RBL's & Forwarders?

Question

I was looking at SpamTitan the other day and came accross this info: 
 The importance of DNS 
 Accurate DNS responses are vital to SpamTitan maintaining a good spam catch rate. SpamTitan queries multiple internet based spam blocking tools using DNS. Due to the very high volume of DNS requests that originate from free/open DNS servers (e.g 8.8.8.8, 8.8.4.4, 4.2.2.1, etc) the test providers will not respond to DNS requests from these servers. Do not configure SpamTitan to use free/open DNS servers, or if you are using your own DNS server do not configure it to use free/open DNS servers as a forwarder. 
 In the above, they do not recommend using free/opendns servers as forwarders 
 I'm wondering how the UTM works with this if you were to put extra RBL's into it. Would using a free/opendns to resolve affect any of these?

DouglasFoster · Answer

I think it is saying the problem flow is: 
 
 SpamTitan client performs RBL lookup via DNS, to see if a mail source is dangerous. 
 Client environment forwards DNS to Google 8.8.8.8 
 Google forwards DNS request to SpamTitan server. 
 SpamTitan server refuses to answer the Google lookup 
 SpamTitan client receives a NXDOMAIN answer, and assumes that the mail source is safe, possibly incorrectly. 
 
 Short version: SpamTitan does not have the resources or the money to provide a free DNS lookup service, but DNS is inherently free. So their workaround is to block Google and Cloudflare and a few others. 
 Solution: Do what they say or use a different RBL. SpamHaus/Zen and Barracuda are both good options. Barracuda asks for registration. SpamHaus asks for money if you are commercial and doing high volume lookups. but I don't know that they have an enforcement mechanism. MXToolbox.com tracks domain reputation using 65 different RBLs, so you can get a complete list of options there. 
 UTM can use any RBL. The issue is their attempt to restrict access to their DNS server; the client device does not matter.