How do I set Outlook as the default mail option in Yosemite?
You can set Outlook as the default mail app following these steps:


Open the “Mail” application in OS X, note that you have to open the Mail app even if you’re wanting to use another mail client like Outlook.
Pull down the “Mail” menu and choose “Preferences”.
Go to the “General” tab. 
Look for “Default email reader” and click on the menu to pull down other mail app options. If the mail app you want to use is not shown in this list choose “Select” to browse the /Applications/ folder and select one manually.