How to digitally sign emails in the Mail app in Windows 8?

I used to use Windows Live Mail before upgrading but now I'm using the Mail app that comes with Windows 8, but I don't seem to be able to figure out how to sign emails with a digital signature.

Before I could sign and verify signed emails when I received them, how to do so in the Mail app?

(if it helps, my private key is already installed)

After exploring the application, I found that there is no such functionality available in current Windows Mail metro app at this time. Even after trying for Certificate Installation process suggested by Microsoft isn't so helpful. So you can go with desktop apps, like Microsoft Outlook if it is must for you.