In both cases, you can override the default, of course, and you can do it comfortably right when composing the message.
Choose the Account Used to Send a Message in Outlook
To specify the account from which to send a message in Outlook:
- Click Account in the message windows (right beneath the Send button).
- Select the desired account from the list.
Choose the Account Used to Send a Message in Outlook 2003
To send an email from an account other than the default in Outlook 2003:
- Click on Accounts in the standard toolbar while composing the message in Outlook.
- If you do not see the Accounts field, go to the View | Toolbars menu and make sure Standard is checked. Maybe you also have to click the down arrow at the right handle of the toolbar and select Add or Remove Buttons | Standard | Accounts.
- Choose the desired account from the drop-down menu.