Why run your own DNS server? (Spamassassin)
If you have a sizable quantity of mail delivery, like greater than 20k messages per day, you are advised to run your own caching DNS server as it benefits your Spamassassin deployment in both speed and reliability. This article describes why it is important to operate your own local caching DNS resolver, and how to enable it on your server.
Why is it important to run your own DNS server?
Spamassassin makes many DNS queries during scanning. It tests various IP-based blacklists, URI-based blacklists and other network queryable rules. If you run these commands you will see how long it takes individual DNS queries to complete.