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 (i.e. dnsbl.sorbs.net).
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 removal http://www.dnsbl.au.sorbs.net/cgi-bin/lookup?IP=%s)
