"URIBL provides public lookups over DNS for low volume usage. If you spam check a large amount of email, or you use a shared DNS platform for resolution, you may receive a response saying the query was refused. That said, Public DNS providers such as OpenDNS or Google Public DNS are effected due to the high volume of queries they generate, as are many other internet service providers (ISP) that use caching nameservers for their customer base."