The easiest way to achieve this would be to setup Exim on CentOS. Then setup a custom transport rule within Exim to deliver all email from the specific domain to the smarthost then all other email by DNS/MX resolution. An outbound route would then be defined within Smartermail to send all outbound email via Exim.
It requires some Linux experience but provides ultimate flexibility in mail routing / handling that far outstrips anything Smartermail can do. If you are not familiar with Exim invest the time to learn as it will pay off.