Outlook doesn't have "contacts autofill". The autocompletion feature for
which John describes the activation doesn't reference your contacts in any
way. It remembers the most recent 1,000 resolved addresses, usually
generated by you sending a message to someone. The remembered recipient
addresses might be in your Contacts because you chose them from there, or
they may somply be addresses you've manually typed in a recipient field.
Here's a complete explanation: http://www.ingressor.com/autocompletetips.htm