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.
This behavior is as designed on Mac OS X Yosemite (10.10) and later versions. Sandboxed applications, such as Outlook 2016 for Mac, cannot call the function to set these applications as default applications in Yosemite and later versions.
To work around this behavior, set Outlook to be the default application in Apple Mail preferences. To do this, follow these steps:
Run the Mail application.
On the Mail menu, click Preferences.
Click the General tab.
For Default email reader, select Microsoft Outlook.
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