Change the default email app to Microsoft Outlook on your Mac computer

Executive summary
- Install and set up Outlook
- Open the Mail app and add an email account of any type (Gmail, iCloud, etc.)
- Go into Mail preferences and change the default email app to Outlook

Steps to change your default email application

1. Open the Applications folder and double-click on the Mail app to launch it.
   
   Note:
   If an email account has not yet been set up in the Mail app, you will need to create one. It is usually simpler to use an existing Gmail or iCloud account over Exchange.
   
   
2. Once you create an account, select the Mail > Preferences... menu options.
3. On the Preferences dialog box, click on the General toolbar icon.
4. On the Default email reader drop-down list box, select Microsoft Outlook.app
5. Close the Preferences dialog box.

Explanation as to why this process is so convoluted...

https://support.microsoft.com/en-us/help/3027171/outlook-2016-for-mac-cannot-be-set-as-the-default-application-in-mac-o