Thanks to sandboxing, there is no setting in Outlook 2016 for Mac Preferences to set Outlook as the default email app. You can read the Microsoft KB: Outlook 2016 for Mac cannot be set as the default application in Mac OS X Yosemite for details on why this option was removed.
To set Outlook as default, you have open Apple Mail > Preferences > General tab to set Outlook as default. However there is a slight hitch to using Mail to set Outlook as default. If you don’t have Mail setup to use an email account, you can’t access preferences.
I suggest adding an iCloud account for easy setup. Open System Preferences > iCloud. Check Mail to add your iCloud email. If you have never setup an iCloud email, it will prompt you to create an iCloud email.
Set Outlook as default:
Open Mail Preferences:
Under General, select Outlook as default from popup options.
1) If you still have Outlook 2011, you can set Outlook as default in Outlook 2011 Preferences > General. The system will default to the newer Outlook 2016.
2) You can download IC-Switch (free) to set default Outlook as default. IC-Switch allows you to change your default emailer, Web browser or news reader in one click.