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.

It would be good to have some clarification directly from ST. IMHO there is no need for service restarting because in my scenario (main SM + gateways) when I changed DNS server in main SM (I don't have there any checks because they are done on gateways) messages started to be sent to gateways and they stuck in spool on gateways with RBL checks timeouts (DNS servers in SM configuration has been also changed) and they started to be delivered correctly only when I changed DNS servers in system. For me it looks like delivery/spool DNS resolving is being done with SM DNS configuration (and then later DNS from system) but RBL checks use DNS directly from system configuration.