Load Testing on QMAIL

[qmailu]

Hi,   I have setup a qmail box running qmail-1.03,vpopmail-4.9.6,courier-imap-0.36a. The authentication method is mysql. I have more than 2 million users. Before I move from my existing setup - I would like to be absolutely sure qmail can take this load. Are there many of you running qmail for more than 2 million users. How can I do the load testing (some load testing tools to the effect ! ) to confirm that my box will perform under maximum load ? Also what would be the ideal IMAP connects that I would need to setup as threshold - in my current setup   we have about 500-600 simultaneous imap connects ! Can courier-imap take this load ?   Raghu