In addition to rule-based filtering, Spamphibian Gateway provides another layer of protection: DNSBL (Domain Name System based black-hole list). DNSBL uses information compiled by third parties as a basis for rejecting connections to Spamphibian Gateway.

DNSBL is an acronym for Domain Name System based black-hole list. What that mouth-full means is that Spamphibian Gateway will check the IP address of incoming connection requests against Internet accessible databases maintained by third parties. Spamphibian Gateway uses an advanced form of DNSBL that takes advantage of the various Filter Actions. Spamphibian Gateway can be configured to block a connection immediately if the incoming connection's IP address is found in DNSBL that is the traditional form of DNSBL. However, Spamphibian Gateway can be configured to perform any of the Filter Actions so that the administrator has greater flexibility in managing messages (instead of delete, the corresponding action for DNSBL is quarantine). This gives Spamphibian Gateway a chance to pass messages through the "Accept" rules before any action is carried out.

Warning: Because these databases are maintained by third-party sources it can be difficult to ensure the quality of the data. Additionally, the criteria for including addresses in the database varies greatly from list to list. For example, some databases only include servers known to have open relays, while other databases attempt to list IP addresses from specific countries. While Spamphibian Gateway includes some pre-configured DNSBL server entries this does not constitute an endorsement of their policies nor does it imply that Outspring has any control over what gets listed in those databases. It is entirely possible that use of DNSBL may increase the number of false positives you receive.

Also, you should be aware that enabling the use of DNSBL on your Spamphibian Gateway may affect the amount of time it takes Spamphibian Gateway to process messages because it takes time for a DNSBL check to be performed.