To send as or on behalf of:
On the Home tab, select New Email.
Click the box and select Other E-mail Address... from the dropdown.
Note: The From field is hidden by default. If the From field is hidden, select Options from within the message you are composing, then select From.
Within the Send From Other E-Mail Addresses window, click From to select the email address of the account you want to send as or send on behalf of.