Information in this article applies to Macs with macOS Catalina through Sierra.

Click theComposingtab at the top of the Composing preferences window that opens.

choose the desired account from the drop-down menu next toSend new messages from.

Path to Mail Preferences on a Mac

wrap up the preferences window to save the change.

Selecting the Composing tab.

Composing screen of Mail preferences

Composing preferences in Mac Mail