Does the DNSBL try to access the DNSBLs directly or do they go through the DNS?
GMS Anti-Spam has the option to check a number of DNS based Block Lists (DNSBLs) to check if any incoming connection is from a server that is known to be the source of spam. If such a source is identified, GMS Anti-Spam allows the administrator to decided what to do with the incoming request to send email.
GMS Anti-Spam queries the allocated RBL servers directly.
If you are operating GMS or GMS Webmail from behind a firewall, you will need to open up the UDP and TCP ports for DNS access (port 53).
A number of DNSBL type servers now work on a subscription basis, this can depend on the IP you are coming from. If you do this via DNS which may be running on a different IP then the lookup would be refused.
Keywords:DNSBL DNS security access