Spamhaus...

Sun Feb 21 06:27:28 UTC 2010

>Maybe I'm mistaken, but it appears each end user has to buy the
>service for their own mail servers, and the ISP isn't allowed to
>bypass that.  For the purpose of the agreements with spamhaus, an ISP
>customer is probably considered a third party, and making a rbldns
>server available to them is disclosing spamhaus' secret DNSBL zone
>information....

In my experience, they're pretty reasonable.  I would talk to them (or
one of their datafeed sales agents) before assuming that they won't
sell you the service you need.

R's,
John