Why not just set Outlook as the default mail client?
1. Open Outlook
2. Click File.
3. Click Options.
4. Under the general tab, Tick the tick-box that says: Make Outlook the default program for e-mail, contacts and calendar.
You can also uninstall yahoo mail if you like (use the programs and features - uninstall a program section in control panel) but if you don't use it again / accept prompts to use it as the default mail client Outlook will do the business from now on.
OP's Update : This link also helped me.