If you have a newer Mac that uses an Intel chip you can use Parallels or BootCamp to run Windows. You then run AirMail in the Windows environment on your Mac. Parallels has the advantage that you don’t have to re-boot to switch operating systems. See www.parallels.com BootCamp also works fine for Mac’s with Intel chips but does require re-booting to change between operating systems. In both cases you need to buy a copy of Windows.
Another approach to running AirMail on a Mac is to use the Mac version of WINE. Using this approach you don’t need to buy a copy of Windows. WINE (specifically Crossover) runs as a program on your Mac, and provides AirMail the environment that AirMail needs in order to run. For details on how to run AirMail on a Mac using WINE/Crossover see: http://tritonmarinesystems.com/Media-Sailmail/ Thanks to Stas’ Lewak for this information.
If you have an older Mac that uses a PowerPC chip (2006 or earlier), you can use VirtualPC.