Options dialog - DNS - Recursion
(Main window -> Options button -> DNS / Recursion section)

 Simple DNS Plus v. 5.0
Copyright © 1999-2008 JH Software ApS
•Perform DNS recursion (resolve non-local domain names)

Specify which IP addresses should be offered recursion.
You can list multiple IP addresses, IP address ranges, and/or IP address subnets.
For DNS servers accessible from the Internet, it is highly recommend that you limit recursion to IP addresses on the local area network - see How to secure your server.

 

•Maximum recursive DNS requests to resolve in parallel
Specifies the maximum number of recursive requests to resolve at the same time.