In Microsoft Outlook 2016 for Mac running on Mac OS X Yosemite (10.10) or later versions, you cannot set Outlook as the default application.
In Outlook Preferences, under General, you enable the Make Outlook the default application for e-mail, calendar, and contacts option. However, when you check the general preferences again, this option is not enabled.
In Outlook Preferences, under General, you enable the Make Outlook the default application for e-mail, calendar, and contacts option. However, when you check the general preferences again, this option is not enabled.