To make us your favorite on the Web/Desktop App...

• Click on the small carrot in the lower left corner and then on Settings in the menu that appears. 

• Click on the Preferences tab on the left side. Making Polymail your default app is the first option there:

To make us your favorite on iOS...

• Go to Settings

• Either scroll down the list of apps at the bottom, or

• Pull down to reveal the search bar and type Polymail

• Tap on Polymail in the list or the search results

• Look for the setting Default Mail App

• Tap Default Mail App

• Once in the menu, select Polymail from the list