While it is impossible to completely simulate the demands of a production system in a test environment, testing remains the best way to achieve confidence in a solution before its deployment.
Setting up a test environment should be done with careful consideration. Simulating the Internet in a lab environment poses many challenges.
Efficient configuration of servers that are used for load testing is likely to be very different than the configuration of production email servers.
It is almost certain that a combination of publicly available and homegrown solutions will need to be amalgamated into a comprehensive package of testing tools.
Examine all testing results with a critical eye. Many pitfalls may be encountered in attempting to accurately test the capabilities of an email server.