You'll need to press Ctrl+K to force autoresolution at any time before sending the message; otherwise Outlook will always resolve the address when you send the message. If a match isn't found, you'll be asked to pick names using the Check Names dialog.
Update: In Outlook 2010 and up, the option to use Automatic name checking is in File, Options, Mail, about halfway down the screen.
Published August 18, 2003. Last reviewed on May 23, 2012.