What performance impact does this have on the mail server?

Actual performance impact will depend on a number of things such as the server hardware, users' WirelessMail schedule and filter settings, and the number of incoming emails received by each user. We have had no reports of performance degradation, and in our own tests on a normally loaded mail server we have not seen any noticeable decrease in server performance when WirelessMail is enabled. For maximum performance, run the agent in 'After new mail has arrived' mode.