http://wiki.apache.org/spamassassin/DnsBlocklists#dnsbl-block
DNS-Blocklists often run on the “free for some” model and/or they may limit the number of queries you can perform to maximize resources.
If you were directed to this link from a rule description, then you have a DNS-Blocklist that is purposefully blocking your queries.
Resolving the block might be as simple as using your own non-forwarding caching nameserver to avoid being lumped together with other users queries; setting up your own mirror of the DNS-blocklist; or paying to use the blocklist. The choice is up to the DNS-Blocklist administrator.
http://www.uribl.com/about.shtml
For low volume end users, we provide public DNS mirrors distributed around the world to answer your queries through products like SpamAssassin which have built in support for URIBL.COM. Many other commercial anti-spam products support realtime DNS blacklists. Consult your documentation on how to configure your software to utilize multi.uribl.com for domain reputation.
For high volume users (see info on abuse http://www.uribl.com/about.shtml#abuse), we offer commerical Data Feed Services over RSYNC and DNS. Datafeed over RSYNC allows companies to run URIBL.COM data in-house, increasing speed and spam accuracy both. Datafeed service also provides extra datasets and prelist data that is not available in public DNS, improving spam accuracy even more! Datafeed over DNS provides the same great information over existing public DNS, without the need for setting up or maintaining your own hardware to download and serve the zone data.
Essentially, Site5’s requests to URIBL are exceeding their set limits for free public access, so URIBL is blocking requests from Site5. I don’t know if URIBL actually resets anything at midnight, but I’m hitting this error by 5AM Eastern time, so a midnight reset means that Site5 is burning through its available free URIBL requests very quickly.
I have submitted a ticket to request that Site5 look into a caching nameserver and/or URIBL’s datafeed service to allow SA to continue to use URIBL to keep the filtering accuracy up.