Is a paid subscription required to use DNSBL services?
This depends on the service. Each IP Allow or Block List Provider (IP-based DNSBL) and URL Block List (URI DNSBL), as well as Sender Score Reputation service, has a different policy for accessing their public mirror.
Some services do not have a public mirror at all and are only available via a paid subscription.
Many services permit low-volume users to access the public mirrors free-of-charge, but you should check the individual policies of the IP Allow List Provider, IP Block List Provider (IP-based DNSBL) and URL Block List (URI DNSBL) services you are using.
Even many hardware appliances which use these services will require a separate subscription to individual DNSBL services for high-volume users.
Many services will implement automatic IP blocking if a threshold of queries to the public mirror is exceeded. In this case, a different return code will be returned. When the providers are configured correctly, this will not result in false positives, but mail filtering efficacy will be greatly reduced, as that provider will no longer be filtering mail.