Setting Outlook as the Default Mail Client on a Mac

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Launch Mail. Choose Exchange and click Continue. You must add an account before you can access the Preferences pane.
Input your APSU email address and password and click Sign In.
Uncheck the boxes beside Contacts, Calendars, Reminders, and Notes and click Done.
Go to Mail in the top bar and choose Preferences.
Click the General tab. Click the dropdown beside Default email reader: and choose Microsoft Outlook.
Quit Mail. Open System Preferences and click Internet Accounts.
Select the account you added in step 1 and click the minus sign.
Click OK.