Alternatives To Thunderbird on Windows & Mac

Mozilla won't be actively #-Link-Snipped-# anymore. That means the die hard fans of Thunderbird will have to look for an alternative. While the traditional Outlook is still the best bet for all the corporate folks; a regular home user needs an alternative to Thunderbird.

I've always failed to find a better performing email app on Windows; but on the Mac, 'Sparrow Mail' is the best app ever. I'd recommend the pro version (only $10) to the business users.

Any other suggestions for the Windows and Mac operating systems?