How to modify default mail client in Windows 8
1. Open the Control Panel (for more details see How to open Control Panel in Windows 8):
2. In the Control Panel dialog box, in the Search Control Panel textbox, enter Default and choose Default Programs:
3. On the next screen, click Associate a File Type or Protocol with a program link:
4. On the Associate a file type or protocol with a specific program screen, scroll down the list until you find Protocols:
5. Under Protocols, choose MAILTO and then double-click it:
6. Choose the client you prefer:
7. Click Close.