Default From
Choose which mailbox a new email starts from.
Left alone, a new email goes out from whichever inbox you are working in at the time. Turn this on when you would rather one address were always chosen for you.

Go to Settings > Default From.
Turn on Default From Email.
Choose the address every new email should start from.
Change the From mailbox on any individual email from within the composer at any time; this setting only decides what a new email opens with.