Set default mail client in macOS without adding an email account?

I'm on OS X el Capitan (10.11.6). I use Outlook to access my email and would like outlook to be the default mail application. All the instructions I find online suggest that I should open mail and choose "Mail -> Preferences" to set the default mail app. Unfortunately, Preferences is greyed out until an account is added. I don't have another account that I want to add to my Mac. How can I set the default mail client to Outlook without adding an account to the mail client?