Setting the default mail app in macOS

macOS requires the default mail client be set using Apple Mail.

To use Swift Mail as your default mail client, open Apple Mail and select Mail > Preferences... in the menu bar. In the General tab of the Preferences window, select Swift Mail.app from the Default email reader pop-up menu.

The full path to Apple Mail is /Applications/Mail.app.

Refer to Apple's support article for additional details.