If you’re outside Mail view (in People view or in a web browser, for example) and you generate a new email, it will by default be sent from the first account you set up in Outlook. You can set another account as the default if you’d rather change it.
Click the File tab.
Click the Account Settings button.
Select Account Settings.
The Account Settings dialog box appears, listing the accounts currently set up. A check mark will appear next to the default account.
Select the account you want to make the default.
Click the Set as Default button.
The check mark moves over to the new default account.
Now, any emails generated outside of Mail view will be sent from the selected account.