Didn't quite catch your meaning.

Hello,

Ironport has its own caching DNS server on board.
If you have to choose between using the root servers and the DNS of your ISP I suggest you use the root servers. Why? Because it's more reliable... if your ISP has more than 3 DNS servers you are lucky and even those can all be unreachable, if you walk the DNS tree yourself there is not much risk that you will be “out of DNS servers“.

Please be aware that Ironport is a HUGE DNS consumer. The Senderbase stuff is fully DNS based. For each incoming connection your device will at least launch 3 DNS queries. For the connections that have an acceptable Senderbase score it will be even more. Especially when you have a busy Ironport infrastructure, your ISP might not be happy with the load you put on their DNS systems.

We use our own (non-Ironport) caching DNS servers because we want to use DNS also for routing to the internal systems.
These DNS servers are used by Ironport and the web proxy systems and the load on those is 90% Ironport, 10% web proxy.

Hope this helps.

best regards
Steven

Thanks Steven for the reply. I've noticed a few quirks on our ISPs DNS servers, so for now I've told the IronPort to use the root servers so the responses it gets are more reliable.

Load wise, I don't think our ISP would really care. We have a single C150 that receives about 600 connection attempts per hour (spam+ham total), not exactly substantial by any measure.