How to use Apple Mail as default email program in Windows 8 running under Parallels?
Yes, this should be possible!
First things first: I have Parallels 8, but I expect this setting to not have moved significantly since.


Open your virtual machine's configuration
Go to options
Select "Programms"
On the right side you'll see "E-Mail" and a drop down list where you can select "Open in Mac" or along these lines.


Mind you, I have a German installation, so this is freely translated by myself, the wording might be slightly different.