This article explains how to change your default account for macOS Mail when you have multiple email accounts. The information in this article applies to Macs running macOS Catalina through Sierra.
How to Set a Default Email Client on Mac
Your Mac Mail account may already have one of your Apple email addresses listed as the default. To change this and specify a new default email account in Mac Mail:
Open the Mail application on your Mac and select Mail > Preferences from the menu bar.
Click the Compose tab at the top of the Compose Preferences window that opens.
