Changing your Default Account
Via Drag and Drop
To change your default account in Postbox, from the main mail window drag and drop the account you want as the default to the top of your account tree. You can also use this technique to re-order your accounts in the account tree.
Via Account Preferences
You can also set the default account via account preferences by going to Preferences > Accounts (Options > Accounts on Windows), selecting the account you wish to make the default and then select the +or Add button and select Set as Default from the menu.