I may be incorrect but here's my understanding of how SmarterMail uses DNS. If you don't define any DNS resolvers in SmarterMail it will default to the server DNS designation. If you define DNS resolvers in SmarterMail it will use those unless they are unresponsive and then will default to the server DNS designation.
When you changed the DNS resolvers in SmarterMail you would probably have to stop and restart the SmarterMail service.
The simplest and best situation is to run the DNS resolver on the SmarterMail server itself. Very fast and you don't run into the blocking issues with some RBL's and URIBL's if you use public or very busy DNS resolvers.