what is the cpu and ram count for your appliance? Since i run the largest smg farm in the world, i can run easily 200,000 emails outbound alone per hour per device. Note, that if you have high queuing rates (either you are throttled or denied sending), this greatly affects your performance. If you loadbalance connections across multiple smg's, u gain even greater performance.
Bottom line, use only virtual appliances with the most ram and cpu you can afford.