You can set up multiple email accounts in Outlook, and they can all have their own email address associated with them. This address will automatically appear in the From: line you send from the account.
But you do not have to set up a new account to send an email from a certain address — to unsubscribe from a newsletter you still get forwarded form an old account, for example.
Send an Email with Any From: Address in Outlook
To send a message with a different email address in the From: line from Outlook:
Create a new message in Outlook.
In Outlook 2007:
Go to the Options tab in the message's toolbar.
Click Show From to make sure it is enabled.
In Outlook 2003:
Select View | From Field from the menu to make sure it is checked.
Type the desired email address in the From... field.