Click on the following menu tabs, Router/SMTP -> Restrictions and controls -> SMTP Inbound Controls.
Scroll down to the "DNS Blacklist Filters" section and enable it.
Enter the Blacklist sites (e.g. dnsbl.sorbs.net - where 'dnsbl.sorbs.net' is the SORBS zone you wish to use.)
Change the desired action (Log only, Log and Tag or Log and reject).
If required, enter a custom SMTP error message (i.e. Your host %s is in the SORBS black list - visit this URL for more information: http://www.sorbs.net/lookup.shtml?%s)