the incoming message and can perform actions such as redirecting, blocking or ignoring messages based on conditions you specify.

The Enable Spam/Content filter on SMTP messages option lets you enable or disable the VPOP3 spam filter. This setting applies to all the SMTP services defined in VPOP3, and is the same setting as the Enable Spam/Content filter on SMTP messages setting in the Spam filter settings.