The DNS blacklist is a feature used for DNS Failover. If you send a call to a gateway and this gateway is not accessible or is responding with a response code type 5xx or 6xx, OpenSER uses a resource called "dns blacklist" and insert your gateway in the blacklist. Your gateway will stay in the blacklist for 4 minutes (may be changed in compile time in blacklists.h) , before you can send traffic to it again. While the gateway is in the blacklist, you will receive the message "473/Filtered Destination", if you try to send calls to this specific gateway. To disable this feature use (it is enabled by default):

disable_dns_blacklist=yes

You can also create your own lists to blacklist gateway permanently or temporarily out of service.