Chapter 4 FortiOS Carrier : MMS Security features : MMS content-based Antispam protection : Overview : White listing
  
White listing
White listing is the practice of adding all critical IP addresses to a list, such as company email and web servers. Then if those servers become infected and start sending spam or viruses, those servers are not blocked. This allows the critical traffic through, even if there might be some malicious traffic as well. Blocking all traffic from your company servers would halt company productivity.