Mac OS X 10.0 through latest

The default account when composing a new message in Mail is the one listed first in the Description column of Mail Preferences. To change this default, you only need to change the order of the names.

Follow these steps:

1. Choose Mail > Preferences.

 

2. Click Accounts.

 

3. Drag account names in the Description column to change their order. The account that is listed first is the default account that is used whenever you open a new mail window.