In the mail server interface click/set the following:
Click 'Anti-Spam'
Click 'DNS Based Spam DB (DNSBL)
In the DNSBL blox click 'New List'
In the 'Lookup Domain' box enter dnsbl.sorbs.net - where 'dnsbl.sorbs.net' is the SORBS zone you wish to use.
Specify the default answer as 'Rejected by SORBS'
Check 'Use TXT DNS records'
Click 'OK'
Click 'Close'
Note: Ensure you check the 'Use DNS TXT Records' otherwise
people trying to send you mail will not know why they are blocked or
how to correct the information.