The TO field remembers what you've typed before. If you've sent anything to
that address, it won't do it for you. However, if the person is in your
contacts (and it's just not popping up), you can type one or two letters,
then click the "Check Names" button. It's to the right of the send and attach
buttons, on the email toolbar. That'll pull up a list and you can select from
there. From this point on, Outlook will remember that address, and it'll just
pop up.